Guidelines for the Selection of Four Optoelectronic Image Detectors for

1 Guidelines for the Selection of Four Optoelectronic Image Detectors for Low-Light Level Applications YAIR TALMI Princeton Instruments, Inc., P.O. Box 2318, Princeton, NJ 08540

Downloaded by 110.14.44.225 on May 9, 2018 | https://pubs.acs.org Publication Date: November 16, 1983 | doi: 10.1021/bk-1983-0236.ch001

KENNETH W. BUSCH Department of Chemistry, Baylor University, Waco, TX 76798

The performance characteristics of four optoelectronic image d e t e c t o r s (OIDs) are discussed. The detectors discussed are the s i l i c o n intensified target v i d i c o n (SIT), the i n t e n s i f i e d SIT, the i n t e n s i f i e d s i l i c o n photodiode array detector (ISPD), and the self-scanned photodiode array detector. The main objective of the paper i s to provide research workers interested in applying OIDs to a particular application with comparative performance information so that the best detector for a particular application may be selected. O p t o e l e c t r o n i c image d e t e c t o r s (OIDs) are c u r r e n t l y of g r e a t i n t e r e s t to a v a r i e t y of r e s e a r c h w o r k e r s whose work i n v o l v e s the d e t e c t i o n of d i f f e r e n t portions of the electromagnetic spectrum. The great d i v e r s i t y of OIDs makes i t d i f f i c u l t f o r researchers who are not n e c e s s a r i l y experts i n the area of o p t o e l e c t r o n i c image d e t e c t i o n to s e l e c t the best image detector f o r a p a r t i c u l a r a p p l i c a t i o n . For example, a detector whose p e r f o r m a n c e i s s a t i s f a c t o r y f o r a p p l i c a t i o n s i n v o l v i n g h i g h - l i g h t l e v e l s may be t o t a l l y u n s a t i s f a c t o r y when used t o detect l o w - l i g h t l e v e l s . The main o b j e c t i v e of t h i s paper i s to provide a framework f o r comparison of a number of OIDs which are expected to have a r e a l impact on spectroscopic d e t e c t i o n i n the foreseeable f u t u r e . The scope of t h i s d i s c u s s i o n w i l l be l i m i t e d t o t h o s e spectroscopic a p p l i c a t i o n s where low l i g h t l e v e l s are involved. I t i s i n t h e s e a p p l i c a t i o n s where most of the p r o b l e m s o c c u r , and where an i n - d e p t h knowledge of d e t e c t o r p e r f o r m a n c e i s r e q u i r e d i n s e l e c t i n g a detector f o r a p a r t i c u l a r a p p l i c a t i o n . By d e f i n i t i o n , i n t h i s c a t e g o r y a r e i n c l u d e d a l l "photons t a r v e d " a r e a s of s p e c t r o m e t r y . I t i s i n s t r u c t i v e t o c o n s i d e r some of the causes of photon s t a r v a t i o n i n spectroscopy.

0097-6156/83/0236-0001 $08.50/0 © 1983 A m e r i c a n C h e m i c a l S o c i e t y

Talmi; Multichannel Image Detectors Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


2

MULTICHANNEL IMAGE DETECTORS

L o w - l e v e l s i g n a l s can r e s u l t f r o m a v a r i e t y of c a u s e s . Among them are: 1. I n e f f i c i e n c y of the s p e c t r o m e t r i c phenomena, e.g., various chemiluminescence, bioluminescence, or thermoluminescence processes. 2. I n e f f i c i e n t s p e c t r a l s o u r c e s or a s s o c i a t e d o p t i c s as e n c o u n t e r e d w i t h the f l u o r e s c e n c e of microscopic samples or i n l a s e r microprobe atomic emission where there i s i n s u f f i c i e n t energy i n the e x c i t a t i o n source. 3. Loss of l i g h t due to high requirements of f i l t e r i n g as i n resonance Raman spectroscopy where a high degree of f i l t e r i n g i s necessary to exclude the resonance e x c i t i n g l i n e . A n o t h e r common r e a s o n f o r p h o t o n s t a r v a t i o n i n v o l v e s s p e c t r a l phenomena of a t r a n s i e n t n a t u r e . The s h o r t e r the measured p u l s e , the more d i f f i c u l t i t i s to keep i t at a s u f f i c i e n t l y high energy l e v e l to be measured. In t h i s category b e l o n g a g r e a t number of l a s e r s p e c t r o s c o p i c s t u d i e s of l i f e t i m e , i.e., s t u d i e s o f m e c h a n i s m s i n v o l v i n g v a r i o u s t r a n s i t i o n s t a t e s , f o r b i d d e n s t a t e s , energy exchange t h r o u g h r a d i a t i o n and r a d i a t i o n l e s s mechanisms, etc. Another example of a spectroscopic technique involving transient signals is picosecond l a s e r spectroscopy which permits d i f f e r e n t i a t i o n of phenomena that would be otherwise unresolved, e.g., s c a t t e r i n g and f l u o r e s c e n c e . Ultra-short spectroscopy also allows measurements to be made i n the p r e s e n c e of e x c e e d i n g l y h i g h c o n t i n u o u s - w a v e (cw) b a c k g r o u n d l e v e l s as e n c o u n t e r e d i n the measurement of plasma temperatures by Thompson s c a t t e r i n g , or measurement of fluorescence of o i l s l i c k s i n d a y l i g h t . In t h i s c a s e , the d i s c r i m i n a t i o n a g a i n s t the h i g h b a c k g r o u n d l e v e l i s performed by synchronizing data a c q u i s i t i o n with pulse t i m i n g . The performance of these procedures can be improved by averaging many i n d i v i d u a l p u l s e s , o b t a i n e d e i t h e r s y n c h r o n o u s l y or asynchronously· L i m i t e d periods of i l l u m i n a t i o n or e x c i t a t i o n of the sample r e p r e s e n t s t i l l a n o t h e r cause of p h o t o n s t a r v a t i o n . Brief p e r i o d s of i l l u m i n a t i o n a r e o f t e n n e c e s s a r y i n a number of spectroscopic a p p l i c a t i o n s to a v o i d d e l e t e r i o u s e f f e c t s . For example, prolonged i r r a d i a t i o n of v a r i o u s f l u o r e s c i n g compounds can o f t e n r e s u l t i n bleaching. S i m i l a r l y , microsample a n a l y s i s by Raman s p e c t r o m e t r y can e a s i l y cause s t r u c t u r a l and c o m p o s i t i o n a l damage t o the sample upon l o n g e x p o s u r e to the intense e x c i t a t i o n r a d i a t i o n . F i n a l l y , the a v a i l a b i l i t y of very l i m i t e d amounts of sample can r e s u l t i n a low r a d i a n t f l u x when samples a r e a n a l y z e d by emission spectroscopy. C l i n i c a l analyses i n v o l v i n g samples from e i t h e r c h i l d r e n or s m a l l animals are an example of a s i t u a t i o n where the amount of sample a v a i l a b l e i s o f t e n very l i m i t e d .

Detectors for Low-Light Level Operation

light

There are a v a r i e t y of OIDs that are s a t i s f a c t o r y f o r lowl e v e l operation. Here, however we w i l l d i s c u s s only the



1.

TALMI AND BUSCH

Low-Light

Level

3

Applications

d e t e c t o r s most a v a i l a b l e c o m m e r c i a l l y , or those t h a t expected to have a r e a l impact i n the foreseeable f u t u r e .

are

S i l i c o n I n t e n s i f i e d T a r g e t (SIT) V i d i c o n and I n t e n s i f i e d (ISIT)

SIT

These are s i l i c o n v i d i c o n (generic name) TV-type d e t e c t o r s t h a t have been e x t e n s i v e l y d e s c r i b e d i n the s p e c t r o s c o p i c l i t e r a t u r e . B a s i c a l l y they c o n s i s t of a t w o - d i m e n s i o n a l (2D) sensing area made of an array of s i l i c o n photodiodes that a l s o s e r v e as t e m p o r a r y s t o r a g e d e v i c e s , and an e l e c t r o n beam s c a n n i n g a r r a n g e m e n t t o r e a d the s i g n a l o f f t h i s t a r g e t * The h i g h s e n s i t i v i t y of the SIT i s a c h i e v e d t h r o u g h an image i n t e n s i f i c a t i o n s t a g e w i t h an i n t e r n a l g a i n o f a p p r o x i m a t e l y 2500. An image, e.g., a s p e c t r u m , i s f i r s t c o n v e r t e d by the photocathode to an equivalent e l e c t r o n image. An e l e c t r o s t a t i c f o c u s i n g s e c t i o n , a c c e l e r a t e s t h i s e l e c t r o n image, w h i l e r e t a i n i n g i t s f i d e l i t y , r e s u l t i n g i n i n c i d e n t e l e c t r o n s of a p p r o x i m a t e l y 8-9 KeV. S i n c e each p h o t o e l e c t r o n p r o d u c e s an a d d i t i o n a l e l e c t r o n / h o l e p a i r i n the s i l i c o n t a r g e t f o r each increase of 3.6 eV, the i n t e r n a l g a i n i s approximately 2500 f o r 9 KeV e l e c t r o n s . I f the r e a d o u t n o i s e i s m a i n t a i n e d a t approximately 2500 electrons/scan/channel, then a S/N = 1 should theoretically be achieved for a signal of 1 photoelectron/channel· The I S I T c o n s i s t s of a f i r s t - g e n e r a t i o n t r i o d e image i n t e n s i f i e r w i t h an o p t i c a l g a i n o f 50-100 w h i c h i s o p t i c a l l y coupled to the photocathode of an SIT tube. The main advantages of an I S I T o v e r an SIT a r e : Gain. T h e o r e t i c a l l y , the ISIT can produce a s i g n a l of at l e a s t 5 d i g i t a l c o u n t s per p h o t o e l e c t r o n . I n f a c t , most of t h i s assumed advantage i s l o s t i n the d i g i t i z a t i o n p r o c e s s and, of course, the maximum S/N f o r a s i g n a l of 1 photoelectron cannot be b e t t e r than 1. However, by l o w e r i n g the g a i n of the p r e a m p l i f i e r to o b t a i n a readout noise l e s s than 1 count rms, an improvement i n S/N i s obtained f o r low l e v e l s i g n a l s (less than 1 photoelectron/scan) when on-target i n t e g r a t i o n i s u t i l i z e d . S p e c t r a l Response. For r e a s o n s stemming from d i f f e r e n c e s i n production p h i l o s o p h i e s , r a t h e r t h a n f u n d a m e n t a l d i f f e r e n c e s , commercial image i n t e n s i f i e r s have more e f f i c i e n t photocathodes, p a r t i c u l a r l y i n the r e d . I n the UV, a U V - t o - v i s i b l e c h e m i c a l converter i s s t i l l necessary. Gat i n g Rat i o . The on/off gating r a t i o i s higher by a p p r o x i m a t e l y 2 o r d e r s o f m a g n i t u d e , making the I S I T a b e t t e r detector f o r pulse spectroscopy.



4


Format, Larger format image i n t e n s i f i e r s , e.g., 25 and 40 mm i n d i a m e t e r , a r e more a v a i l a b l e and a f f o r d a b l e t h a n SITs. Such i n t e n s i f i e r s can be o p t i c a l l y coupled to an SIT through a format reducing o p t i c a l - f i b e r coupler, e.g., 40 to 18 mm (SIT standard format). This method w i l l a l l o w a wider s p e c t r a l coverage w i t h l i t t l e , i f any, l o s s i n r e s o l u t i o n . The main d i s a d v a n t a g e s of the I S I T a r e : 1. S u b s t a n t i a l l y c o s t l i e r . 2. Less r e l i a b l e ; s h o r t e r l i f e t i m e . 3. More d i f f i c u l t to o p e r a t e ; more o p e r a t i o n a l p a r a m e t e r s t o c o n t r o l . 4. L e s s immune t o sudden " l i g h t " shocks. 5. Worse p i n cushion d i s t o r t i o n ; worse r e s o l u t i o n and more n o n l i n e a r wavelength c a l i b r a t i o n . 6. More d i f f i c u l t i e s on c o o l i n g , i.e., more l a g a t l o w e r t e m p e r a t u r e s . T h e r e f o r e , the I S I T i s even l e s s amenable than the SIT to long term s i g n a l i n t e g r a t i o n (on-target). MCP/SPD ( I S P D - I n t e n s i f i e d S i l i c o n Photodiode Array Detector) The ISPD or i n t e n s i f i e d s i l i c o n photodiode array d e t e c t o r c o n s i s t s of a m i c r o c h a n n e l p l a t e (MCP) image i n t e n s i f i e r o p t i c a l l y c o u p l e d t o a s i l i c o n p h o t o d i o d e a r r a y (SPD) as the m u l t i c h a n n e l r e a d o u t / s t o r a g e d e v i c e . The MCP (1_) i s a d i s k shaped c o n t i n u o u s dynode e l e c t r o n m u l t i p l i e r imager. It c o n s i s t s of m i l l i o n s of hollow microchannels formed i n a g l a s s substrate, where each channel, which i s 12-25 y m i n diameter, i s s e m i - c o n d u c t i n g . Each c h a n n e l a c t s as an i n d i v i d u a l e l e c t r o n m u l t i p l i e r w i t h an absolute geometric r e g i s t r a t i o n between the input and output p l a t e s of the device. An o p t i c a l gain of 10-50 χ 10 i s t y p i c a l of MCPs. C a s c a d i n g a few MCPs w i t h c h a n n e l s a r r a n g e d i n a c h e v r o n o r i e n t a t i o n can produce g a i n s o f 10 , s u f f i c i e n t f o r m u l t i c h a n n e l photon c o u n t i n g . An MCP image i n t e n s i f i e r c o n s i s t s of an input photocathode f o l l o w e d by an MCP wafer which i s i n turn f o l l o w e d by a phosphor output. Focusing of the e l e c t r o n image produced by the photocathode can be done by p r o x i m i t y , e l e c t r o s t a t i c , and m a g n e t i c methods. Only the f i r s t two a r e a v a i l a b l e c o m m e r c i a l l y at an a f f o r d a b l e c o s t . This d i s c u s s i o n w i l l concentrate on the MCP/SPD, although other OIDs can be used as readout devices Q ) . Self-Scanned S i l i c o n Photodiode Array

(SPD)

At t h i s time, the self-scanned s i l i c o n photodiode array i s t h e o n l y c o m m e r c i a l l y a v a i l a b l e d e v i c e t h a t has been s p e c i f i c a l l y designed as a p a r a l l e l s p e c t r o m e t r i c detector. The performance c h a r a c t e r i s t i c s o f t h i s d e t e c t o r and i t s a p p l i c a b i l i t y t o s p e c t r o p h o t o m e t r i c and s p e c t r o f l u o r o m e t r i c measurements have been r i g o r o u s l y d i s c u s s e d e l s e w h e r e (.2-4.). The self-scanned photodiode array i s a m o n o l i t h i c s i l i c o n wafer c o n s i s t i n g of an a r r a y of d i o d e s , each a c t i n g as a l i g h t - t o charge t r a n s d u c e r and a s t o r a g e d e v i c e . Two s h i f t r e g i s t e r s


1.

TALMI AND BUSCH

Low-Light

Level

5

Applications

r e a d out the c h a r g e s i g n a l s s t o r e d i n each of the odd and even diodes* Low n o i s e and, c o n s e q u e n t l y , h i g h dynamic range a r e a c h i e v e d t h r o u g h an a p p r o p r i a t e d e s i g n o f t h e readout p r e a m p l i f i e r and s i g n a l p r o c e s s i n g e l e c t r o n i c s e c t i o n * One c o m m e r c i a l l y a v a i l a b l e d e v i c e , an S-type s i l i c o n p h o t o d i o d e array made by EG&G Reticon, i s arranged so that each i n d i v i d u a l d i o d e i s 2*5 mm h i g h and 25 μ m wide* T h i s 100:1 a s p e c t r a t i o i s a compromise design corresponding to t y p i c a l aspect r a t i o s of conventional spectrometer entrance s l i t s .


The Ideal Multichannel (Parallel) Detector The s e l e c t i o n of a p a r t i c u l a r OID f o r a g i v e n a p p l i c a t i o n i n v o l v e s a number of important c o n s i d e r a t i o n s . In e v a l u a t i n g the performance of these devices, i t i s i n s t r u c t i v e to compare t h e i r a c t u a l p e r f o r m a n c e c h a r a c t e r i s t i c s w i t h t h o s e of a h y p o t h e t i c a l i d e a l OID. I n t h i s way, i t i s p o s s i b l e to d e t e r m i n e the e x t e n t t o w h i c h a g i v e n d e t e c t o r compares i n performance to an " i d e a l " OID. Table I l i s t s some of the major c h a r a c t e r i s t i c s of an i d e a l OID. These c r i t e r i a a r e d i v i d e d i n t o two c a t e g o r i e s : operation and performance c h a r a c t e r i s t i c s , and s p e c t r o m e t r i c performance* In the d i s c u s s i o n which f o l l o w s , each detector under c o n s i d e r a t i o n w i l l be evaluated against the c r i t e r i a l i s t e d i n Table I. To be most u s e f u l to the reader i n l o c a t i n g i n f o r m a t i o n about a p a r t i c u l a r performance f e a t u r e of a g i v e n d e t e c t o r , the f o l l o w i n g d i s c u s s i o n w i l l be o r g a n i z e d around the format developed i n Table I. Thus, the reader who i s i n t e r e s t e d i n a p a r t i c u l a r c h a r a c t e r i s t i c — immunity t o l i g h t shocks, f o r e x a m p l e — can r e f e r to that s e c t i o n f o r a d i s c u s s i o n of t h i s feature f o r the d e t e c t o r s under consideration*

Operation and Performance Characteristics Cost and

Availability

S I T &. I S I T * C o m m e r c i a l l y a v a i l a b l e from manufacturers at a moderate cost*

a variety

of

ISPD* C o m m e r c i a l l y a v a i l a b l e i n 18, 25, and 40 mm diameters (MCP i n t e n s i f i e r s ) . SPDs a r e a v a i l a b l e as 64, 128, 512, 1024, and 4096 element arrays. SPD* SPDs a r e c o m m e r c i a l l y a v a i l a b l e , a l t h o u g h due t o production y i e l d c o n s i d e r a t i o n s t h e i r p r i c e i s s t i l l r e l a t i v e l y high. R e l i a b i l i t y and R e p e a t a b i l i t y SIT & ISIT.

Reasonably good.


6

MULTICHANNEL IMAGE DETECTORS Table

I . The

Ideal Spectrometric Multichannel ( P a r r a l e l ) Detector


O p e r a t i o n and 1.

Low

Performance

c o s t and w i d e c o m m e r c i a l

Characteristics

availability.

2.

High

3.

Long s h e l f l i f e , l o n g o p e r a t i o n l i f e , Day-to-day r e p r o d u c i b i l i t y .

d e v i c e - t o - d e v i c e performance r e l i a b i l i t y

4.

A v a i l a b i l i t y i n a l a r g e v a r i e t y of formats, s i z e s , p i x e l ( p i c t u r e c e l l ) d i m e n s i o n s , and a r r a y d e n s i t y . B o t h l i n e a r area a r r a y s are necessary.

and

and

repeatability.

l o n g term

stability.

and

5.

S i m p l i c i t y o f o p e r a t i o n , m i n i m a l number o f a d j u s t a b l e paramet e r s t h a t a f f e c t the performance of the d e t e c t o r .

6.

M e c h a n i c a l and e l e c t r o n i c r u g g e d n e s s . T o l e r a n c e t o and o p e r a b i l i t y under h o s t i l e environments, i . e . , h e a t , h u m i d i t y , m a g n e t i c f i e l d s , RF, e t c . Minimum m i c r o p h o n i c e f f e c t s , i . e . , s t a b i l i t y o f r e a d o u t under v i b r a t i o n and o t h e r p r e s s u r e wave oscillations.

7.

Immunity t o sudden " l i g h t s h o c k s , " i . e . , r a p i d r e c o v e r y f r o m sudden e x p o s u r e t o h i g h l i g h t l e v e l s . N o n - d e s t r u c t i v e under v e r y h i g h l i g h t l e v e l s as l o n g as e x c e e d i n g l y h i g h t e m p e r a t u r e s are not reached.

8.

Low

9.

High t o l e r a n c e f l a t n e s s a c r o s s the e n t i r e a r r a y to a v o i d l o s s of r e s o l u t i o n at the f l a t f o c a l p l a c e of the s p e c t r o m e t e r . A v a i l a b i l i t y of curved a r r a y s to f i t curved f o c a l p l a n e s .

weight,

low power c o n s u m p t i o n , e a s y and

Spectrometer

adjustable cooling,

Performance

1.

Wide s p e c t r a l r e s p o n s e : x - r a y t o IR. At l e a s t i t i s d e s i r a b l e to have d e t e c t o r s c o v e r i n g t h e UV t o NIR, x - r a y t o UV, and NIR t o medium IR.

2.

Arrays with a variable s p a t i a l (spectral) resolution. This can be a c h i e v e d by v a r y i n g e l e c t r o n i c a l l y t h e d e n s i t y o f t h e array. Good r e s o l u t i o n .

3.

Random a c c e s s r e a d o u t o f i n d i v i d u a l p i x e l s . Pseudo random access, i . e . , fact access, i s possible with l i n e a r arrays. R e a l random a c c e s s i s p o s s i b l e w i t h a v a r i e t y o f 2D a r r a y s . To enhance t h e S/N o f low l e v e l s i g n a l s i n t h e p r e s e n c e o f h i g h l e v e l s i g n a l s , t h i s a c c e s s s h o u l d be p o s s i b l e w i t h o u t a d e s t r u c t i v e readout.

4.

U l t r a r a p i d e l e c t r o n i c s h u t t e r i n g ( g a t i n g ) f o r measurements o f v e r y f a s t t r a n s i e n t phenomena. H i g h o n / o f f g a t i n g r a t i o s are necessary.


1.

TALMI AND

BUSCH

Low-Light


Table

Level

I

1

Applications

(Continued)

5.

Minimum number o f b l e m i s h e s and d e f e c t s . Most d e f e c t s b e l o n g to e i t h e r o f two c a t e g o r i e s ; s e v e r e l y r e d u c e d ( o r no) r e s p o n s e , and u n u s u a l l y h i g h d a r k c h a r g e .

6.

Minimum v a r i a t i o n s i n d a r k - c h a r g e and r e s p o n s e a c r o s s t h e array. A l s o , minimum v a r i a t i o n i n s p e c t r a l r e s p o n s e , i . e . , c o n s t a n c t response ( w i t h h i g h e f f i c i e n c y ) w i t h wavelength. T h i s e x c l u d e s , by d e f i n i t i o n , " p h o t o n s e n s o r s . "

7.

Low d a r k - c h a r g e t o e n a b l e l o n g s i g n a l i n t e g r a t i o n and s t o r a g e p e r i o d s and f o r l o n g - t e r m s t a b i l i t y . Dark c h a r g e n o i s e s h o u l d be n e g l i g i b l e compared w i t h e i t h e r p r e a m p l i f i e r / r e a d o u t o r photon shot n o i s e .

8.

Low p r e a m p l i f i e r n o i s e , and v e r y h i g h g a i n ( 10^) t o d e t e c t v e r y low l i g h t l e v e l s i g n a l s , i d e a l l y t h r o u g h photon-counting techniques. Readout s y s t e m s h o u l d bave an u l t r a r a p i d r e s p o n s e time.

9.

V e r y w i d e dynamic r a n g e . Usually a transfer characteristic c u r v e ( i n p u t - t o - o u t p u t ) w i t h a s l o p e ( ) o f 1 i s i d e a l . However, a c a p a b i l i t y t o a l t e r t h i s s l o p e e l e c t r o n i c a l l y c a n be a d v a n t a g e o u s when v e r y h i g h dynamic r e s e r v e s a r e n e c e s s a r y w i t h p i x e l s of l i m i t e d c a p a c i t a n c e .

10. Long s i g n a l i n t e g r a t i o n p e r i o d s f o r enhancement o f S/N. f l e x i b i l i t y i n a l t e r i n g the l e n g t h of these p e r i o d s .

High

11. Long s i g n a l s t o r a g e p e r i o d s . P a r t i c u l a r l y u s e f u l i n 2D OIDs where a l a r g e amount o f d a t a must be t e m p o r a r i l y s t o r e d b e f o r e b e i n g t r a n s f e r r e d t o memory. 12. N e g l i g i b l e r e a d o u t l a g , i . e . , i n c o m p l e t e r e a d o u t o f s i g n a l . This i s s p e c i f i c a l l y important f o r t r a n s i e n t spectrometry. 13. N e g l i g i b l e b l o o m i n g , i . e . , p i x e l - t o - p i x e l c r o s s t a l k , due t o c h a r g e s i g n a l o v e r s p i l l t o a d j a c e n t p i x e l s . 14. Long t e r m e l e c t r i c a l , r a d i o m e t r i c and g e o m e t r i c (wavelength c a l i b r a t i o n ) . 15. Minimum d i s t o r t i o n o f image ( s p e c t r a l Constant m a g n i f i c a t i o n .

mostly

stability

l i n e s ) a c r o s s the a r r a y .

16. E q u a l m o d u l a t i o n t r a n s f e r f u n c t i o n (MTF) a c r o s s t h e a r r a y . This w i l l r e s u l t i n equal r e s o l u t i o n c h a r a c t e r i s t i c s . 17. C a p a b i l i t y t o v a r y s p e c t r a l d i m e n s i o n s e i t h e r o p t i c a l l y ( w i t h o p t i c a l f i b e r c o u p l e r s ) o r e l e c t r o n i c a l l y (by d e m a g n i f i c a t i o n ) to e n s u r e c o m p a t i b i l i t y w i t h f o c a l p l a n e f o r m a t . 18. C a p a b i l i t y o f v a r y i n g s c a n

time.



8

ISPD. Reasonably good, although they vary widely with respect to photocathode s e n s i t i v i t y (yA/lumen) and u n i f o r m i t y . SPD. D e v i c e - t o - d e v i c e r e l i a b i l i t y i s g e n e r a l l y good, although s u b s t a n t i a l v a r i a t i o n s i n dark charge are s t i l l s i g n i f i c a n t . Long Term S t a b i l i t y


SIT &. ISIT.

Reasonably

satisfactory.

ISPD. E l e c t r o s t a t i c a l l y f o c u s e d MCPs a r e b e t t e r i n t h i s r e s p e c t . P r o x i m i t y - f o c u s e d MCP i n t e n s i f i e r s show an a v e r a g e g a i n r e d u c t i o n of 40-50% a f t e r 2-3,000 hours of n o n - s t o p e x p o s u r e t o v a r y i n g low l i g h t l e v e l s . T h i s i s e q u i v a l e n t t o approximately 1-1.5 years of continuous o p e r a t i o n (4 hours/day). A l s o , the gain can be r a i s e d again, r e s u l t i n g i n a r e a l l i f e t i m e of 6,000-8,000 hours. SPD. The SPD, t y p i c a l of other i n t e g r a t e d c i r c u i t devices, has good s h e l f and o p e r a t i o n a l l i f e . Day-to-day r e p r o d u c i b i l i t y of p e r f o r m a n c e i s good. A p o t e n t i a l p r o b l e m , not y e t t h o r o u g h l y i n v e s t i g a t e d , i s v a r i a t i o n i n response a f t e r prolonged exposure to high l e v e l UV r a d i a t i o n . At what energy dose that phenomenon becomes s i g n i f i c a n t and whether o r n o t a n n e a l i n g can be p e r f o r m e d i s not y e t f u l l y u n d e r s t o o d . However, when used i n conventional UV-spectrometric measurements, t h i s p r o b l e m does not appear to be serious. Formats SIT & I S I T . A v a i l a b l e i n a few f o r m a t s , e.g., 18diameter, but non-standard ones are expensive.

and 25-mm

ISPD. MCP wafers are a v a i l a b l e i n a v a r i e t y of formats, shapes, and s i z e s , e.g., c i r c u l a r w a f e r s of up t o 13 cm i n d i a m e t e r and r e t a n g u l a r w a f e r s of up t o 8 χ 10 cm a r e r o u t i n e l y f a b r i c a t e d . MCPs a r e a l s o a v a i l a b l e w i t h i n d i v i d u a l c h a n n e l s b i a s e d a t a s p e c i f i c angle. This i s very u s e f u l f o r grazing incidence VUVX-ray spectroscopy. MCPs can be stacked, i n tandem, to achieve h i g h g a i n s , or o t h e r w i s e the i n d i v i d u a l c h a n n e l s a r e bent i n e i t h e r a " J " - or "C"-shape t o a l l o w o p e r a t i o n a t h i g h v o l t a g e s (2500-3000 V). At t h e s e v o l t a g e s , a s i n g l e MCP may a c h i e v e enough g a i n f o r photon c o u n t i n g (1_). MCP i n t e n s i f i e r s a r e c o m m e r c i a l l y a v a i l a b l e o n l y i n 18, 25, and 40 mm diameter formats. SPD. S-type SPDs a r e a v a i l a b l e i n f o r m a t s of 64, 128, 512, and 1024 e l e m e n t a r r a y s . A new 4096 e l e m e n t a r r a y SPD d e t e c t o r ( a s p e c t r a t i o 40:1) i s now a v a i l a b l e . A r e a a r r a y s w i t h up t o 200 χ 200 e l e m e n t s a r e a l s o a v a i l a b l e . I t i s our o p i n i o n (as


1.

TALMI AND BUSCH

Low-Light

Level

9

Applications

w e l l as others), however, that charge-coupled devices (CCDs) and c h a r g e - i n j e c t i o n type i m a g e r s (CIDs) h o l d g r e a t e r p r o m i s e f o r two-dimensional measurements. S i m p l i c i t y of

Operation


SIT &. ISIT. No. Magnetic c o i l parameters, cathode v o l t a g e , image s e c t i o n must a l l be c a r e f u l l y c o n t r o l l e d .

and

ISPD. Very simple operation with minimum v a r i a b l e parameters. MCP photocathode v o l t a g e i s a d j u s t a b l e from approximately 160240 V, but i s t y p i c a l l y f i x e d . Phosphor v o l t a g e i s a d j u s t a b l e f r o m 5-6 KV, and i s a l s o t y p i c a l l y f i x e d . MCP v o l t a g e can be v a r i e d t o a d j u s t the g a i n . As a r u l e of thumb, a 50 V i n c r e a s e i n v o l t a g e w i l l d o u b l e the g a i n . W i t h r e s p e c t t o SPDs, the phases and p r e a m p l i f i e r g a i n need u s u a l l y be adjusted only once. Scan time i s u s u a l l y f i x e d . SPD. The SPD i s r a t h e r s i m p l e to o p e r a t e . Adjustments are mainly those of phases and odd-even p r e a m p l i f i e r gain s e t t i n g . The a r r a y does not p r o v i d e any a m p l i f i c a t i o n g a i n (as SIT or MCP/SPD d e v i c e s do). Scan r a t e can a l s o be a d j u s t e d f r o m a few KHz to a few MHz. Ruggedness SIT ISIT. Rather good mechanical and e l e c t r i c a l ruggedness. These d e v i c e s a r e s u s c e p t i b l e to m a g n e t i c f i e l d s , and s u f f e r from microphonic e f f e c t s . ISPD. R a t h e r compact and r e s i l e n t e l e c t r o n i c a l l y rugged. Magnetic MCPs and SPDs are non-microphonic, under high humidity and temperature

d e v i c e s . M e c h a n i c a l l y and f i e l d s have l i t t l e e f f e c t . and are designed to operate conditions.

SPD. E l e c t r o n i c a l l y and m e c h a n i c a l l y very rugged. Tolerant of h i g h t e m p e r a t u r e , h u m i d i t y , v i b r a t i o n , and e l e c t r i c a l or magnetic f i e l d s . L i g h t Shocks SIT & ISIT. Can t o l e r a t e a r e a s o n a b l e l i g h t f l u x w i t h o u t s u b s t a n t i a l l y adverse effects. However, p r e c a u t i o n s necessary to avoid a c t u a l damage to the photocathode.

any are

ISPD. MCPs s h o u l d not be exposed t o l i g h t l e v e l s above 4x10"^ f o o t - c a n d l e f o r l o n g p e r i o d s of t i m e . P r e c a u t i o n s s h o u l d be s i m i l a r to those taken w i t h PMTs. Exposure to high l i g h t l e v e l s w i l l t e m p o r a r i l y r a i s e the n o i s e l e v e l of the d e t e c t o r . To p r o t e c t the MCP and warn the u s e r , ISPD d e t e c t o r s a r e u s u a l l y


10


equipped w i t h an automatic b r i g h t - c o n t r o l c i r c u i t r y that shuts o f f the p h o t o c a t h o d e v o l t a g e when p h o t o c u r r e n t o r phosphor c u r r e n t s above a t h r e s h o l d l e v e l a r e r e a c h e d . A l s o , a b e e p e r alarm warns the user of t h i s s i t u a t i o n (5). SPD. H i g h immunity t o " l i g h t shocks". R e c o v e r s r a p i d l y f r o m exposure to very h i g h l i g h t l e v e l s . Not adversely a f f e c t e d by l i g h t e x c e p t when l e v e l s a r e s u f f i c i e n t l y h i g h t o h e a t the s i l i c o n wafer above i t s m e l t i n g point. Low Weight and Power Consumption


SIT & I S I T . No. ISPD. MCP i n t e n s i f i e r s , p a r t i c u l a r l y the p r o x i m i t y f o c u s e d t y p e , a r e v e r y compact and rugged. They r e q u i r e l i t t l e power f o r o p e r a t i o n and are easy to c o o l . SPD. S m a l l s i z e and low w e i g h t . Uses v e r y l i t t l e power, and can be cooled to l i q u i d n i t r o g e n temperature. Flatness SIT & I S I T . A r r a y i s f l a t . Curved f o r m a t s a r e not a v a i l a b l e . E l e c t r o s t a t i c focusing causes pin-cushion d i s t o r t i o n and a l o s s of geometric r e s o l u t i o n at the detector's edges. ISPD. Both MCPs and SPDs a r e v e r y f l a t and have e x c e l l e n t g e o m e t r i c (wavelength) r e g i s t r a t i o n . E l e c t r o s t a t i c focusing, however, causes p i n - c u s h i o n d i s t o r t i o n and t h e r e f o r e a l o s s of g e o m e t r i c r e g i s t r a t i o n , p a r t i c u l a r l y a t the d e t e c t o r ' s edges. The p r o x i m i t y MCP produces a near p e r f e c t geometric r e g i s t r a t i o n and does not s u f f e r f r o m r e d u c e d s e n s i t i v i t y a t the edges ( s h a d i n g ) . Concave-shaped MCPs a r e a v a i l a b l e f o r non-photon a p p l i c a t i o n s (see below). SPD. Very f l a t a r r a y s w i t h e x c e l l e n t g e o m e t r i c t y p i c a l of i n t e g r a t e d c i r c u i t technology.

resolution,

Spectrometries Performance S p e c t r a l Response SIT & I S I T . S p e c t r a l r e s p o n s e depends on the s e l e c t i o n o f the p h o t o c a t h o d e and can be o p t i m i z e d f o r the b l u e o r the n e a r IR. T y p i c a l p h o t o c a t h o d e s a r e S-20 and S-20R (extended r e d m u l t i a l k a l i ) which a l l o w an adequate response up to 900 nm. Because the e l e c t r o s t a t i c a l l y focused image s e c t i o n r e q u i r e s a s p e c i a l o p t i c a l - f i b e r f a c e p l a t e window ( c u r v e d i n s i d e t o match t h e f o c u s i n g f i e l d c o n t o u r s ) made out of g l a s s , the c u t - o f f


1.

TALMI AND BUSCH

Low-Light

Level

Applications

11


wavelength f o r SITs i s approximately 350 nm. To overcome t h i s problem, a t h i n l a y e r of a proper o r g a n i c s c i n t i l l a t o r ( f l u o r e s c i n g agent) i s deposited on the o p t i c a l - f i b e r f a c e p l a t e . With such a s c i n t i l l a t o r an o v e r a l l quantum e f f i c i e n c y (QE) of 1-2% can be a c h i e v e d t o 100 nm or l o w e r . The e f f e c t i v e QE o f S I T d e t e c t o r s i s l o w e r t h a n t h a t o f PMTs because of t r a n s m i s s i o n losses through the f i b e r f a c e p l a t e . ISPD* The MCP i n t e n s i f i e r o f f e r s an u n u s u a l l y wide s p e c t r a l response, unmatched by any other OID. Each s p e c t r a l region w i l l be discussed i n turn. In the U V - V i s i b l e . the e l e c t r o s t a t i c MCP i n t e n s i f i e r has the same s p e c t r a l c h a r a c t e r i s t i c s as the SIT, i . e . , S-20 o r S20R p h o t o c a t h o d e s , and r e q u i r e s a c h e m i c a l c o n v e r t e r f o r UV measurements. The p r o x i m i t y focused MCP i n t e n s i f i e r has a good response i n the 200-850 nm s p e c t r a l range. In the VUV, two p o s s i b i l i t i e s e x i s t . F i r s t MgF£ or L i F windows can be used w i t h a v a r i e t y of photocathodes, mostly of the s o l a r - b l i n d type. Second, the photocathode m a t e r i a l can be d i r e c t l y deposited on the input of the MCP. For instance, C s l , KBr, and CsTe p h o t o c a t h o d e s a r e good t o a p p r o x i m a t e l y 100 nm, whereas L i F , BaF > M g F 2 , and CaF2 respond below 100 nm. Third, the channels themselves respond d i r e c t l y to photons of energies below approximately 100 nm. MCPs have been used e x t e n s i v e l y f o r d i r e c t measurement of p a r t i c l e s : f o r example, ions i n the 3 eV-20 KeV range, e l e c t r o n s i n the 100 eV to 100 KeV r a n g e , p o s i t r o n s , and m e t a s t a b l e s and n e u t r a l s such as He, Ne, A r , K r , Xe, e t c . T h e r e i s a s u b s t a n t i a l l i t e r a t u r e on t h i s mode of d e t e c t i o n , but i t s c o m p l i l a t i o n here i s beyond the scope of t h i s paper. 2

SPD. SPDs have good r e s p o n s e i n the s p e c t r a l r e g i o n f r o m 185 (or l e s s ) t o 1100 nm (2.). F o r use i n the VUV and EVUV r e g i o n s , i t i s p o s s i b l e t o d e p o s i t a f l u o r e s c i n g agent d i r e c t l y on the d i o d e s w h i c h w i l l a c t as a w a v e l e n g t h - u p c o n v e r t e r , t h e r e b y e n s u r i n g a good r e s p o n s e to w a v e l e n g t h s l e s s t h a n 185 nm. In the x-ray s p e c t r a l region, i n o r g a n i c phosphors can be deposited on the o p t i c a l - f i b e r window c o u p l e r ( S F - t y p e SPDs). W i t h an a p p r o p r i a t e t h i n n i n g of the p r o t e c t i v e S1O2 o v e r c o a t , the SPD can d i r e c t l y respond to h i g h energy p a r t i c l e s such as e l e c t r o n s . Some a d v e r s e s i d e e f f e c t s , m a i n l y due t o an i n c r e a s e i n d a r k charge under continuous bombardment, have been reported. Also, there i s a s i g n i f i c a n t a b s o r p t i o n of e l e c t r o n s w i t h e n e r g i e s below 6 KeV, even w i t h an o v e r c o a t i n g o f S1O2 t h a t i s 1 Pm thick. T h i s a b s o r p t i o n makes the d e v i c e i n s e n s i t i v e t o e l e c t r o n s at these energies.


12



Resolution SIT & I S I T . The r e s o l u t i o n of SIT d e t e c t o r s can be d e f i n e d i n s e v e r a l w a y s — a) p o s i t i o n ( s p a t i a l ) r e s o l u t i o n , b) R a y l e i g h r e s o l u t i o n , and c) modulation t r a n s f e r f u n c t i o n (MTF). S p a t i a l r e s o l u t i o n i s one channel which i s t y p i c a l l y 25 μ m wide. S p e c t r a l r e s o l u t i o n i s the p r o d u c t of the c h a n n e l w i d t h and the r e c i p r o c a l d i s p e r s i o n of the s p e c t r o m e t e r . For example, a s p e c t r o m e t e r w i t h a f o c a l l e n g t h of 0.25 m and g r a t i n g of 152.5 grooves/mm t y p i c a l l y p r o d u c e s a r e c i p r o c a l l i n e a r d i s p e r s i o n of 25 nm/mm. Therefore a 25 μηι channel w i l l c o v e r 0.64 nm. A 305 g/mm g r a t i n g u s e d w i t h t h e same spectrometer would produce a r e s o l u t i o n of 0.32 nm/channel. R a y l e i g h r e s o l u t i o n i s the s e p a r a t i o n r e q u i r e d f o r two images to be recognized as such. With OIDs t h i s w i l l t y p i c a l l y mean t h a t s e p a r a t i o n i s such t h a t c r o s s t a l k between l i n e s i s 50%. I f z e r o c r o s s t a l k i s r e q u i r e d , a t l e a s t one s e p a r a t i n g c h a n n e l i s n e c e s s a r y between the two l i n e s . Under i d e a l c o n d i t i o n s , cross t a l k between SIT channels i s approximately 40 to 50%, so that Rayleigh r e s o l u t i o n i s about 50 Pm (2 channels). I f a n a r r o w s l i t i s used w i t h a h i g h q u a l i t y s p e c t r o m e t e r , Rayleigh r e s o l u t i o n i n wavelength u n i t s becomes 50 μπι times the reciprocal dispersion. A s i m p l i f i e d v e r s i o n of w a v e l e n g t h r e s o l u t i o n which w i l l be used throughout t h i s manuscript i s the number of channels (or p i x e l s ) at FWHM of a s p e c t r a l l i n e . For an SIT, t h i s r e s o l u t i o n i s a p p r o x i m a t e l y 2-3 c h a n n e l s . The r e s o l u t i o n , however, i s not u n i f o r m a c r o s s the d e t e c t o r and worsens when a p p r o a c h i n g t h e edges. The r e a s o n f o r t h i s i s a pin-cushion geometric d i s t o r t i o n , t y p i c a l of e l e c t r o s t a t i c image sections. A d i s t o r t i o n o f 5% i s t y p i c a l and w i l l r e d u c e r e s o l u t i o n and cause e r r o r s i n the wavelength c a l i b r a t i o n curve of the d e t e c t o r . The l a t t e r , however, can be c o m p u t e r corrected. A n o t h e r means of e x p r e s s i n g the f i d e l i t y of an o p t i c a l s y s t e m i s i n terms of i t s m o d u l a t i o n t r a n s f e r f u n c t i o n . The m o d u l a t i o n t r a n s f e r f u n c t i o n d e s c r i b e s the a b i l i t y of the o p t i c a l system to a c c u r a t e l y reproduce an object whose p a t t e r n of luminance v a r i e s i n à s i n u s o i d a l manner. An o p t i c a l system w h i c h can p r e c i s e l y d u p l i c a t e the m o d u l a t i o n p a t t e r n o f t h e object i n the modulation p a t t e r n of the image has a modulation t r a n s f e r f u n c t i o n equal to 1.0; t h i s represents the performance of a p e r f e c t system. The g r e a t e r the d i f f e r e n c e between the m o d u l a t i o n p a t t e r n i n the image compared w i t h t h a t i n the object, the l o w e r the m o d u l a t i o n t r a n s f e r f u n c t i o n . In p r a c t i c e , the modulation t r a n s f e r p a t t e r n i s u s u a l l y evaluated w i t h a square-wave p a t t e r n produced by a p e r i o d i c a r r a y of lines. The modulation t r a n s f e r f u n c t i o n approaches zero as the s p a t i a l f r e q u e n c y of the l i n e s i n c r e a s e s . The limiting r e s o l u t i o n i n terms of t h e m o d u l a t i o n t r a n s f e r f u n c t i o n i s



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determined by the number of l i n e p a i r s per m i l l i m e t e r which give a modulation t r a n s f e r f u n c t i o n which approaches zero. When two c l o s e l y spaced s p e c t r a l l i n e s of equal i n t e n s i t y s a t i s f y the Rayleigh c r i t e r i o n , the v a l l e y between the l i n e s i s approximately 19% of the peak i n t e n s i t y . This corresponds to an MTF o f about 0.10. Based on the d a t a p r e s e n t e d under t h e d i s c u s s i o n of R a y l e i g h r e s o l u t i o n , a MTF of 0.10 would be expected to r e s u l t from a p a t t e r n w i t h about 20 lines/mm. The r e s o l u t i o n of t h e SIT i s d e f i n i t e l y v a r i a b l e ; any number of adjacent channels can be grouped together and read out as a s i n g l e r e s o l u t i o n element. A l s o , the number of diodes read out i n each channel can be v a r i e d from four (approximately 100 ym) t o 400 ( a p p r o x i m a t e l y 10 mm). Thus, the t w o - d i m e n s i o n a l target can be d i v i d e d i n t o a number of h o r i z o n t a l tracks, each of w h i c h can c o n t a i n any number of r e s o l u t i o n e l e m e n t s (max. 512) depending on the grouping setup. ISPD. W i t h p r o x i m i t y MCP i n t e n s i f i e r s the r e s o l u t i o n i s p r a c t i c a l l y constant across the e n t i r e array. By our s p e c t r a l r e s o l u t i o n d e f i n i t i o n (see SIT), the r e s o l u t i o n i s 3-4 diodes at FWHM. Near f i e l d s t r a y l i g h t , however, widens t h e wings of s p e c t r a l l i n e s . There are a few phenomena b e l i e v e d to cause t h i s apparent s t r a y r a d i a t i o n energy ( v e i l i n g g l a r e ) . These i n c l u d e : (a) L i g h t t r a n s m i t t e d through the semi-transparent photocathode i s r e f l e c t e d from the MCP wafer back to the photocathode. This process i s enhanced by the closeness of the photocathode to the i n p u t f a c e p l a t e of the MCP, i . e . , 200 ym. (b) H i g h e n e r g y photoelectrons c o l l i d i n g w i t h the dead area between MCP channels (10,000 V/cm f i e l d ) can p r o d u c e s e c o n d a r y e l e c t r o n s t h a t a r e c o l l e c t e d by c h a n n e l s w i t h i n a p p r o x i m a t e l y a 200 ym r a d i u s (8 diodes). With e l e c t r o s t a t i c i n t e n s i f i e r s t h i s s t r a y energy (estimated at approximately 3% of photoelectron energy) can be c o l l e c t e d by a s p e c i a l g r i d e l e c t r o d e p l a c e d between t h e p h o t o c a t h o d e and the MCP. T h i s i s i m p o s s i b l e t o do w i t h proximity intensifiers. (c) S e c o n d a r y e l e c t r o n s a r e s p r e a d during m u l t i p l i c a t i o n across the channel. This w i l l reduce the r e s o l u t i o n w i t h both types of i n t e n s i f i e r s . S i n c e the r e s o l u t i o n w i t h the proximity-focused phosphor i s approximately i n v e r s e l y p r o p o r t i o n a l to the gap between the phosphor and the o u t p u t f a c e p l a t e o f t h e MCP, and s q u a r e r o o t i n v e r s e l y p r o p o r t i o n a l to the v o l t a g e between them, i t could be somewhat i m p r o v e d by r e d u c i n g t h e gap and/or r a i s i n g t h e v o l t a g e . However, corona discharge problems set a p r a c t i c a l l i m i t on t h i s approach. The e f f e c t of n e a r f i e l d s t r a y r a d i a t i o n on t h e s p e c t r a l l i n e p r o f i l e i s shown i n F i g u r e 1. SPD. The s p e c t r a l r e s o l u t i o n of an SPD i s governed by the same f a c t o r s d i s c u s s e d f o r t h e S I T , i . e . , i t d e p e n d s on t h e g r a t i n g / s p e c t r o m e t e r c o m b i n a t i o n . The s p a t i a l r e s o l u t i o n i s degraded by a degree of d i o d e - t o - d i o d e c r o s s - t a l k , i.e., i t i s




Figure 1. The effect of near field stray radiation on the spectral line profile (546-nm Hg emission line). The detector was a microchannel-plate image intensified diode array (Reticon model RL-512SF, Princeton Instruments, Inc. model IRY-512). Key: a, full line profile; b, upper half (FWHM) of line; and c, lower portion of line, 0-10% relative emission intensity.



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v e r y d i f f i c u l t t o c o n t a i n the e n t i r e energy of a v e r y n a r r o w l i n e w i t h i n the b o u n d a r i e s o f a s i n g l e d i o d e . Typically, however, 2 d i o d e s or l e s s can c h a r a c t e r i z e the FWHM o f a spectral line. The c r o s s - t a l k of SPDs i s a c t u a l l y an advantage because i t r e d u c e s a l i a s i n g p r o b l e m s (2). This " a n t i - a l i a s i n g " e f f e c t of c r o s s - t a l k i s p a r t i c u l a r y i m p o r t a n t f o r h i g h resolution applications. A d e m o n s t r a t i o n of the e x c e l l e n t r e s o l u t i o n of SPDs i s shown i n F i g u r e 2. The s p a t i a l and s p e c t r a l r e s o l u t i o n of the SPD can be e l e c t r o n i c a l l y v a r i e d by summing a d j a c e n t d i o d e s i n groups o f 2,4,6,8, 10 e t c . The d i o d e s a r e r e a d out ( r e c h a r g e d ) a t v e r y h i g h r a t e s , e.g., 2 MHz, but o n l y the group-summed t o t a l i n t e g r a t e d s i g n a l i s d i g i t i z e d , and a t a r e g u l a r s c a n r a t e , e.g., 60 KHz. Diode grouping i s u s e f u l i n k i n e t i c s a p p l i c a t i o n s where s e q u e n t i a l scans must be r a p i d l y s t o r e d , but where s p e c t r a l r e s o l u t i o n can be reduced by grouping without any l o s s i n e x p e r i m e n t a l r e s u l t s . Grouping i s a l s o u s e f u l i n smoothing (low-pass f i l t e r i n g ) the odd/even g a i n v a r i a t i o n p a t t e r n t y p i c a l of SPDs, and thus making t h e i r r e a l - t i m e d i s p l a y more amenable to user i n t e r p r e t a t i o n . Random Access C a p a b i l i t y SIT £ ISIT. Real random-access i s p o s s i b l e because the e l e c t r o n r e a d o u t beam can be r a n d o m l y d i r e c t e d t o any p o r t i o n o f t h e target, skipping a l l others. T h i s mode of o p e r a t i o n i s extremely u s e f u l i n that i t a l l o w s a ( d e s t r u c t i v e ) readout of v e r y i n t e n s e s p e c t r a l l i n e s , t h e r e b y a v o i d i n g severe blooming problems, while p e r m i t t i n g weak s i g n a l s to i n t e g r a t e f o r long p e r i o d s of t i m e , on t a r g e t , to improve t h e i r S/N r a t i o s . The random-access c a p a b i l i t y of the SIT makes i t i d e a l f o r a v a r i e t y o f 2D a p p l i c a t i o n s , e.g., é c h e l l e s p e c t r o m e t r y , total l u m i n e s c e n c e s p e c t r o m e t r y , e t c . The m a i n l i m i t a t i o n on t h i s mode of o p e r a t i o n i s s e t by dark c h a r g e b u i l d u p t h a t , u n l e s s reduced by c o o l i n g , w i l l l i m i t the time a v a i l a b l e f o r on-target s i g n a l integration. ISPD. Random access c a p a b i l i t y depends only on the p r o p e r t i e s of the readout device. With the SPD only a pseudo-random, f a s t access (skip) readout i s p o s s i b l e (2,3). SPD. The d i o d e s i n an SPD must be s e q u e n t i a l l y scanned by the s h i f t r e g i s t e r s and t h e r e f o r e a r e a l random a c c e s s cannot be a c c o m p l i s h e d . However, a pseudo-random a c c e s s ( f a s t a c c e s s ) readout mode i s p o s s i b l e . In t h i s mode, diodes that c o n t a i n no r e l e v a n t s p e c t r a l i n f o r m a t i o n are skipped (not read) at a high scan r a t e , e.g., 2 MHz, whereas t h o s e c o n t a i n i n g d e s i r e d i n f o r m a t i o n a r e d i g i t i z e d a t a r e g u l a r s c a n r a t e , e.g., 60 KHz. T h i s o p e r a t i o n p e r m i t s a f a s t a r r a y scan without s a c r i f i c e i n r e s o l u t i o n , as grouping does.



(a) CHART SPEED 0.2 IN/MIN SCAN SPEED 1 nm/MIN DOUBLE BEAM CONVENTIONALSPECTROPHOTOMETER

ω ο ζ


< CD cr ο < / > œ

4000 can be achieved w i t h such s i g n a l s . Indeed, SPD d e t e c t i o n systems are a b l e t o a c h i e v e p h o t o m e t r i c p r e c i s i o n l e v e l s b e t t e r t h a n 10"" AU. Dynamic Range SIT &. ISIT. Under the d e f i n i t i o n of dynamic range as the r a t i o of the l a r g e s t readable s i g n a l to the rms noise of the d e t e c t i o n system, there are a c t u a l l y at l e a s t four kinds of dynamic range values. We w i l l confine our d i s c u s s i o n to the f o l l o w i n g three types of dynamic range. S i n g l e c h a n n e l dynamic range i s u s u a l l y e q u a l t o the dynamic range of the A/D converter used, since readout n o i s e i s a r b i t r a r i l y set equal to one d i g i t a l count. A range of 16,384:1 (14 b i t s ) i s e a s i l y achieved f o r t h i s range which describes the maximum and minimum s i g n a l each channel can measure. I n t r a s p e c t r a l dynamic range i s the r a t i o of the l a r g e s t and s m a l l e s t input s i g n a l s that can be simultaneously measured i n a s i n g l e s c a n ( r e g a r d l e s s of t h e l e n g t h of s i g n a l i n t e g r a t i o n t i m e ) . T h i s d e f i n i t i o n i s one of the l e a s t u n d e r s t o o d . T h e r e are a v a r i e t y of r e a s o n s why t h i s range i s s u b s t a n t i a l l y l e s s than 1 6 , 3 8 4 : 1 ; f o r example: (a) S t r a y l i g h t r e f l e c t e d f r o m the i n t e n s i f i e r e n c l o s u r e can o b s c u r e low l e v e l s i g n a l s , (b) H i g h d a r k c u r r e n t and/or background s i g n a l s upon w h i c h the a n a l y t e s i g n a l i s s u p e r i m p o s e d can r e d u c e t h i s f o r m of dynamic range. Even though b o t h a r t i f a c t s can be a c c u r a t e l y s u b t r a c t e d , the noise a s s o c i a t e d w i t h them can cause obscuration of the s i g n a l . For example: i f an a n a l y t e s i g n a l w i t h an i n t e n s i t y of 9 p h o t o e l e c t r o n s / s c a n i s s u p e r i m p o s e d on a l i n e wing of an adjacent l i n e w i t h a s i g n a l magnitude of 91 photpelectrons/scan, the S/N f o r t h e a n a l y t e s i g n a l i s 9 / ( 9 1 + 9 ) = 0.9. In the absence of the l i n e wing, the net S/N = 3. Memory dynamic range depends on the s i z e of the a v a i l a b l e memory. Of c o u r s e , v e r y wide d i g i t a l memories can be used t o s t o r e v e r y l a r g e s i g n a l v a l u e s , i.e., t h e r e s u l t of o n - t a r g e t and in-memory r e a d o u t modes. Depending on the n o i s e of the s i g n a l and detector, the o p t i m a l memory width can be s e l e c t e d , but a minimum range of 10 :1 i s u s e f u l . 1 / 2



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ISPD. The ISPD p r o v i d e s a l i n e a r dynamic range (1-3%) of a t l e a s t 16,384:1 Ç14 b i t A/D c o n v e r t e r ) . A t i l l u m i n a t i o n l e v e l s above 4 χ 10 foot candle (with a corresponding output b r i g h t n e s s of approximately 1 foot lambert), the MCP saturates and t h e r e a f t e r has a n o n - l i n e a r t r a n s f e r characteristic. However, t h i s l e v e l i s way above the A/D c o n v e r t e r s a t u r a t i o n l e v e l and has no i m p l i c a t i o n s on low l i g h t l e v e l spectroscopy. SPD. The dynamic range o f SPDs w i l l be d i s c u s s e d i n terms o f t h e i r s i n g l e c h a n n e l dynamic range and t h e i r i n t r a s p e c t r a l dynamic range. S i n g l e channel dynamic range i s p r a c t i c a l l y l i m i t e d by the range o f a f f o r d a b l e A/D c o n v e r t e r s t h a t can o p e r a t e a t the p r o p e r scan r a t e s . W i t h 14 b i t A/D c o n v e r t e r s ( 1 6 , 3 8 4 : 1 ) , l i n e a r i t y i s a c h i e v e d t o w i t h i n a t l e a s t 2-3% o v e r the e n t i r e range. The V a r i a b l e I n t e g r a t i o n Time (VIT) t e c h n i q u e (2.) g r e a t l y increases the p r a c t i c a l dynamic range. T h i s i s done by v a r y i n g the on-target i n t e g r a t i o n time periods so that h i g h and low i n t e n s i t y s i g n a l s can be r e a d w i t h c o m p a r a b l e S/N r a t i o s . The r e c i p r o c i t y , i . e . , the p r o d u c t of s i g n a l i n t e n s i t y and s i g n a l i n t e g r a t i o n time i s l i n e a r w i t h i n a count range exceeding 10 : 1 . The VIT t e c h n i q u e a l l o w s d e t e c t i o n of low l i g h t l e v e l s i g n a l s w h i c h would o t h e r w i s e be d e t e c t a b l e o n l y by h i g h g a i n d e t e c t o r s , e.g., PMTs o r I S P D s . A good example of t h i s c a p a b i l i t y i s g i v e n elsewhere i n t h i s book (4). The i n t r a s p e c t r a l dynamic range of the SPD i s a f f e c t e d by a c e r t a i n degree of s t r a y l i g h t , although s u b s t a n t i a l l y l e s s than the ISPD. F u t h e r m o r e , s i n c e d e t e c t o r heads have t h e i r own sealed quartz windows, i t i s p o s s i b l e to use the SPD without i t s own q u a r t z window and t h u s r e d u c e the degree of i n t e r n a l reflection. A d d i t i o n a l l y , i n t e r n a l r e f l e c t i o n s between t h e s i l i c o n target and the detector quartz window can be reduced by using o f f - a x i s o p t i c s . S i g n a l Storage C a p a b i l i t y SIT ISIT. S t o r a g e t i m e of SITs depends o n l y on the d a r k c h a r g e l e v e l , w h i c h depends on the d e t e c t o r t e m p e r a t u r e . At room temperature, a storage time of 1-2 seconds i s t y p i c a l . At d r y - i c e temperature, 20-50 minutes of storage time are p o s s i b l e . ISPD. S t o r a g e t i m e depends on c o o l i n g t e m p e r a t u r e . F o r l o n g s t o r a g e p e r i o d s t h e MCP m u s t a l s o be c o o l e d t o r e d u c e spontaneous photoemission. However, c o o l i n g of the photocathode w i l l reduce the red response of the photocathode. SPD. to

S i g n a l storage time i s s m a l l unless the detector i s cooled below -50 °C (30-60 minutes).


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Lag


SIT & I S I T . V i d i c o n d e t e c t o r s r e q u i r e some t i m e to a d j u s t to l a r g e v a r i a t i o n s i n s i g n a l magnitude because of readout d i s c h a r g e l a g , i.e., i n c o m p l e t e r e a d o u t of s i g n a l i n a s i n g l e scan. A c t u a l l y , b e t t e r t h a n t h e o r e t i c a l (based on shot n o i s e ) S/N p e r f o r m a n c e i s a c h i e v e d i n c v ( c o n t i n u o u s wave) o p e r a t i o n because lag causes s i g n a l averaging. However, discharge l a g i s a d i s t i n c t d i s a d v a n t a g e i n s i n g l e shot measurements. The p r e s e n c e of l a g means t h a t a t l e a s t 5-10 scans a r e r e q u i r e d t o read a l l the charge o f f the t a r g e t . This i s not a problem when high r e p e t i t i o n r a t e p u l s e s , i.e., a few per second or more, are measured. ISPD. The SPD r e a d o u t d e v i c e i s p r a c t i c a l l y d i s c h a r g e l a g f r e e . Any l a g of the ISPD w i l l o r i g i n a t e from the decay time of the output phosphor (although at very high r a t e photon counting, the e l e c t r i c a l r e s p o n s e t i m e o f the c h a n n e l s becomes an important f a c t o r ) . As explained w i t h the SIT (under discharge l a g ) , slow phosphor decay t i m e w i l l have l i t t l e e f f e c t on the a c c u r a c y o f m e a s u r e m e n t s o f s i n g l e p u l s e s or w i t h h i g h r e p e t i t i o n r a t e p u l s e s or cw. I t becomes a p r o b l e m , however, with low r e p e t i t i o n r a t e pulses where each pulse i s superimposed on the decay t a i l o f the p r e c e d i n g p u l s e . F o r t u n a t e l y , the phosphor decay t i m e d e c r e a s e s s i g n i f i c a n t l y as the e x c i t a t i o n (measured) pulse becomes shorter. Decay time i s a l s o a f f e c t e d by the energy l e v e l of the p u l s e and by the g r a n u l a r i t y of the phosphor. I t seems, however, t h a t f o r s h o r t p u l s e s < 1 U s , decay time may not become an accuracy l i m i t i n g f a c t o r . SPD. When high q u a l i t y devices are used, readout l a g of SPDs i s very s m a l l f o r l i g h t l e v e l s below the capacitance s a t u r a t i o n of the d i o d e s , i . e . , a p p r o x i m a t e l y 2 χ 10 photons/diode. Above t h a t l i g h t l e v e l , c h a r g e d i f f u s e s t o the s u b s t r a t e and a few a r r a y scans may be n e c e s s a r y t o f u l l y r e a d i t o f f the t a r g e t . I t i s i n t e r e s t i n g t o n o t e t h a t the v i d i c o n l a g i s due to incomplete readout of s m a l l s i g n a l s , whereas the reverse i s true f o r SPDs. Blooming SIT &, ISIT. Blooming w i l l occur when diodes are saturated, i.e., s i g n a l w i l l s p i l l o v e r t o adacent d i o d e s . T h i s i s p a r t i a l l y r e d u c e d by the e l e c t r i c a l i s o l a t i o n between a d j a c e n t d i o d e s . A n o t h e r phenomenon w i t h s i m i l a r r e s u l t s i s h a l a t i o n o r l i g h t r e f l e c t e d back to the p h o t o c a t h o d e f r o m the t a r g e t (the photocathode i s semi-transparent). T h i s i s , i n essence, a s t r a y l i g h t phenomenon t h a t p r o d u c e s an h a l o around h i g h i n t e n s i t y spectral l i n e s .


26



ISPD. B l o o m i n g p r o b l e m s a r e v e r y s m a l l and s i m i l a r to t h o s e discussed f o r the SPD (see below), i.e., blooming i s l i m i t e d to charge o v e r s p i l l to adjacent diodes only, once the capacitance l e v e l ( a p p r o x i m a t e l y 8 χ 10 e l e c t r o n s ) i s exceeded. Stray energy r a d i a t i o n problems have been p r e v i o u s l y discussed. SPD. W i t h q u a l i t y d e v i c e s , b l o o m i n g seems t o be c o n f i n e d t o adjacent diodes, i.e., upon s a t u r a t i o n of a diode, s i g n a l charge w i l l p a r t i a l l y s p i l l o v e r to the n e a r e s t u n s a t u r a t e d d i o d e . L o c a l i z e d blooming i s e s s e n t i a l i n order to m a i n t a i n an adequate s p a t i a l r e s o l u t i o n across the array and to e f f i c i e n t l y u t i l i z e the VIT r e a d o u t t e c h n i q u e , i.e., the s m a l l e r the b l o o m i n g , the s m a l l e r the e f f e c t of h i g h l e v e l s i g n a l s on n e a r - b y low l e v e l signals. Long Term S t a b i l i t y SIT &. ISIT. Long-term s t a b i l i t y i s p o s s i b l e , but d i f f i c u l t , to a c h i e v e . I n s t a b i l i t i e s w i l l o c c u r as a r e s u l t of t e m p e r a t u r e variations. Examples of p a r a m e t e r s a f f e c t e d by environmental c o n d i t i o n s and whose v a r i a b i l i t y w i l l d e g r a d e d e t e c t o r performance are: t a r g e t dark charge, c o i l o p e r a t i o n , magnetic i n t e r f e r e n c e w i t h c o i l operation, i n s t a b i l i t i e s i n high v o l t a g e s u p p l y w h i c h cause v a r i a t i o n s i n f o c u s i n g and d i s t o r t i o n , v a r i a t i o n s i n c a t h o d e v o l t a g e w h i c h a f f e c t r e a d o u t and l a g p a r a m e t e r s , e t c . In g e n e r a l , the l a r g e r the number of s e t - u p parameters, the greater the i n s t a b i l i t y . ISPD. Good l o n g t e r m s t a b i l i t y p r o v i d e d t h a t the MCP has not been p r e v i o u s l y exposed to high l e v e l i l l u m i n a t i o n and that the SPD i s a c c u r a t e l y thermostated. SPD. Long t e r m r a d i o m e t r i c s t a b i l i t y i s good. Geometric s t a b i l i t y i s u n a f f e c t e d by e x t e r n a l p e r t u r b a t i o n s , e.g., m a g n e t i c f i e l d , and i s a l w a y s the same. E l e c t r i c a l s t a b i l i t y depends on the design of the p r e a m p l i f i e r and s i g n a l processing e l e c t r o n i c s and i s u s u a l l y not a problem. In the past, long i n memory i n t e g r a t i o n periods r e s u l t e d i n o u t p u t s e p a r a t i o n i n t o f o u r s p e c t r a ( f o r each phase); t h i s has not been o b s e r v e d w i t h the new (SPD) d e t e c t o r head (Princeton Instruments, Inc., Model RY-512). Long t e r m b a s e l i n e s t a b i l i t y depends on c o o l i n g and p r e c i s i o n thermostating, and can be maintained at or below the l e v e l of photon-shot noise. Distortion SIT &. ISIT.

Subject

to p i n cushion d i s t o r t i o n (see above).

ISPD. E l e c t r o s t a t i c i n t e n s i f i c a t i o n has a maximum d i s t o r t i o n (pin-cushion) of approximately 5%. P r o x i m i t y i n t e n s i f i e r s have


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no g e o m e t r i c d i s t o r t i o n . M a g n i f i c a t i o n r a t i o s (input/output) can vary from detector to detector i n the 0.9 to 1.04 range. SPD. SPD a r r a y s have no g e o m e t r i c d i s t o r t i o n . Variationi n r e s o l u t i o n , r e s p o n s e , dark c h a r g e , p a t t e r n n o i s e , e t c . a r e random across the array. Modulation T r a n s f e r Function


SIT &. I S I T . P i n c u s h i o n d i s t o r t i o n ( s e e above) r e s u l t s i n a v a r i a t i o n of the modulation t r a n s f e r f u n c t i o n across the t a r g e t of t h e d e t e c t o r w i t h a maximum d i s t o r t i o n p r e s e n t i n t h e periphery. ISPD. pair):

T y p i c a l MTF v a l u e s

f o r MCPs a r e as f o l l o w s ( l p = l i n e

lp/mm

proximity

electrostatic

2.5

86%

90%

7.5

58%

60%

15

20%

25%

The MTF of the SPD i s s u b s t a n t i a l l y b e t t e r than that of the MCP i n t e n s i f i e r s and t h e r e f o r e c o n t r i b u t e s l i t t l e to the o v e r a l l MTF of t h e ISPD. V a r i a b l e Dimensions SIT

ISIT.

The dimensions of the SIT and ISIT are f i x e d .

ISPD. A 40 mm MCP can be coupled to a 25 mm SPD (1024 elements) through a 40/25 mm format reducing o p t i c a l - f i b e r coupler and thereby r e s u l t i n a p r o p o r t i o n a l increase i n the s p e c t r a l window of the detector. SPD. Format reducing o p t i c a l - f i b e r couplers, e.g., 40 mm to 25 mm, can be used as s p e c t r u m d e m a g n i f i e r s . Such d e v i c e s can increase the s p e c t r a l coverage of an SPD at the s a c r i f i c e of UV response. U V - t o - v i s i b l e chemical converters can be deposited on the i n p u t o p t i c a l - f i b e r f a c e p l a t e , b u t a c e r t a i n degree o f l a t e r a l c r o s s - t a l k could result i n a d e t e r i o r a t i o n i n resolution. F l e x i b l e bundles of coherent (imaging) o p t i c a l f i b e r s can be used to bind together (side by side) any number of SPDs i n a contiguous manner. This can allow an a d a p t a b i l i t y of SPDs to any f o c a l plane and any s p e c t r a l coverage required.


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V a r i a b l e Scan Time SIT &. ISIT, V a r i a t i o n of s c a n t i m e i s p o s s i b l e w i t h i n a reasonable range, i.e., lowest p o r t i o n l i m i t e d by discharge l a g , highest p o r t i o n by dark charge l e v e l . V a r i a t i o n s between 20200 ys are e a s i l y achieved.


ISPD. Rather than vary the scan time, i t i s p r e f e r a b l e to vary the o n - t a r g e t i n t e g r a t i o n t i m e . T h i s can be c o n v e n i e n t l y a c c o m p l i s h e d over the range f r o m 16 ms to 120 seconds ( w i t h c o o l i n g to -10 ° C ) . SPD. Scan t i m e can be v a r i e d f r o m a few KHz to a few MHz, but v a r i a t i o n of scan t i m e t h r o u g h a d j u s t m e n t of e x p o s u r e t i m e i s advantageous.

Conclusions The p r i m a r y purpose of t h i s paper has been to p r o v i d e a c o n v e n i e n t r e f e r e n c e f o r r e s e a r c h e r s who a r e i n t e r e s t e d i n a p p l y i n g OIDs f o r l o w - l i g h t l e v e l s p e c t r o s c o p y and who need a s o u r c e of d e t a i l e d i n f o r m a t i o n about t h e s e d e v i c e s to e n a b l e them to s e l e c t the proper detector f o r a p a r t i c u l a r a p p l i c a t i o n . T h i s paper has p r e s e n t e d an i n - d e p t h c o m p a r i s o n o f f o u r o p t o e l e c t r o n i c image devices w i t h 25 performance c r i t e r i a . In using the g u i d e l i n e s presented i n t h i s paper f o r the s e l e c t i o n of a p a r t i c u l a r OID f o r l o w - l i g h t l e v e l a p p l i c a t i o n s , one should d i v i d e the v a r i o u s c r i t e r i a discussed i n t o two c a t e g o r i e s based on the p a r t i c u l a r a p p l i c a t i o n under c o n s i d e r a t i o n . The f i r s t category should i n c l u d e those c r i t e r i a w h i c h a r e of paramount importance f o r the p a r t i c u l a r a p p l i c a t i o n . The other category should i n c l u d e those c r i t e r i a which are of secondary importance i n the p a r t i c u l a r a p p l i c a t i o n . Care s h o u l d be t a k e n a t t h i s stage to avoid mutually e x c l u s i v e s i t u a t i o n s . For example, i t w i l l be d i f f i c u l t to f i n d a detector which i s s e n s i t i v e to very l o w - l i g h t l e v e l s and i s simultaneously immune to l i g h t shocks. Based on the p a r t i c u l a r c r i t e r i a i d e n t i f i e d i n the f i r s t c a t e g o r y , a p a r t i c u l a r OID may emerge as b e i n g the most s a t i s f a c t o r y f o r the g i v e n a p p l i c a t i o n . A l t e r n a t i v e l y , no c l e a r - c u t winner may emerge from the comparison which may mean t h a t the s e l e c t i o n of a p a r t i c u l a r OID may not be c r i t i c a l t o the given a p p l i c a t i o n or that the current s t a t e - o f - t h e - a r t OIDs are unable to meet the requirements of the experiment. I t should be c l e a r from the d i s c u s s i o n of the performance c r i t e r i a of OIDs t h a t no one d e t e c t o r i s b e s t s u i t e d f o r a l l p o s s i b l e s p e c t r o s c o p i c a p p l i c a t i o n s . Proper selection, t h e r e f o r e , depends on an i n - d e p t h u n d e r s t a n d i n g o f d e t e c t o r c h a r a c t e r i s t i c s as discussed i n t h i s paper. F o r t u n a t e l y , there are a l r e a d y a d i v e r s i t y of OIDs a v a i l a b l e and more a r e b e i n g developed. T h i s i s a r a p i d l y changing a r e a and second-


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generation devices with improved performance s p e c i f i c a t i o n s are being c o n t i n u a l l y developed. Therefore, i f current s t a t e - o f - t h e a r t devices are not s a t i s f a c t o r y f o r a given a p p l i c a t i o n , i t may not be much longer before improved devices are developed.

Literature Cited 1.


2. 3. 4.

5. 6.

Siegmund, Oswald H. W.; Malina, Roger F. in "Image Devices in Spectroscopy," Talmi, Y., Ed.; ACS SYMPOSIUM SERIES No. ACS:Washington, D.C., 1983. Talmi, Y.; Simpson, R. W. Appl. Opt. 1980, 19, 1401. Talmi, Y. Appl. Spectrosc. 1980, 36, 1. Grabau, F.; Talmi, Y. i n "Image Devices i n Spectroscopy," Talmi, Y., Ed.; ACS SYMPOSIUM SERIES No. 102 ACS:Washington, D.C., 1983. "Model IRY Detector Head Manual," Princeton Instruments, Inc., 1982. Talmi, Y.; Baker, D. C.; Jadamec, J. E.; Saner, W. A. Anal. Chem. 1978, 50, 936A.

R E C E I V E D July 26, 1983


Guidelines for the Selection of Four Optoelectronic Image Detectors for

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