Chemical Instrumentation 5. 2. LEWIN, N e w York University, Washington Square, New York 3, N. Y.
(cf. Figures 15 and 161, the sensitivity to light can he made considerably greater than that achievable with p h o t o ~ l t s i s cells.
T h i s series of articles presentsa SUTuey ofthe basic principles, characterislics, and limitations of those imtrumenls which find important applications i n chemical xork. The emphasis i s on commercially available epipment, and approxi~nateprices are quoled to show the order of magnitude of cost of the various types of d e s i p and constmction.
7. Photometers and Colorimeters (Conf'dl The photovoltair or harrier layer photooell instruments already described have the advantage of ruggedness, simplicity of circuitry, economy, and freedom from bulky or unstable power supplies. but they have certain inherent disadvantages. They show small but definite time delays in responding t o intensity variations; they are subject t o fatigue effects, they require sensitive galvanometers for the reednut; and the relationship between light intensity and output signal often deviates greatly from linearity. These features, which may in some mensarement situations he unacceptable, can he svoided by the use of vacuum (photoemissive) phototuhes.
liberated from the surface of the cathode. The tuhe current is then "saturated," or emission-limited, i.e., i t is practically unaffected by further increases in the applied voltage. Thus, when the applied voltage is great enough (50 to 100 volts. generally), the tube current depends only on the light intensity, and is insen~itive to variations in the power supply.
Figure 15. Simplitled schematic of a typicol photometer circuit with o single stage of mnpliflcation. V ~ C U U phototube ~
A typical circuit for use with s vacunm phototube is shown in Figure 15. Light striking the photo-cathode liberates elertmns, causing an electron current to flow bhmugh the phototube resistor, Rt, in the direction shown. The IR-drop in this resistor is in the sense of making its top (in the Figure) positive with respect to its bottom. This make8 the grid of the triode more poeitivr, causing an increased current to flow through the t111,r and he read on the meter. The grid b i a ~battery, E.. is used to sdiust the triode current to
E .
VOLTS
Figure 14. Variation of vocuum phototube current with applied vollege ot several radiotion level. (Mi = millilumen.1.
values of the applied voltage, the rate of electron emission due to the action of the light on the photoseneitive surface of the cathode may he greater than the rate a t which the electrons are swept across to the mode under the influence of the applied voltage. Under these conditions, a space charge of electrons is built up around the crtthode, causing the tuhe current to be very sensitive to the applied voltage. As the tube voltage is increased, the point is soon reached where the potentid difference is great enough to sweep all eleetmn~to the anode as soon as they are
AMPLIFIER
Figure 16. SimpiiAed null-bdonce circuit for ure with a vacuum phototube.
Vacuum Phototube Photometers ~h~ form f, the current-voltnge chraeteristic for a. tube is shown in Figure 14. At small
TO
A N D DETECTORrr
phatotube. A null-balance type of circuit utilizing the output of a vacuum phototubc is shown in Figure 16. Here, the IR-drop genemted in the phototobe resistor, R,, is opposed by the voltage tapped off a slidewire potentiometer. When the two voltages are exactly equal, no net current is detected hy the amplifier-detector device. The position of the sliding contact can be calibrated in terms of light intensity. Vacuum phototuhes rexpond very rapidly to flucturttions in inrident light illtensity, and are excellently suitcd to mene~trements with "chopped" light beams. Since electronic ~mplificationcan be employed with these phototuhea
An example of a vacuwn phototube photometer is the Model 501 M Elm tronic Photometer ($315) of Photovolt Corporation, New York 16, N. Y. This instrument consists of a search unit, which may utilise phototubes with any of the spertral sensitivities nvrtilrthle (see Figure (i),and a high gain (X50,000) twostage dr amplifier. Tho amplifier is &.abilked sgsinst line voltage fl~tctuationshy use of a constant-voltage input t r a n s former. Read-out is on n IY.4rsonvaltype indicating meter, full-scale deflection corresponding, on the most sensitive range, to about 0.01 foot-czndlc. This is shout 5000 timrn the sensitivity that could be obtained directly from a, photovoltaic cell with th.2 same read-out meter without. electronic amplification. A bntterypowered vemion of this photometer ia also availalde (Model 514 M, $320). This photometer is readily adapted to use in either. transmission or reflection dennitometry. A search unit for t r a n s mission mensurementa (Model 52 Transmission-Density Unit, $190) is shown in diagram in Figure l i . An example of a vacuum tube photometer employing two detectors in a tw* channel arrangement for comparison reflection photometry ie the Elrepho photometer of Carl Zeiss, Inc., New York 15, N. Y. ($2712). The optical design is illustrated in Figure 18. Light r e fleeted from the diffusely-illuminated specimen s t A is condensed by objective 0, on phototubo Ph,, and light reflected from the standard at R is aimilsrly imaged onto Ph*. Zero adju~tment is accomplished by means of the adjustable diaphragm operated by control MH. Nullbalance measurement is achieved by inserting the neutral grey wedge GK into thesample path until theoutputsof the two ' phototubesareequnl. Eleetronicampliati ion is empluyed, giving a ren~itivityof 5 scale divirima on the wad-oot meter for (Continted on page AS791
Volume 37, Number 5, May 1960
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dism~ssionnof n H meters. Manv sneatro-
;
TXC*IVfllBLL
rn.TU.E
5 ..
W"TE T W ?."EL
U M SPOT CE-
Figure 17.
Diagram
of
-
Photovolt Corp. Tronrmirsion-Density photometer based upon
use of
vacuum
phototube ar rentor.
Dc amplifcccation has the disadvantage that the magnitude of the output signal depends upon the magnitude of the voltages applied to the amplifying tubes, and hence the problem of "drift" may he encountered. This is minimized in the doublebeam, or balanced tuhecircuits, but t,hen the omble~nof exact matchine of the tube char'acteris1,ics comes t o the f o k Ac amplification eliminates these problems,
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by using a mechanical interrupter, or "lighkhoppor" to intercept the light heam periodically and create the alternetion in output signd. These instruments will be disoussed in detail subsequently. I t is, however, appropriate a t this point to call attention to the unique photometer manufactured bv W. M. Welch Scientific Company, C k g o 10, Illina~s, which "chaps" the vacuum phototube
output eleotromagnetieally. The Densichron (Model 1, with blue-sensitive prohe, $250) is based upon a speciallydesigned vacuum phototube which ie mounted in the gap between the pole pieces of an electromagnet, as shown sohe matically in Figure 19. The 60-cycle voltage applied to this electromagnet causes its magnetic field t o vary in step; t h a t is. as the msenetizine current increases. the magnetic field increases, reaching s. maximum, then falling away t o zero as the current drops during the ac cycle. When the magnetic field is weak, the phota-electrons ejected from the cathode reach the anode t ~ n da current flows in the external circuit. Each time the magnetic field approaches its peak value (twice during each complete cycle of the ac), the photoelectrons are deflected by this field sufficiently so that they do not strike the anode, and the external phobcurrent drops markedly. Thus, the 60-cycle voltage applied t o the electromagnet produces a 120-cycle modulation in the output of the phototube. I n the Dennichron. this alternatine outout is ~.uplifivd i n .L ~ I ~ ~ ~ p l y - t u fiw-itape, w~l, feecll~n~k-sr:al,ilirrd amplifier, a d tla. WG put is presented on a 0-1 ma. meter movement. This instrument shows very good st* hility, and has a highsensitivity by virtue of the high-gain amplifier used. The standard instrument will give full-scale deflection for an illuminstion of about 0.005 foot-candle. The signal-to-noise ratio is better than that obtained with (Continued o n page Ab74) ~
~
Chemical Instrumentation
A Figure 18. Two cross sections of the optical layout of the Zeiss Elrephe reflection photometer.
do amplifiers, sinre the use of sharp tuning ~ e r m i tthe s rejection of most of the random noise frequencies. However, an ac a m p lifier of this type is snbject ta the pickup of stray signals of the rtmplifier f r t quency, such as are likely ta be radiated from fluorescent light fixtures, motors, and other electrical npparatos. 4180,
Figwe 19. Illurtratim of the primiple invdvtd in the "MognephdS phototube employed in the Welch Dentichron photometer.
difficulty may be encountered in using this type of photometer for menswing the light from lamps pomred by ac, if the lamp output Huctuates with the same frequency as the magnetic field producing the phototube modolrttion. The Densich(Conlinued on page $676)
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Chemical Instrumentation ron photot,uhes arc nvailnhlr: xith the standard S-l and S-4 photo~onsit~ive s~rfaces (see Figure 6).
Photomultiplier Tube Photometers The commercial availsbilit,y of stable. reproducible photomultipli~r t,nhes has ushered in n ncw era in light intensity measoremmts. These tubes are extremely sensitive as well as extremely fast,, and are widcly u ~ e dwhere such properties are el;smtinl, as in mierodmsitomctry, high resolut,ion spectrophotomotry, and fluorrscenee or scintillation detect,ion. The upper limit of light int,ensity that can he emplayd n i t h photomdt,iplier tuhes is detrrmined hy t,he onset of fatigue effects and thr deterioration of clertmdc surfaccs due to local heating effects; thc lonw limit is set by t,he inhpwnt dark current due to thermionir mninshm, m d the random noise signals. 3Iodrrn photomultipliers are generally u s ~ f n from l nhout l 0 F 4 up to ahout 10+lumen (continuow). The continuous current Ron-ing fmm the tuhe anode should Ire kept hrioa I milliampere to avoid fnt.igur etTcctrr: for maximum stability, the mrrcnt sho~ildhe in the range 0.1 to 0.001 ma. At suib able light level^, a photomult,ipli~rbubc will respond to light pulses as Iwicf as 10-8 t,o 10-o seconds in durat ion.
Figure 20. Simple voltage divider orrangemen1 for s u d v i m each i t a m of the ohotomultiplier tube with a higher voltage t h m the preceding stage. The tube current Roving to the anode creates on ill-drop in R, that server or the output signal.
.. . -
-
A simplifi~dcircuit for use wiLh s photomultiplier t i h e i~ a h o m in Figure 20. Since the amplification of a photomultiplier tuhe depends on the multipliration effect produced a t each of the clynodes by secondary emission, thc ovcr;dl m sitivity is very dopendont upon the necelerating voltage per stag?. Tho eNcet of supply volt,;ige on tub? scnsitivity is shown for a, typical tube in F'iguro 21. To prevent excessive drift,, special measures are taken either t,n maintain strict constanry of the npplicd voltage, or by electronic means to st,al,iliac the amplification in spite of volt,agc Hnctuat,ions. ~-~ A number of manufacturers oNcr photomultiplier photomet,ers, consisting es(Continued on paye Ab78) ~
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Chemical lnstrumentation aentially of a photomultiplier tube mounted in n probe unit, and connected t o a power supply, amplifier. and rend-oot
Figure 21. Sensitivity of a typical photomultiplier tube or o function of the supply voltage applied acrorr the tube.
meter. American Instrument Co., Silver Spring, Maryland, markets nuch an instrument, called the Photomultiplier Microphotometer $583). The power supply is stsbilizcd h y moans of n constsntvoltage input transformer; the output of the P h l tube is icd into a t v w ~ t a g e , feedbark-stabilized amplifier. Seniiitivity is 0.1 X 10PPhunens (corresponding to minimum detect,able deflection), and the meter reading is linear to & 4% over the mtire range uf four dcesdos of sensitivity. Connections arc provided for suhstituting an oscilloscope o r a millivolt reeonlcr in place of t,he D'Arsonval mcter rend-out. Farrand Optical Co., NPW Yolk 66, P;. Y., makes a portxhle Elertron Multil'lier Photometer wit,h a hnttcry supply ( 5 xit,hout galvanometer ior readout,). Good atnhility i~ achieved by sing H, separate hi~tteryor hattery pack for each of the dynodes. A bucking battery is wed for balancing out tho photomnltiplirr tube dark current in itdjwting the instrument rcra. An Electron Multiplier Photometer wibh a n ao power supply ($560, without rnirroammet~r fol. rendo u t ) i~ now also bring produced. The powt.r supply is electronically stabilized, xitb a constant rcfcrenrr voltage supplied t o the control circuit hy n rener diode. A dc microammeter suitable for this photometer is available ($150). In these instrummts, the photomultiplier, battery, and read-out form separate unibs, and are connected t o each other by cables. Phoenix Precision Instrument Co., Philadelphia 40, Pennsylvania, mitnuiartnres a n Eleetron-Mnltiplier-Photomrtpr ($870 without galvanometer) t h a t employs a n sc ~tabilised poxer supply, and that can be attached t o most gnlvznometers or recorders for read-out of the anode current. The photomultiplier tuhc housing, shown in Figure 22, has several inkresting features of dosign. I t is provided with two cylindrical scroencd myriers that can he fiUed with a desiccant, t o reduce the noise and dark current in humid iltmosphere~,or a cooling agcnt such as dry ice to rcduce the thermal (Johnson) noise Ievcl. A photographic shutter with iris diaphragm and time ex(Conlimed on page .4Z80)
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Chemical Instrumentation posorc control is mounted a t the front, and the rcsr has s, removable light-tight optical alignment pwp-plug screw Photovolt Corp., Yew York 16, N. Y., offers r. Multiplier I'hotometer (Model 520 A , X560 complete) that incorporates zn el~rtronicnllystsbiliscd power supply, and a two-stage, feedback-stahilize11, halnnccd de amplifier with D'Arsonvsl meter read-out. The instrument shows good stability and on its most sensitive range gives full-scale deflection for lo-' foot-candle (2 X 10-'lumen).
Figure 22.
Photomultiplier tube housing used Instrument Co.
by Phoenix Precision
E l d o r ~ d oElectronics Co., Berkeley 10, Cdiforniil, offers several photometers in this category. Their Universal Photomultiplier Photometer (Model PH-200, $495) is acoperated and has a IYArsonval meter read-out with provision for sobstitilting an oscilloscope or a recorder. A precision diffcrcntial photometer (Model 210, $985) utilizing two photomultiplier tubcs is avnilrrblr. I t ran measure either input independently, or present the difference between the two. The sensitivity adjustable over a 10 to 1 is cantinuo~~sly range; stability is 0.1 7, per channel pt-r day. tJltmsonir Engineering Co., Maywoad, Illinois, msnufnrtr~rcsa n Electron Molt,iplicr Photometcr consisting of a portshle PM unit which is attached by leads to a unit containing an ae stabilized power supply and a balanced dc amplifier r-ith 1 )'Arsonvxl meter read-out.
Other Photoelectric Photometers I t is sometimes advantageous to employ other sensors for radiant energ), than the photovoltaic or photoemissive cells described above. Far example, the latter do not function in the infrared region, and for photometry of such radiations it is necessary to employ thermocouples, bolometers, pneumatic (Golay) cells, or photoconductive detectors. A variety of varunm thermocouples and thermopiles manufactured by Kipp and Zonen, Delft, Holland, is available in the U. S. through James G. Diddle Co., l'hiledelphia 7, Pennsylvania. The Golny (Continued on page A882)
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Chemical lnstrumentution cell is manufactured by Eppley Laboratories, Newport, Rhode Island. V~rious types of photoconductive cells are made hy Enstman Kodak Co., Rochester, New York; General Electric Ca., Scheneetady, S e w York; Clairex Corp., New York,
x. Y.
Instruments for Color Specification Strictly speaking, the term colorimeler should be reserved for those instruments that are employed in specifying the visual eolor sensation produced by s. specimen. This would avoid tho ambiguity and oonfusion inherent in the current common practice of using bhis term both for such devices and for quite different types of instruments designed for absorptiometric measurements. The following paragraphs will deal with the restricted category of "tmc" calorimeters; vie., those instrumcnts int,cndod for the evaluation of visual eolor sensation. In order to discuss the design of these rolorimeters, it is necessary to bear in mind the definitions of terms used in this field. A speclral eolor is a eolor that occurs in the natural spectrum of white light, such as red, ormge, yellaas, green, hlue, violet, and all the intermediate shade8 between these. A w n - s p e c l ~ a l color can be ohtsined by mixing spectral colors; e.g., a non-spectral purple is oht,ained hy mixing red light and hlue light. The hue is the color itself, i.e., red, green, etc. The s a t z ~ a t i o n is the purity of the color; e.g., if a spectral color alone is considered, i t is said to he fully saturnled; if i t is mixed with white light or with other colors, it becomes less saturated. C O ! quality ~ refers to the combination of hue and saturation, and is equivalent to a, full description of the type of eolor. However, although the color quality specifies the kind of color(s) and their relative proportions, i t does not specify the total amount of light present in the color. This is described as the hcminanee. In determining or matching colors, it is necessary to measure or equate the three independent variables: hue, saturation, and luminance. Two colors t h a t have the same hue and saturation, hut different hminznces, give quite different visusl impressions, and the same is true of any other pair of these three parameters. The simplest type of calorimeter is the visual color-matching device. This merely consists of s, means of viewing aidebyside the specimen, and s. standard sgainst which i t is to be matched. An example is the Pocket Comparator of Hellipe, Inc., Garden City, Nem York ($50, eomplete with one color disc), shown in Figure 23. With this instrument color quality is determined by visually eomparing the hue and saturation of the light transmitted by the specimen with that passing through the color ("roulette") wheel. The tube mounted in line with the color disc is filled with the same medium ss the test liquid, but no colorgenerating reagent has been added, so (Continued m page A8841
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Chemical instrumentation as to provide eomponswtion for any tnrhidit,y or extraneous color in the original specimen; i.e., to match the saturation of the colors. This type of device is most appropriate for such applications as the calorimetric determinxtion of pH, metal ions, hemoglobin, etr., where t h r valw of the hminnnre docs not play n critical role.
Figure 23. Front and back inride v i e w of Heilige Pocket Comparator for virual color matching.
Similar visoal color comparators for R wide range of test proredurrs me manofseturod by Tintometer, Ltd., Salisbury, England. zvnilalde in t,ho T.S. through Hayps G. Schimp Co., Alhertson, Kew York. Another manr~farturer in this field is the Lahlottc Chemical Products Co., Chcstertown, Maryland. 14
2 1 3 8
Figure 24. Optical layout of C e National InstruSpectron viruol dorimeter. 2, enment Co. trance slit; 3, lens; 4, friportite prirm; 8, comporiron prirm for rimultoneour viewing of reference source; 13, wovelength scale.
A more prpcise visual comparison of color qnalitirs is possible with the Spectron. m~nufaetured by National Instrument Co., Baltimore 15, Maryland (%135), shown diagrammctticnlly in Figure 24. Light from the specimen is dispersed into a spectrum by passage through the tripartite direct vision prism. A comparison prism permits light from a standard to be dispersed similarly, and (Continued on page A386)
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Chemical instrumentation the two field8 of view are presented sidehy-side t o the operator. An attachment is provided that permits a test-tube cuvette t o be mounted in front of t,he entrance slit.
Tristimulus Colorimetry Since it is often difficult or imlmssible to ohtain stable eolor standards that closely match wide rangas of unknowns in hue, saturation, and luminance, much effort ha8 heen expended in developing system8 of precise, objective rolor description, so that colors may be specified in tcrms of stnblc units, without cmplaying the matching technique dcscribcd above. I n 1031, the C.I.E. (Commission Internationale de I'Eclzirago) agreed upon an objective system that is now in general use and sorves ns thc hasis for nonmatching colorimctors. The C.I.E. sy-s tern is hesed a n the fact that s n appvozimation to moat colors can bo ohtsined by a suitable mixture oi throe primary, fully saturated colors, vie., red, green, and hlue. However, in order t,o match all possihle spectral colors ~ r a r l l ~i/t , would be neeemary to use ss the three fund* mental colors, a red, n green. and a blue that are more .fully satwaled than the spectral colors. Such colors cannot, of course, l,6 produced in practice, hut they provide a. convenient abstraction for the specifi
