US 3807875 A
This invention relates to electro-optical densitometry. More specifically, this invention provides apparatus suitable for spectrophotometry which employs incoherent or coherent injection electroluminescent diodes as light sources for providing pulsed light emission. Such apparatus can measure, for example, chemical concentrations, sedimentation rates, absorption, and light scattering phenomena.
Claims available in
Description (OCR text may contain errors)
United States Patent [191 Fischer et al.
[451 Apr. 30, 1974 DENSITOMETRY APPARATUS Inventors: David J. Fischer, Corning, N.Y.;
Charles D. Jamerson, Jr.; Hans J. Kunz, both of Raleigh, NC.
 Assignee: Corning Glass Works, Corning,
 Filed: June 9, 1971  Appl. No.: 151,309
 U.S. Cl. 356/201, 250/217 SS, 250/218, 250/220 R  Int. Cl. G0ln 21/22  Field of Search 250/217 SS, 218, 211 J, 250/220 R, 199; 356/93-97, 201-208, 41
 References Cited UNITED STATES PATENTS 3,317,730 5/1967 Hilsum 356/103 UX 3,524,066 8/1970 Blakkan.... 250/218 3,558,974 l/l97l Stewart..... 250/217 SS 3,588,253 6/1971 Wittmann 356/93 3,629,589 12/1971 Gleixner 250/218 OTHER PUBLICATIONS Schawlew; Scientific American, Vol. 209, No. 1, July 1963 pages 34-45 Craig et al.; IBM Technical Disclosure Bulletin Vol. 10 No. 4 September 1967 pages 442 and 443 Gamblin, IBM Technical Disclosure Bulletin, Vol. 12 No. 4, September 1969 Epstein et al.; Electro-Technology Vol. 82, No. 1, July 1968 page 35-41 I-Iinkley, Applied Physics Letters, Volume 16, Number 9, May 1, 1970, pages 351-354 Reynolds, Laboratory Practice, Vol. 16, No. 9, September 1967 pages 1119-1121 Primary Examiner-Ronald L. Wibert Assistant ExaminerF. L. Evans Attorney, Agent, or F irm-Clinton S. lanes, .lr.
[ 1 ABSTRACT This invention relates to electro-optical densitometry. More specifically, this invention provides apparatus suitable for spectrophotometry which employs incoherent or coherent injection electroluminescent diodes as light sources for providing pulsed light emission. Such apparatus can measure, for example, chemical concentrations, sedimentation rates, absorption, and light scattering phenomena.
14 Claims, 8 Drawing Figures POWER SUPPLY SAMPLING I CHAMBER 7 5 DIODE 2 6 MEASURING Ac gggzgks MSA APPARATUS IV ,4 db- 1E 3' 3| PATENTEUAPR 30 1914 SHEET 4 BF 7 INVENTORS David J. Fischer Charles D Jamerson, Jr Hans J Kunz SHEETSHF? SIGNAL AND NOISE SPIKES RIDING 60h: PICK-UP SIGNAL TIME MUM PATENTEBAPR 30 m4 OUTPUT INVENTORS. David J. Fischer Char/es D. Jamersomw: Hans J. Kunz BY I AT RNEY TIME Fig.
AMPLIFIER 86 PATENTED APR 30 I974 SHEET 7 [IF 7 mDhqm 4 JOE.
DENSITOMETRY APPARATUS BACKGROUND OF THE INVENTION Standard methods of electro-optical densitometry, especially spectrophotometry, involve apparatus comprising a light source, a prism or grating, and a detector which is usually a photomultiplier tube. These components together with a standard power supply normally occupy a volume of at least one cubic foot.
Several attempts have been made at miniaturizing spectrophotometers and colorimeters. Spectrophotometers allow selection of a narrow spectral width for optical density measurements Colorimeters employ broader bands of light, resulting from use of single or multiple filters, for making optical density measurements. One feature in previous size reduction approaches is that remote light sources are used rather than the development of a smaller intense light source. Light is selected from a normal source and then reflected through mirror arrangements or piped through fiber optics to the sample measuring chamber. Colorimeters employing this approach may be as small as several cubic inches. Spectrophotometers, however, due to the narrowness of the spectral width needed, have not been substantially miniaturized.
SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide miniaturized, simple, economical high speed apparatus for making densitometry measurements which overcomes the heretofore noted disadvantages of the presently available apparatus.
Briefly, the apparatus for conducting electro-optical densitometry measurements with this invention comprises a sampling chamber for containing the materials to be measured and at least one injection electroluminescent diode for emitting light having frequencies within a selected band width. Also included is a photodetector for sensing the intensity of light, emitted by said injection electroluminescent diode, arriving at said photodetector, and apparatus responsive to the phototransistor for measuring the intensity of the received light. The apparatus also includes means for furnishing operating power to the injection electroluminescent diode and the intensity measuring apparatus. In further embodiments, the apparatus comprises at least one electroluminescent diode and a single photodetector capable of being positioned in various controlled locations, or the apparatus comprises an array of electroluminescent diodes and at least one photodetector, said photodetector capable of being positioned in various controlled locations.
- BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of densitometry measuring apparatus incorporating the teachings of this invention.
FIGS. 2a-2c provide an electrical schematic of a preferred embodiment of this invention.
FIG. 3 is a diagram of a typical signal measured by the receiver of the preferred embodiment of FIGS. 2a-2c.
FIG. 4 is a diagram of the input and output wave forms of the peak detect-and-hold circuit of the receiver of the preferred embodiment of FIGS. 2a-2c.
FIG. 5 is a diagram showing the percentage of light emitted by a gallium arsenide diode transmitted through varying concentrations of a copper sulfate and water solution. (Percentage of light transmitted [percent light] versus Grams of Copper Sulfate per liter of water [grams CuSO /liter H 01).
FIG. 6 is a block diagram illustrating an array (plurality) of electroluminescent diodes and an array (plurality) of photodetectors.
DETAILED DESCRIPTION OF THE INVENTION The invention have discovered that electro-optical densitometry of the type shown in FIG. 1 may be miniaturized to occupy a volume less than 0.02 cubic inch by the employment therein of specialized devices and microcircuitry. Referring now to FIG. 1, there is shown control apparatus 1 which controls the pulsating of light emission of injection electroluminescent diode 2. When pulsed on, diode 2 emits light rays 3 within a selected frequency band. These light rays enter a sampling chamber 4 which contains sample 5 upon which densitometry measurements are to be made. A photodetector 6 receives light rays 3', which are the same as light rays 3 except the intensity has been attenuated by sample 5. A significant phase shift may also exist between light rays 3 and 3. Phototransistor 6 produces an electrical current corresponding to the intensity and the phase of light rays 3'. The parameters of said current are measured by apparatus 7. Power supply 8 supplies power as required to illuminate diode 2, and to operate measuring apparatus 7. In the preferred embodiment hereinafter described, this invention discloses apparatus that, because of its unique pulsing operation generates less heat within the samples than heretofore has been experienced by available densitometry instrumentation. Further, the apparatus of this invention is not sensitive to normal environmental temperature fluctuations.
The light source and associated apparatus such as prisms or gratings inherent in the bulky previously available spectrophotometers which are used to produce a narrow. spectral width, have been replaced herein by a small injection electroluminescent diode which may have an outside diameter as small as 3 mils. The injection electroluminescent diodes are very similar in their electrical characteristics to the common diodes used for normal electronic purposes except for the fact that they emit electromagnetic radiation such as visible and infrared light. The emitted light may be incoherent or coherent. These diodes are desirable because the narrow band of light they produce can function as the necessary monochromatic light source of a spectrophotometer. The selection of the correct semiconductor material and diffusion of the proper dopant to prepare the appropriate junction for the diodes is quite critical to insure that light of the proper wavelength will be emitted. Should it be desirable to decrease the band width of the emitted light further, filters, which are produced simply by coating the surface of the diodes, may be employed. The surface of the semiconductor material can be cut or etched to function as a lens for focusing the emitted light. Alternatively, if special needs arise, a separate miniaturized lens with the appropriate coatings may be employed to concentrate the light and to further narrow the band width.
Each injection electro-luminescent diode will be selected for that wavelength whichis appropriate for the characteristic phenomenon of the compound or cornpounds which are to be measured. Thus, an array of miniaturized light sources individually activated on a selective basis will perform like a spectrophotometer rather than a colorimeter. Each injection electroluminescent diode in this array will emit a different wavelength. Each emitted wavelength will be that wavelength readily absorbed or scattered by a particular chemical species. Therefore, as each injection electro-luminescent diode in the array is individually turned on and off the concentration or other characteristic of a particular chemical species may be measured by the degree to which the species affects the associated emitted light wavelength. Assembly of different diodes for additional wavelengths will proportionally increase the size of the light source subsystem.
This invention employs a semiconductor photodetector being similar in size to that of the light source in place of a bulky photomultiplier tube presently used in existing systems. The material used in the photodetector is based upon its ability to respond to and measure rapid intensity changes in the light emitted by the array of diodes. This type of spectrophotometer, aside from the reduction in size, differs from the conventional because the use of the selective pulsing approach for either a single diode or an array of different electroluminescent diodes makes possible a large number of optical density measurements in less than a second. In fact, the limit on the number of optical density measurements will come from the utilizing apparatus and not from the light detector measurement apparatus. Ordinarily, spectrophotometers and colorimeters employ mechanical rather than high speed electronic techniques for selecting wavelengths and making optical density measurements.
PREFERRED EMBODIMENT Referring now to FIGS. 2a-2c, there is shown a schematic of a preferred embodiment of the present invention. For convenience, this schematic is separated into three parts: (1) the transmitter, comprising the trigger circuit, the monostable multivibrator, and the gating circuit; (2) the receiver, comprising a phototransistor, a range multiplier, a peak-detect-and-hold circuit, a meter, and (3) the power supply. The transmitter, item 1, the receiver, item 2, and the power supply, item 3, will now be taken in sequence.
(1) The Transmitter (FIG. 2a)
The heart of the transmitter is the monostable multivibrator. The magnitude and duration of the transmitted light pulse is determined by this circuit. The switching action is as follows: Transistors and 12 initially are off. Therefore, no current will flow through light emitting diode 14. Transistor 16 is biased on through resistor 18 and diode 20. When a positive trigger pulse is applied to the base of transistor 10, it begins to turn on and its collector voltage starts to drop. This dropping voltage is coupled through diode 20 and capacitor 22 to the base of transistor 16, causing the base of transistor 16 to go negative such that transistor 16 is turned off. As transistor 16 starts turning off, its collector voltage rises and starts to turn transistor 12 on, which further reduces the collector voltage of transistors 10 and 12. Thus, the turn-on is regenerative and transistorlZ quickly saturates. Since transistor 12 is fully turned on,
the current through diode 14 is limited and determined by resistor 24 or the parallel combination of resistors 24 and 26. This full on condition of transistor 12 is maintained until the resistor l8-capacitor 22 time constant permits the base of transistor 16 to once again go positive. When this occurs, transistor 16 turns on which turns transistor 12 off, and the current through diode 14 goes back to zero. This current remains zero until another positive trigger pulse is applied to the base of transistor 10.
The minimum acceptable transmitted pulse width is determined by the rise-time of the phototransistor discussed hereinafter. When operated at the extremely low signals encountered by maximum absorption, the rise-time of v the phototransistor is about 250 psec. Thus, the. resistor l8-capacitor 22 time constant is chosen to give a transmitted pulse width of approximately 300 sec. Y
Resistor 26 is switched into a parallel connection with resistor 24 to provide additional diode current when working with high absorption materials, thereby providing an extra order of magnitude to the range of absorption measurements. Diode 20 is required to prevent the emitter-base junction of transistor 16 from breaking down when transistor 16 is turned off. Resistors 28 and 30'are biasing resistors for transistors 10 and 12 respectively, and resistor 32 is a biasing and current limiting resistor-for transistor 16.
The trigger circuit provides the positive pulses that initiate the one-shot cycle of the monostable multivi brator circuit, and so determines the repetition rate of the transmitter. For two reasons it is desirable to have alow repetition rate. First, a low repetition rate minimizes the effects of heat generation in the sample material, and second, it permits driving light-emitting diode 14 much harder than its dc rating and thereby gives extra measurement range. The chosen repetition rate of 60 Hz gives a duty cycle of only 1.8 percent with the resistor l8-capacitor 20 time constant as stated. Said 60 Hz value was chosen after considerable investigation, and it was found that by synchronizing the trigger to the power line, a significant signal-to-noise advantage is realized, as explained below.
For highest-absorption measurements, the signal received from the phototransistor output is in the microvolt range, so for accurate measurements the noise must normally be below this level. This would ordinarily require considerable isolation and shielding. However, in this application, only relative measurements are made. Relative" because the absolute signal level is never measured. In operation, the absorption scale is arbitrarily set to zero for some particular condition. In the CuSO experiment, described in the specific example to follow, zero corresponds to no CuSO in the H 0. Noise does not affect the relative measurements nearly so much so long as the noise and signal phase are the same. For example, suppose the received signal is as sketched in FIG. 3, with the signal riding a relatively large 60 Hz pickup that is covered with noise spikes.
So long as the signal is measured at exactly the same point on each cycle, the relative measurements can be accurately made for signals smaller than the noise level.
Referring again to FIGS. 2a-2c, the synchronized trigger signal is taken directly from the transformer of the power supply circuit and applied by line 33 through resistor 34 to the emitter of the unijunction transistor 36. Transistor 36 remains off until the emitter becomes forward biased. At this point, the turn-on of transistor 36 is regenerative and results in a large current pulse which is coupled through capacitor 38 to transistor to trigger the monostable multivibrator. As the input sinewave goes past its positive peak and approaches zero, transistor 36 turns off, such that it is reset for the next pulse. Diode 40 prevents the emitter of transistor 36 from breaking down during the negative part of the 60 Hz input signal. Resistor 42 limits the current flow through unijunction transistor 36 and resistor 44 provides the proper bias. The receiver circuit discussed hereinafter is gated on and off by a gating circuit. As explained heretofore, transistor 16 is normally biased on which in turn biases transistor 46 off through resistor 47. Therefore, when the monostable multivibrator circuit turns off transistor 16, the collector voltage of said transistor rises and turns on transistor 46. When transistor 16 is again turned on, the collector of transistor 46 is off. Since transistor 46 is normally off, FET (field effect transistor) 48 is normally biased on through the combination of resistors 49, 50, and 51. While FET 48 is on, a large current is delivered to said receiver circuit by line 53 through current limiting resistor S2 and PET 48. However, when transistor 46 is biased on its collector voltage decreases which results in FET 48 being turned off which in turn stops the current flow to said receiver circuit.
(2) The Receiver (FIG. 2b)
The phototransistor 54 is biased on by the light received from light emitting diode 14. The output current from the phototransistor 54 generates a voltage across resistor 56. An operational amplifier 58 is used to amplify this photo-response to a useable level wherein a feedback resistor 60-76 selected by switch 77 is connected to set the correct amplification range.
Due to the high impedance of the feedback resistors 60-76 and the high feedback ratio of the feedback resistor to resistor 56, the amplifier 58 input current and voltage offsets very possibly cause considerable do drift at the output of said amplifier. Typically, about 0.5 volt may be experienced over several hours depending upon the feedback resistor selected. However, in this embodiment only the pulse response is important and dc drift is of little consequence, therefore, capacitor 78 is used to block all drift effects and to couple the response of the phototransistor 54 through to the peakdetect-and-hold circuit. Resistor 80 limits the current through plate transistor 54. Resistor 82 is a biasing resistor for amplifiers 58, and resistor 84 provides an adjustment for setting the output of amplifier 58 to zero.
To prevent the peak-detect-and-hold from capturing unwanted noise spikes, the peak-detect-and-hold circuit is gated by the transmitter gate circuit. When no pulse is being transmitted, FET 48 as explained heretofore is biased on, and a large current is driven into the input of amplifier 86. This causes the output of amplifier 86 to go negative. Diode 88 is reverse biased, and so no noise signal can get to the output V When a pulse is transmitted, FET 48 is turned off and the peakdetect-and-hold circuit can then capture any input signal.
The input signal to the peak-detect-and-hold circuit is denoted as V,-,, as shown in FIG. 2b and FIG. 4. This is a negative-going pulse. As V, starts negative, the
output V of operational amplifier 86 goes positive thereby charging capacitor 90. As V goes through its peak and starts decreasing in magnitude, V tries to decrease. However, as the output of amplifier 86 begins to decrease, diode 88 becomes reverse-biased, due to the charge stored in capacitor 90, thereby isolating amplifier 86 from those portions of the circuit connected to the opposite side of diode 88.
Thus, V is a dc level corresponding to the peak magnitude of the input. The reading from dc meter 96 is directly proportional to V and so is directly related to the amplitude of V Diode 98 is required to provide a feedback path for amplifier when diode 88 is reverse-biased. This occurs just after the peakinput signal is passed as seen above and for all time between pulses due to the gate circuit input. Resistors 100 and 101 provide for full scale adjustment of meter 96. Resistor 102 provides proper biasing for amplifier 86, and resistor 104 provides an adjustment for setting the output of amplifier 86 to zero. Resistors 106 and 108 allow for the proper input signal range to amplifier 86. Resistors 110-114 provide proper biasing for transistors 92 and 94.
(3) The Power Supply (FIG. 2c)
Since the preferred operating voltages'of operational amplifiers 58 and 86 are typically :15 volts, power supply output voltages of ilSV will be assumed for the remainder of this discussion. The other circuits discussed above are designed to operate at the same values as said amplifiers to minimize power supply requirements. The operational amplifiers are operated-double-ended throughout and so are relatively insensitive to supply voltage variations (typically 0.4 mv./volt). However, in this preferred embodiment both the phototransistor and light-emitting diode are operated single-ended from the positive 15 volt supply. The phototransistor output signal is very strongly a function of the positive 15V supply. As an example, for an intermediate range of absorption, measured values of phototransistor output voltage vs supply voltage are as shown in Table I.
TABLE I Phototransistor Output Supply Value (normalized to unity Since the signal response is so strongly dependent upon the positive supply,-care must be taken in provid ing a constant, well-regulated positive supply.
The secondary voltage of transformer 116 is rectified by the full wave rectifier output. Due to the noncritical nature of the negative 15V supply, a simple shunt regu lator comprised of a zener diode 130, and resistor 132 is satisfactory. The negative 15V supply is further filtered by capacitor 134. The positive 15 volt supply is a conventional series-regulated supply, using negative feedback to maintain the correct output voltage. Transistors 136 and 138, zener diode 140, and resistors 142, 144, and 146 comprise said series regulator. Capacitor 148 further filters the output of said positive 15 volt supply. Drift measurements madeon the positive supply indicates acceptable low-drift characteristics: After a two-minute warm-up, the drift of the positive supply of such a power supply was less than 12 mv for a 30- minute observation period, while the drift for the negative supply was less than 24 mv for the same period.
This invention may be employed to measure chemical concentrations by absorption measurements and to detect precipitants and determine sedimentation rates by measuring the amount of light scattered or reflected. If the absorption coefficients are known, absorption data would provide accurate thickness measurements o1, for example, thin semiconductor slices. Also, flow rates 01 liquids could be measured with the apparatus by injecting small quantities of absorbing material into the liquid. The time between the absorption peaks for two different locations at a known distance from each other in the flow path would indicate the flow rate.
In addition to absorption, scattering and reflective phenomena could be measured. Scattering measurements could be used to detect particles suspended in a fluid. For this application an array of ph'otodetectors or emitters may be used instead of a single detector or emitter.
SPECIFIC EXAM PLE The following example describes an embodiment of the invention employing a gallium arsenide (GaAs) light emitting diode. Said example is particularly well suited for making measurements of substances which absorb light having a wavelength in the region of about 0.9 microns. This specific example is apparatus of the type described in the preferred embodiment and illustrated in FIGS. 2a2c. Table 11 lists the necessary components comprising the specific example.
TAB LE II.- RESISTORS Reflective measurements could be made, for example, to determine refractive index. Particles could be suspended in the liquid and if their refractive index is matched to the refractive index of the liquid, less scattering would occur compared to a situation where the refractive indices of particles and liquid are not matched. The Tyndall effect may also be useful when measuring concentrations of extremely small particles. The detector in these scattering measurements would possibly be arranged at a right angle or other suitable angle relative to the light source.
The narrow band widths possible and the fast switching speed of this apparatus provide a useful method for investigating the input and output spectra of fluorescent and phosphorescent materials, and the decay times of the chemiluminescent and phosphorescent materials.
Because measurements are made in fractions of, a second by a unique pulsing operation, inaccuracies associated with heat produced by the light sources in older apparatus are minimized in this invention.
These combinations of light emitters and detector may be powered and connected for readout through a coaxial cable which is a probe subsystem, or with a slight increase in size (total volume still less than 0.02 cubic inch) for allowing inclusion of miniaturized batteries, oscillator, and antenna, a multiple purpose completely independently operated system, sometimes Part No. Resistance Wattage Tolerance Part No. Resistance Wattage Tolerance 680K fizwu 5%.
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CAPACITORS Part No. Capacitance Voltage 20 .1 at 50 VDCWV 3s .Olpf vocwv 78 2 p.f 5O VDCWV 90 .1 pf 50 VDCWV 126 500 pf 50 VDCWV 128 500 pf 50 VDCWV 134 500 pf 50 VDC 50 148 500 pf 50 VDC TRANSISTORS Part No. Manuf. Part No. Manufacturer 10 MP8 3710 (MOT) :2 MP8 6531 (MOT) MP8 3710 M 36 2N 4871 OT) 46 MPS 3710 (MOT) 48 CM 640 (Crystalonics) 54 L 14A 502 (6,5,) 92 MP8 3710 (MOT) 94 MP5 3710 (MOT) 136 2N 3766 138 MP5 6531 (MOT) DIODES Part No. Manuf. Part No. Manufacturer 14 M 120C (MONSAN'I'O) 22 1N 3064 40 IN 3064 8 IN 3064 98 IN 3064 118 1N 4721 120 IN 4721 122 IN 4721 124 1N 4721 1N 4744 IN 755 called an Endo Capsule, would exist. The probe or TRANSFORMERS Part No. Manuf. Part No. 116 TRlAD-F92A OPERATIONAL AMPLIFIERS Part No. Manuf. Part No. Manufacturer 58 SQ a (NEXUS) 86 SO lOa (NEXU S 7w" COMPONENT DESCRIPTION Switch Part No. Manufacturer 77 9 Position Simpson 96 3 /4" 100 A Meter This example demonstrates the sensitivity and accuracy of absorption measurements using the concepts of this disclosure. In this particular example, the electroluminescent diode is an infrared emitting (incoherent) GaAs diode, and the phototransistor is made of silicon. With appropriate amplifier feedback resistors, a range of transmission intensities could be measured for concentration extremes having a ratio of one to 10 As an example of results obtained with this model, absorption measurements were made on aqueous copper sulphate solutions of varying concentrations. Transmission measurements were made on varying concentrations of CuSO in H O (See FIG. 5), from which it is seen that the expected exponential characteristic (linear curve on semi-log plot) does exist for over five orders of magnitude, thereby demonstrating the accuracy of the apparatus.
1. An apparatus for conducting electro-optical densitometry measurements of a material comprising a sampling chamber for the material, the optical density of which is to be measured,
at least one injection electroluminescent diode for emitting light within a selected band width through the material in the sampling chamber, a source for providing a pulsed electrical current for said electroluminescent diode,
at least one photodetector for sensing the intensity of the light emitted by the electroluminescent diode and through the material in the sampling chamber, and
signal measuring equipment responsive to said photodetector.
2. An apparatus according to claim 1 comprising an array of electroluminescent diodes and a single photodetector.
3. An apparatus according to claim 1 comprising a single electroluminescent diode and an array of photodetectors.
4. An apparatus according to claim 1 comprising at least one electroluminescent diode and a single photodetector capable of being positioned in various controlled locations.
5. An apparatus according to claim 1 comprising an array of electroluminescent diodes and a complementary array of photodetectors.
6. An apparatus according to claim 1 comprising an array of electroluminescent diodes and at least one photodetector, said photodetector capable of being positioned in various. controlled locations.
7. An apparatus according to claim 1 and further comprising a transmitter circuit for controlling said source of electrical current to said electroluminescent diode.
8. The apparatus of claim 7 wherein said transmitter circuit comprises a trigger circuit, and
a monostable multivibrator activated by said trigger circuit to supply electrical current of constant amplitude and direction to said electroluminescent diode.
9. An apparatus according to claim 8 wherein a gating circuit driven by said monostable multivibrator and electrically connected to said measuring equipment activates said measuring equipment while said electrolu minescent diode is receiving electrical current.
10. An apparatus according to claim 1 wherein said measuring equipment comprises a peak-detect-and-hold amplifier, and
a range multiplier connected between said photodetector and said peak-detect-and-hold amplifier.
11. An apparatus according to claim 10 wherein said photodetector and range multiplier combination supplies a signal consisting of voltage pulses to said peak-detect-and-hold amplifier.
12. An apparatus according to claim 11 wherein said peak-detect-and-hold amplifier is connected to an instrument for measuring the signal received from said peak-detect-and-hold amplifier.
13. An apparatus according to claim 1 wherein said source of electrical current is a power supply, and said power supply also supplies power to said measuring equipment.
14. An apparatus for conducting electro-optical densitometry measurements comprising an array of electroluminescent diodes,
a power supply for supplying a source of pulsed electrical current to said electroluminescent diodes,
a monostable multivibrator to switch said current to said electroluminescent diodes,
a trigger circuit to activate said monostable multivibrator,
a sample chamber for holding a sample to be measured,
a photodetector for receiving light emitted by said array of electroluminescent diodes,
a peak-detect-and-hold amplifier,
a range multiplier connected between said photodetector and said peak-detect-and-hold amplifier, an instrument connected to said peak-detect-andhold amplifier for measuring the signal received from said peak-detect-and-hold amplifier, said photodetector, range multiplier, monostable multivibrator, gate circuit, and peak-detect-and-hold amplifier being supplied operating power by said power supply.