Light intensity measurements I: Large area bolometers with microwatt

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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6 , MAY 1979

Light Intensity Measurements I: Large Area Bolometers with Microwatt Sensitivities and Absolute Calibration of the Rhodamine B Quantum Counter D.

G. Taylor‘

and J. N. Demas”

Department of Chemistry, University of Virginia, Charlottesville, Virginia 2290 7

A large area ( 1 inch X 1 inch) bolometer with microwatt senskivity has been developed. Construction, calibration, and evaluation details are included. One of these bolometers was then used to calibrate the spectral responsivity of a Rhodamine B quantum counter (5 g/L in methanol). This counter Is shown to be flat to f2% over the 360-590 nm range.

Relative and absolute light intensity measurements are crucial for many spectroscopists, photochemists, and analytical chemists. For all but the most rudimentary relative intensity measurements, quantitative detectors are divided into two classes (1)thermal detectors and (2) quantum counters ( I , 2 ) . Thermal detectors which include bolometers, thermopiles, and pyroelectric radiometers, are thermally activated and respond by converting incident photons into heat by absorption in the sensor surface or volume. The incident flux is related to the sensor response to this heating. Bolometer, thermopile, or pyroelectric detectors sense temperature changes by variations in resistance, in developed EMF, or in capacitance, respectively. T h e key advantage of thermal detectors is that by making the absorber uniformly black a t all wavelengths, the detector is energy flat in response (Le., equal response for equal powers a t different wavelengths) (1-6). Spectroscopy, however, is more often concerned with the photon flux rather than the beam power. Only if the spectral distribution is known precisely, can the quantum flux be calculated from the incident power. Further, thermal detectors rarely have the large area, fast response time, sensitivity, or dynamic range required in spectroscopy. Although small in area (1 x 1 cm2), the recently developed pyroelectric null radiometer avoids many of these problems (3-6). Quantum flat quantum counter detectors (described below) have proved very useful in the spectral calibration of optical detectors and monochromator-source excitation systems, and the determination of absolute photoluminescence yields (7, 8 and references therein). The first two calibrations are important in luminescence work to correct excitation and emission spectra. Absolute luminescent quantum yield determinations are hampered by the wavelength-dependent responses of photocells and phototubes while thermal detectors are not quantum flat and lack adequate sensitivity. Quantum counters avoid these problems. An ideal quantum-flat detector is characterized by a response solely proportional to the incident photon flux regardless of its spectral distribution. Based on an idea of Vavilov (9, IO), Bowen (11)introduced a detector which very closely approximated this description. He gave the name “quantum counter” to the combination of an optically dense fluorescent screen placed before a phototube. The screen had a luminescence spectrum and quantum yield which was in-

’

Current address, School of Chemical Engineering, Purdue University, West Lafayette, Ind. 47907.

dependent of excitation wavelength within its useful range. The phototube current was then directly proportional to the photon flux incident on the screen. Since this response was independent of wavelength within the useful range, the signal was proportional to the integral of the photon distribution incident on the counter which has led to the term “integrating screen”. By combining a highly fluorescent dye counter and a photomultiplier, a very sensitive quantum-flat detector results. The standard luminescent counter material for -20 years has been the xanthene dye Rhodamine B (RhB) (8,12) with a useful range from the UV out to -600 nm. Luminescence research on red-IR emitting dyes and transition metal complexes has increased the need for quantum-counting materials with deeper red and IR sensitivity. In addition, the response of existing and new counters should be calibrated as accurately as possible. This paper and the subsequent one (13)are devoted to the accurate calibration of several existing and some new quantum counter systems. This present paper describes the development, construction, and evaluation of large area (one square inch) bolometers. These bolometers are relatively simple and inexpensive to construct, may be built to accommodate almost any beam cross section, are sensitive (- 10 kW), highly linear over more than a decade of intensity ( 5 ppt), and precise (3 ppt). One of these bolometers was used to recalibrate the RhB quantum counter with what we believe is the highest accuracy to date. In the following paper (13),we describe the development and operation of a new design of quantum counter comparator. This system permits the rapid calibration of one optically dense quantum counter against another with a precision and accuracy of better than 0.5%. Using this comparator and the bolometer calibrated RhB counter, a critical evaluation of other RhB counters and of several new quantum counter systems is presented.

EXPERIMENTAL Materials. The chemicals and solvent preparations are given in the subsequent paper (13). Excitation System. The excitation source was a 1000-W Xe arc lamp run from an Oriel constant current power supply. The excitation beam was passed through 5 cm of aqueous CuS04.5H20 (100 g/L) into an 0.25-m Bausch and Lomb grating monochromator (66 A/mm resolution) equipped with an achromatic output lens. To further ensure low infrared stray light, an additional CuS04solution filter (1cm) was placed over the exit lens. General Description of Bolometer. The bolometer sensor was a 1 inch X 1 inch square metal foil target with a positive temperature coefficient thermistor attached to it. The target wm blackened to absorb and convert light energy into heat. The bolometer response was measured as the change in thermistor resistance upon irradiation. The bolometer was maintained at a constant temperature to minimize spurious signals and drift due t o external temperature changes. Absolute calibration was achieved by electrical substitution whereby a known electrical power was dissipated on the target with a resistive heater. We

0003-2700/79/0351-0712$01.00/00 1979 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979

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Figure 1. "U" bracket 1 inch X 1 inch silver foil bolometer sensor. (A) Back view. Thermistor is soldered inside "U" bracket at the top. The fine copper wires soldered at each corner are for mounting. (B) Details of thermistor and "U" bracket now describe the bolometer target, the isothermal bolometer housing and temperature controller, and the performance of the system. B o l o h e t e r Sensor. In our initial attempts at making large area sensors, we mounted thermistors directly onto the backs of 1inch X 1 inch targets (thin aluminum sheet or 0.001-inch silver foil). Variations in surface sensitivity of the blackened sensor were probed with an -1-mm diameter, 1-mW CW He-Ne laser. The sensitivity, which was -10-20% less a t the edges than directly over the centrally located thermistor, was inadequate for our work. An improved version (-5% deviation) was obtained by mounting the thermistor on a small "U"-shaped silver bracket (Figures 1 and 2A) soldered onto the silver plate. The positive temperature coefficient (-20% /"C) thermistors in this and the subsequently described unit were from Pennsylvania Electronics Technology Inc. (Pittsburgh, Pa.) and gave 4-5 times the response of normal negative coefficient thermistors. These units were specially processed by the manufacturer for high resistances at their specified 30 "C operating point. One electrical contact of the thermistor bead was a flat copper surface which was sweat soldered onto the target, while a fine nickel wire made the other connection. A #40 enameled manganin wire calibration heater was threaded through the sensor plate (Figure 2A). The benzene-soot coated sensor was suspended by four 540 copper wires, one of which was used for a thermistor connection. The support wires were fastened onto an insulating support ring which was mounted on a '/,-inch copper plate by electrical standoffs (Figure 3). Entire sensor assemblies could then be interchanged in the bolometer head. To further improve uniformity of surface sensitivity, the sensor of Figure 2B was constructed from 0.0025-inch copper foil. The rationale behind this design was to lengthen and increase the number of heat-flow paths to the thermistor. This detector surface was blackened with several thin coats of Krylon 1602 flat black spray paint which is much easier to apply and more durable than benzene soot. The maximum deviation in response across this device's surface was -2.5%. For the relatively uniform -3/8-inch diameter excitation beam used in the current work, response uniformity was probably an order of magnitude better and was deemed acceptable. This detector, shown in Figures 2B and 3, was used to calibrate the standard Rhodamine B quantum counter. Because of the fragile nature of the calibration heater mount, no attempt was made to convert this unit into an absolute bolometer.

B Figure 2. Diagram of bolometer sensing plates. (A) Absolute silver-foil (0.001-inch) sensing plate with manganin wire calibration heater. T-thermistor ("U" bracket a s in Figure 1) and lead, H-manganin wire heater and leads, S-copper support wires and thermistor return lead, ////-solder connections. (B) Copper foil (0.0025-inch)sensor used in the RhB calibration. T-thenistor and lead, S c o p p e r wires and thermistoc return lead, ////-solder connections

Flgure 3. Assembled copper foil sensor (Figure 2B) shown mounted on interchangeable copper plate with insulating plastic mounting The thermistor resistances were measured with a Wheatstone bridge made from decade resistance boxes. Bridge power was from a 1.35-V RM42R Hg cell or from a 6-V lead acid battery with a series dropping resistor. The null detector was a Keithley Model 155 microvoltmeter. T h e Bolometer Head. The bolometer head (Figure 4) was a solid aluminum cylinder 31/4inches in diameter by 3 inches long. The ends were turned out to form two cavities 2l/, inches in diameter by 1'/8 inches deep. The rear cavity housed a small signal preamplifier (See Figure 5) which was placed in the strictly temperature-controlled bolometer head to improve its already low (0.3pV/"C) temperature drift. Head temperature was sensed by a Maier bridge (14) constructed from 540 copper wire (sensors R3 and R4) and k40 manganin (references R5 and R6) enameled wire wound around the forward section of the housing. The temperature control resistive heater (R15) was wound around the rear section of the head. A similar auxiliary heater was wound around the front section and was used to reduce the initial warm-up period. The thermal mass of the aluminum housing helped to smooth out heating and temperature variations a t the detector. The aluminum head was carefully isolated by an abundance of thermal insulation. The head was fit into two heavy woolen boot socks (large, Sears, Roebuck & Co.) to form six insulating layers except over the entrance window. This assembly was enclosed in a doubled up woolen stocking cap (R. W. Taylor,

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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979

A

B

Figure 4. Bolometer head. (A) Top view of bolometer head showing the sensor on interchangeable mount (Figure 3) installed, and the inner quartz window. (B) Rear view of the bolometer head showing the Maier bridge, preamplifier, and feedback network. The lower narrow winding is the auxiliary heater, the central broad band is the Maier bridge, and the upper narrow band is the control heater. The tape covered copper foil is an electrical shield which fits over the Maier bridge Model 1974) and mounted snugly in a 1-foot cubic box made of '/,-inch plywood. The box was insulated inside by a minimum of 1-inch thickness of sheet Styrofoam. A 'I8-inch thick quartz window on the head proper was complemented by a matching window a t the light entrance to the box. This arrangement provided a very high thermal resistance which aided enormously in maintaining low background fluctuations. Heating power dissipated in R15 a t 10 'C above room temperature was 2-3 W. T e m p e r a t u r e Controller. The proportional temperature controller was a modification of the design of West et al. (15,161 (Figure 5). The Maier bridge sensor was wound on the outside of the sensor cavity of the aluminum head. The bridge imbalance corresponding to the temperature error was amplified by A1 (effective gain of -lo5). Power for the bridge and preamplifier,

heater current, and the boosted Maier bridge signal was passed through an umbilical cable to the main controller. Ideally, the proportional temperature controller operated about the set point temperature of the copper-manganin bridge (i.e., the temperature a t which the bridge imbalance is zero). The preamplified bridge imbalance was inverted (A2) with a gain of 5.6. A fraction of this amplified bridge imbalance voltage, adjustable by R2, was summed a t unity gain by A3 with an adjustable bias set by R1. A3 controlled a current booster (T1and T2) which varied the heating current supplied to R15 from a 15-V series pass voltage regulator (D, T3). One of the series resistors (R13) could be shunted to provide faster warmup while the other provided a measure of short circuit protection. For optimum temperature control, R1 and R2 were adjusted after the head had equilibrated near the controller set point. R1 was adjusted to null M1 to within a few hundredths of a volt. R2 was then adjusted to increase the fraction of imbalance signal summed by A3 until any further increase caused the controller to oscillate. R1 and R2 interacted and several adjustments were initially required. The long response time of the system required -0.5-2 h for transient effects to subside after major adjustments, but, once adjusted, the system was stable for weeks. If power was interrupted for more than a few seconds, R2 was backed off and readjusted as the system again settled into the non-oscillatory operating range; therefore, the controller was run continuously in this work. Measurement of the Reflectance of t h e Bolometer Absorber. The reflectance of Krylon 1602 flat black enamel was measured a t 10-nm intervals using the Xe arc excitation system. An aluminum plate was blackened on one side with the paint and freshly smoked on the other with MgO. Excitation was normal to the surface, and reflected intensities were monitored a t 45' (6 inches from the surface) using a United Detector Technology PIN lODP photodiode operating as a current source. Currents were measured either directly on a Keithley Model 160 digital voltmeter, or after current-to-voltage conversion (Analog Devices 40 J operational amplifier with a 20-Mfl feedback resistor). At each wavelength, signals were measured for the MgO coated and the blackened sides of the plate with both in the same orientation. The reflectance of the enamel, (r.fb)xwas the ratio of the

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Flgure 5. Schematic of the proportional temperature controller. Components within the dashed lines are located in or on the bolometer head. R1, l O k R 20-turn trimpot; R2, 20kR 20-turn trimpot; R3, 11742 #40 enameled Cu wire: R4, 1 1 7 4 #40 enameled Cu wire; R5, 127-12 #40 enameled manganin wire; R6, 1 1 6 4 #40 enameled manganin wire; R7, 10 MQ; R8, 100 k R ; R9, 560 k R ; R10. 10 k-R; R11, 1 kfl; R12, 2.7 kR; R13, 10 R, 3-W; R14, 330 n; R15, 28R #40 enameled Cu wire; all resistors 1 / 4 W unless specified. CI, 0.1 pF; C2, 0.047 pF; D. 1N4744 15-V Zener; TI, GE 22318; T2, T3, Sylvania ECG 121; A l , Analog Devices 233J: A2, A3, Analog Devices 741K. The 2.5 V is derived from a 7805 three-terminal regulator and a series dropping resistor

ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979

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Flgure 6. Schematic diagram of the Rhodamine B calibration apparatus. (B) Bolometer; (D) front surfaced deflection mirror; (M) excitation light source and monochromator; (C) comparator consisting of RhB counter cell and photomultiplier. (See Ref. 13 for details)

black-to-Mg0 photodiode signals a t each wavelength; the paint and MgO are both near Lambertian reflectors, and MgO is a near perfect reflector over the wavelength range of interest (17,18). Measurement of Window Transmittance. The transmittances of the bolometer windows and quantum counter cells were measured on a Cary 11 recording spectrophotometer using the 0 4 . 1 absorbance slidewire. In each case, the air vs. air spectrum was run at the same instrument settings and subtracted from the sample spectrum. The quantum counter cell contained methanol and the effective absorbance of a single cell window was taken as half the measured absorbance of the cell. Absolute Calibration of Rhodamine B. The standard reference cell and solution of RhB (5 g/L methanol) was calibrated by measuring the responsivity of the quantum counter vs. the bolometer using the apparatus shown in Figure 6. This sealed cell and solution was also used throughout the quantum counter comparator measurements of the next paper. The excitation beam was alternately directed onto the quantum counter cell or the bolometer by means of a front surfaced mirror with adjustable positioning stops. Measurements were made at 10-nm intervals over the range 320-600 nm using a 19-nm (FWHM) optical slit width. This bandwidth was larger than that used with the quantum counter comparator (13) to provide measurable bolometer response at the spectral extremes. The bolometer thermal time constant was -30-50 s and, after a change in illumination, the resistance was stable to within 1 2 fl after - 3 min. Bolometer equilibrium was therefore assumed to attain within 4 min. Compensation for slow drifts in excitation intensity and bolometer temperature was effected by a semisymmetrical dark-lightaark measurement cycle at each calibrated wavelength. Data collection efficiency was optimized through the multiple use of bolometer dark resistance values for base-line correction of adjacent illuminated bolometer readings. The complete measurement cycle required 8 min per calibrated wavelength and was subdivided into three periods of 2, 4, and 2 min, respectively. Figure 7 illustrates the measurement cycle for calibration a t A,. Upon entering Period $1 (from Period $3) the bolometer had only half completed equilibration to darkness so the average a W

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illuminated quantum counter signal V L ~ (was i ) recorded in the interim. The bolometer dark resistance R l ~ ( iwas ) then recorded and the excitation beam deflected to illuminate the bolometer, thus initiating Period #2. Quantum counter dark signal V2&) was recorded at Period #2 midpoint, and the bolometer illuminated resistance RzL(i) was recorded as Period #2 was terminated by illumination of the quantum counter. During Period #3 the average illuminated quantum counter signal a t hi was again recorded as V3L(i).Period f 3 ends by advancing the excitation wavelength to Ai+l, thus restarting the measurement cycle a t Period #1 for the new Xi+l. As the cycle began again, the quantum counter was illuminated a t Ai+l and the bolometer was half equilibrated to darkness. The bolometer response (AR/Ro)A,and the quantum counter response V(A,) were calculated using the following equations:

P(XJ = 0.5 [ V l ~ ( i+) V3L(i)]- Pz~(i)

(2)

Rhodamine B Calibration D a t a Treatment. (AR/Ro)A is directly proportional (see below) to the incident power. The quantum counter signal V(X) is proportional t o the incident number ofphotons per unit time under the same conditions. Each (4R/Ro)Awas converted to relative quanta per unit time by multiplying by A. The apparent variation in relative luminescence quantum yield (quanta emitted per quanta absorbed) for Rhodamine B ( I ’ ( A ) ) was calculated from

I l V = V(A)/[(aR/R,)A.~l

(3)

Z’(X) was not the true relative luminescence quantum yield; the transmittances of the bolometer and quantum counter windows, and reflectance from the bolometer’s black paint varied slightly with wavelength. The true relative response of the quantum-counter quartz windowed dye-cell combination, ZQ(X), was obtained from ~ Q ( A )=

z’(A){(tbw)Az[l

- (rd~ll/N

(4)

where (tbwIA was the transmittance of a single bolometer window a t A, (rfi)Awas the reflectance of the flat black paint a t A, and N was a normalization factor making I Q ( A ) average unity over the wavelength range reported. ZQ(A) represented the real responsivity of a quartz windowed RhB counter. The true relative luminescence quantum yield of Rhodamine B, @(A), differed slightly from I ( A ) , because of variation in the transmittance of the quantum counter cell window, (tcw)A. @(A) is given by @(A) = [z(A)/(tC!W)Al/ N ’ (5) where N’ is a new normalization factor for $(A). Since tcw increased by only -0.3% from 360 to 590 nm, I ( X ) and &(A) differ only slightly.

-xi

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Flgure 7. Measurement cycle for calibration of the standard RhB counter against the large area bolometer. X’s indicate recorded measurements

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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979

RESULTS AND DISCUSSION Bolometer-Controller P e r f o r m a n c e . The temperature controller's performance was judged by the drift in the bolometer's dark resistance vs. time. When fully equilibrated, detector drift over the 12 min required for a dark, light, dark measurement cycle was a t the very worst 300 p " C and nominally 75-150 p"C (-4-9 maximum change in the 70-k9 thermistor). The bridge noise corresponded to -A2 Q,so the temperature drift and measurement "noise" were comparable. Using the detector with calibration heater, short term drift and noise corresponded to wA5-10 pW of incident power. Also, this absolute version had constant sensitivity within 0.5% over the power range 300-4000 pW (AR/Ro 0.2-470) as determined using the internal calibration heater. The relative bolometer had very nearly the same sensitivity as the absolute one, a n d since the two thermistors came from the same supplier and batch, we assumed the relative bolometer to be equally linear over the same range of (ARIR,). Over the region 340-590 nm in the RhB calibration, AR ranged over 400 to 2500 9. We estimated an upper bound on the measurement precision to be A'/,% based on the null noise level of f 2 9. T h e mean relative deviation of the pairs of resistance changes

-

-

580nm

Flgure 8. Relative variation in luminescence quantum yield 4 (A) of the rear viewed standard Rhodamine B counter (5 g/L)in methanol l c m optical path ~-

Table I. Relative Response of a Standard Rhodamine B Quantum Countera

-

(6)

where Ro and RT are resistances a t Toand T , respectively, and B is the thermistor sensitivity factor. When used over a narrow temperature range, the exponential can be expanded in a power series and truncated after the squared term to yield

For very small temperature changes, the second term may be ignored and (ARIR,) is proportional to T - Toand thus the incident power. For somewhat higher values of (AR/Ro), the second term represents an error in the linearity of the bolometer response. T h e largest (ARIR)encountered in the R h B calibration was -3.6% ( T - To 0.2 K), and our assumption of a linear power dependence in (AR/Ro)is in error by no more than 2%. While Equation 6 is accurate over a wide temperature range for negative coefficient thermistors, it is not an exact representation of the actual sigmoidal R vs. T response of positive temperature coefficient thermistors. Thus, the real error was less than the maximum 2% figure derived above. The measurements made with the absolute bolometer suggest t h a t the errors were, in fact, under 1 % . Thus, we conservatively estimated our measurement accuracy in the RhB calibration a t better than 2%. Rhodamine B Calibration. The results of our bolometer calibration of the standard RhB (5 g/L in methanol) quantum counter are shown in Figure 8 and Table I. This counter was the one used as the reference counter in the following paper. I&) represents the responsivity of a quartz windowed counter; @(A) is the true relative luminescence yield corrected for cell reflections. Quantum counters are most commonly used to calibrate the wavelength dependence of the intensities of spectral sources. The use of ZQ(X) and @(A) occurs in these calibrations

-

380nm

x

in the RhB calibration (370-590 nm) was 0.17% for 23 measurements with 410 fl 5 AR 5 2530 9 (mean resistance 1665 9). This mean relative deviation agreed quite well with an estimated mean uncertainty of 2/1665 = 0.12% using the 2-9 observed noise level. Thus, the upper bound figure of is certainly conservative a t the higher power levels. We turn now to the bolometer accuracy, which we estimate a t 1-2%. The response of a thermistor is given by Equation 6 (19).

(RT- R o ) / R o= ( a R / R o )= exp[B(T,--l- T1)l- 1

480nm

a

A, n m

IO (A 1

590 580 570 560 550 540 530 520 510 500 490 480 470 460 450 440 430 420 410 400 390 380 370 360

1.009 1.007 0.999 1.001 0.993 0.998 0.994 1.001 0.999 0.997 1.006 1.013 1.016 1.012 1.009 1.005 1.007 1.010 1.002 0.997 0.992 0.978 0.981 0.974

@(A) 1.009 1.007 0.998 1.000 0.993 0.997 0.994 1.001 0.999 0.996 1.006 1.013 1.016 1.013 1.010 1.005 1.007 1.011 1.003 0.998 0.993 0.980 0.983 0.976

Methanol solution of RhB (5 g/L).

in two different cases. T h e basic equations for the use of quantum counters are

V(

= K F e x t (A) tcw (A) 4 (A)

(W

= KFext(X)IQ(X)

(8B)

= KFint(X)@(A)

(ac)

where V (A) is the quantum counter's photomultiplier signal, K is a proportionality constant, F,&) is the relative free air quantum flux of the source, tcw(X) is the transmittance of the front window of the quantum counter cell, Fint(X)is the flux delivered to the sample within the quantum counter cell, and d(X) and ZQ(X) are as defined previously. Equation 8A is used to calculate the relative free air spectral distribution of the source given knowledge of the cell window transmittance and $(A). Equation 8B is used to directly calculate F,,,(X) when the quantum counter solution is contained in a high quality quartz cell as was used by us to obtain IQ(X). Equation 8C yields the light flux delivered in a cell when the quantum counter and unknown solutions are contained in equivalent cells. The @(A) vs. X data yield a standard deviation (36Ck590 nm) of 1.04% about the mean with a maximum spread of 4.2%. As discussed above, we conservatively estimate our experimental precision to be f0.5% except toward the long and short wavelength limits where our diminished arc intensity might increase errors slightly. T h e accuracy rests on the linearity of thermistor response, which we conservatively estimate to be 1-270.

ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979

We believe that our present calibration is a t least a factor of 2 more accurate and precise than previous thermopile calibrations. Errors in earlier calibrations accrued from the failure to correct for the reflection and transmission properties of the optical paths, the use of thermopiles, the variable collection efficiency of the detector, and purity problems with RhB (see ref. 13). Distortions in beam splitter systems can arise from variations in reflection coefficients caused by beam polarization. Thermopiles are small area detectors with very nonuniform sensitivities. Thus, even if the excitation beam underfills the thermopile sensor, changes in beam size with wavelength caused by chromatic optics (e.g., lens) produce differing sensitivity factors for the detector. For large area beams which overfill the sensor, chromatic aberrations in the optics produce large variations in the collection efficiency. Thus, conditions such that a sampled beam accurately represents the whole beam are very difficult to obtain and verify. In the current measurements, however, the excitation optical train was relatively achromatic, optical losses were corrected, and the entire excitation beam was intercepted by both the quantum counter cell and a uniformly sensitive bolometer. Thus, this calibration makes a sound reference for the examination of other counter materials.

ACKNOWLEDGMENT We gratefully acknowledge R. B. Martin for use of his Cary 11, E. D. West for the gift of the manganin wire, and A. Norvelle for his skill and assistance in machining the bolometer head.

717

LITERATURE CITED Parker, C. A. "Photoluminescence of Solutions"; Elsevier: New York, 1968. Calvert, J. G.; Pins, J. N., Jr. "Photochemistry"; J. Wiley & Sons: New York, 1966. Geist. J.; Blevin, W. R. Appl. Opt. 1973, 12, 2532. Doyle, W. M.; McIntosh, B. C.; Geist, J. Opt. Eng. 1976, 15(6), 541. Geist, J.; Lind, M. A,; Schaefer, A. R.; Zalewski, E. F. "SpecVal Radiomeby: A New Approach Based on Electro-Optics"; Natl. Bur. Stand. ( U . S . ) Tech. Note 954, 1977. Geist, J. Natl. Bur. Stand. (U.S.) Tech. Note 954-1, June 1971. Demas, J. N.; Crosby, G. A. J . Phys. Chem. 1971, 75, 991. Melhuish, W. H. J . Res. Natl. Bur. Stand.. Sect. A 1972, 76, 547. Vavilov, S . J. Z . Phys. 1927, 4 2 , 3 11 Vavilov, S. J. 2. Phys. 1924, 22, 266. Bowen, E. J. Proc. R . SOC. London, Ser. A 1936, 154, 349. Melhuish. W. H. N . 2. J . Scl. Techno/., Sect. 6 1955, 3 7 , 142. Taylor, D. G.; Demas, J. N. Anal. Chem., following paper in this issue. Maier, C. G. J . Phys. Chem. 1930, 3 4 , 2860. West, E. D.; Case, W. E.; Rasmussen, A. L.; Schmidt, L. 8. J . Res. Natl. Bur. Stand. 1972, T6A, 13. West, E. D., National Bureau of Standards, Boulder, Colo., private communication, 1975. Killick, D. E.; Bateman, D. A,; Brown, D. R.; Moss, T. S.; de la Perrella, T. E. Infrared Phys. 1966, 6 ,85, Figure 10. Middleton, W. E. K.; Sander, C. L. J . Opt. SOC. Am. 1951, 4 1 , 419. Gunn, S. R., J . Chem. Educ. 1973, 50, 515.

RECEIVED for review September 14,1978. Accepted January 23,1979. Support by the National Science Foundation (MPS 74-17916 and CHE 77-20379), the Air Force Office of Scientific Research (78-3590),the Research Corporation, and the donors of the Petroleum Research Fund, administered by the American Chemical Society, is gratefully acknowledged. This work was taken in part from the M.S. Thesis of D. G. Taylor a t the University of Virginia, 1976.

Light Intensity Measurements 11: Luminescent Quantum Counter Comparator and Evaluation of Some Luminescent Quantum Counters D. G. Taylor' and J. N. Demas' Department of Chemistry, University of Virginia, Charlottesville, Virginia 2290 1

The design, construction, and evaluation of a very precise and accurate instrument for comparing optically dense luminescence quantum counters is described. The system Is capable of making intercomparisons with an accuracy and precision of better than 0.5%. A series of Rhodamine B, Rhodamine 6G, and blue dye quantum counters are calibrated over the 360690 nm region. The Rhodamine B counters offer the best spectral flatness of response, but are sensitive to photolysis. The blue dyes, especially Nile Blue A, promise to extend quantum counting to -700 nm.

The term "quantum counter" was originally used by Bowen in 1936 (1) to describe a spectrally flat quantum responsive detector composed of a fluorescent screen placed before a photodetector. Using a thermopile for the calibrations, his original measurements gave suitable results for a 1-mm C u r r e n t address, S c h o o l of C h e m i c a l Engineering, P u r d u e U n i v e r i t y , W e s t L a f a y e t t e , Ind. 47907. 0003-2700/79/0351-0717$01 .OO/O

thickness of crystalline uranyl ammonium sulfate in paraffin (252-436 nm) and aesculin (1-cm cell, 1 g/L in water; 252-367 nm), but not so for fluorescein owing to an absorption minimum around 367 nm. The independence of luminescence efficiency with wavelength, however, was a concept demonstrated qualitatively by Vavilov ( 2 , 3 ) a decade earlier. With great insight, Vavilov described the very application of this property which was developed by Bowen and Sawtell ( 4 ) and finds most extensive use today. In 1955, Melhuish ( 5 ) introduced Rhodamine B (RhB) as a new quantum counter material which provided a useful range to -600 nm. Using a standard lamp, he found that the luminescence yield of a front viewed 4 g/L solution in glycerol was wavelength independent with the exception of an -5% reduction in yield around 450 nm. Weber and Teale (6) followed with a thermopile calibration of RhB (9.6 g / L in ethylene glycol) in a side viewed counter; they found a similar 10% reduction in yield a t 440 nm (6). Melhuish (7), attributing this dip to the dye's absorption minimum, introduced a mixed-dye counter of acriflavin (1g/L) and RhB (4 g/L). This system was based on the strong absorption of acriflavin a t -450 nm and its demonstrated efficient energy transfer 0 1979 American Chemical Society