Optical densitometer

In a preferred embodiment of the present invention, an optical densitometer is provided having an improved output scale resolution. An output scale reading from 0.000 to 4.000, having a resolution to 0.001, is obtained by improved method and apparatus for obtaining the reference operating characteristics for "zero-mode" parameters. An analog reference anode signal is formed by making adjustments in a generally antilogrithmic relationship to the difference between the reference anode signal being generated and the anode signal produced under the zero-mode conditions. The antilogrithmic relationship improves scale resolution near the zero point and increases densitometer stability during a subsequent measurement cycle. The combination analog reference anode signal generating circuit and an improved exponential characteristic selection circuit enables the improved output resolution and accuracy to be obtained. The exponential characteristic selection circuit enables the measurement circuitry to be adjusted in accordance with the actual photomultiplier tube in the circuit to produce and maintain a measurement having an improved accuracy.

FIELD OF THE INVENTION 
This invention relates to optical densitometers generally for use in the 
photographic industry and, more particularly, to optical densitometers 
utilizing photomultiplier tubes for converting input light intensities 
from an optical density sample to an electrical signal. 
BACKGROUND OF THE INVENTION 
Optical densitometers are in wide use in the graphic arts industry to 
obtain measurements of relative light intensity reflected from, or 
transmitted through, a selected surface or portion of a surface. 
Measurments may be taken from photographs, photographic negatives, or 
other image source and the measurements are used to control subsequent 
photographic exposures. The standard scale in the photographic industry 
for optical density measurements has a logarithmic base and the present 
invention includes improved circuitry for converting linear light 
intensity inputs to the industry standard logarithmic scale. 
The standard scale for optical density used in the graphic arts industry is 
a scale ranging from 0.00 to 4.00. Because of the logarithmic nature of 
this scale, optical density variations in the range of 1:10,000 may be 
accommodated. The standard logarithmic scale was developed in the 
Nineteenth Century when it was discovered that an exponential relationship 
existed between the mass of metallic silver in a developed photographic 
negative and the opacity of the negative to light transmission. As the 
silver mass per unit area increased linearly, opacity increased 
exponentially. It was also discovered that a linear relationship existed 
between the opacity of the developed image and the light exposure. Thus, a 
quantitative determination of the mass of silver present in the developed 
image provided a measurement of light exposure, the silver mass being 
logarithmically related to the light exposure. 
Electronic instruments are now used to measure optical densities, in lieu 
of quantitative chemical analyses, but the original scale relationships 
have been maintained. Thus, "opacity" is still defined as the reciprocal 
of the percentage light transmission of a given sample and "optical 
density" is defined as the common logarithm of opacity. Input light 
intensities may be obtained by either light transmission through a given 
sample or light reflecting from a sample. The above definitions apply in 
either situation. 
It is conventional to use photomultiplier tubes in electronic optical 
densitometers for converting input light intensities to electrical 
signals. The amplification factor of a photomultiplier tube is, among 
other factors, related to the voltage applied across the dynode system of 
the photomultiplier tube. The electron flow is increased by the same 
factor at each dynode so that the final amplification factor may be quite 
large. Conventional photomultiplier tubes used in the graphic arts 
industry thus require large voltages for operating the dynode system and 
obtain anode voltages which may be in the neighborhood of 500-1,000 volts. 
In one prior art optical densitometer described in U.S. Pat. No. 3,765,776 
to Bravenec, a resistor-capacitor (RC) network is used to provide an 
exponentially decaying voltage across the dynode system. Thus, the lower 
the input light intensity, the higher the dynode voltage at which a given 
anode current is obtained. This feature is used to trigger a counter, 
initiating a count cycle which terminates when the dynode voltage has 
discharged to a predetermined level. In this system, maximum dynode 
voltage is always supplied across the photomultiplier tube. The resulting 
large voltage swings are detrimental to the photomultiplier tube and to 
the associated circuit components. 
In the prior art, signals generated by the photomultiplier tube in 
combination with an RC circuit are generally compared with internal 
reference signals, to produce outputs which may be used to actuate 
counting apparatus for a time period functionally related to the logarithm 
of the input light intensity. The need to match the photomultiplier tube 
and the RC circuit characteristics with internal references has required 
that photomultiplier tubes for use in optical densitometers having 
operating characteristics within a very narrow range. The production yield 
of tubes having such a narrow range of parameters is quite small and it 
would be very desirable to accommodate a wider range of photomultiplier 
tube characteristics. 
Generally, standard optical density samples are available in the graphic 
arts industry for calibrating optical densitometers. The basic reference 
sample is the "white" sample which produces a scale output reading of 
0.00. That is, the "white" sample produces a relative light intensity of 
1. At the other end of the scale, a "dark" sample produces a relative 
light intensity of 1.times.10.sup.-4, or a relative opacity of 
1.times.10.sup.4. 
According to one aspect of the present invention, the operating parameters 
of the photomultiplier tube are set during an "automatic-zero" cycle where 
light from a "white" optical density sample is input to the 
photomultiplier tube. A reference dynode voltage is obtained, the 
corresponding anode current is determined, and a reference signal is 
derived corresponding to the anode current. The reference signal is then 
retained in the optical densitometer to maintain a constant current during 
a subsequent measurement cycle. In most instances, the reference condition 
is set using a "white" sample having an optical density of 0.00. In this 
region, it is very desirable to provide increased sensitivity in the 
automatic zeroing circuitry to obtain an accurate and stable reference 
anode signal. The disadvantages of the prior art are overcome by the 
present invention, however, and improved methods and apparatus are 
provided for obtaining accurate optical density measurements. 
SUMMARY OF THE INVENTION 
In a preferred embodiment of the present invention, an improved method for 
converting an input light intensity representing an optical density to a 
standard industry scale reading is provided, along with an apparatus for 
measuring optical density. The dynode voltage of a photomultiplier tube is 
controlled to maintain a constant anode current during a measurement cycle 
to obtain a dynode voltage functionally related to an input light 
intensity. The resulting dynode voltage is compared with the voltage from 
a reference exponential characteristic, which may be a decaying RC 
circuit, to obtain a first output. The exponential characteristic signal 
is then compared with a reference signal to obtain a second output. The 
first output may be conveniently used to start a counter and the second 
output may conveniently be used to stop a counter, the counter circuit 
producing a reading according to the industry scale. In one embodiment, 
improved scale resolution is obtained, the scale output reading from 0.000 
to 4.000. 
According to one embodiment of the present invention, an RC circuit 
produces a family of exponential decay curves within a predetermined 
envelope. An exponential characteristic is selected in cooperation with 
the photomultiplier tube to produce a reading of 4.000 when exposed to a 
light intensity from a known 4.000 optical density sample. Any exponential 
decay characteristic within the envelope may selected, all of the 
characteristics having a common crossing point. 
In another aspect of the present invention, exponential characteristics 
representing the envelope boundary are detected and the crossing time is 
detected to obtain the second output. A reliable reference is thus always 
available. 
In yet another aspect of the present invention, the optical densitometer 
sets the photomultiplier tube operating characteristics when exposed to a 
known optical density sample, which may generally be a "white" sample. An 
anode reference current level is detected and an analog reference signal 
is generated corresponding to the anode current. Exponential circuitry is 
provided to increase the sensitivity of the analog reference anode signal 
about the "white" optical density reference sample. The improved 
resolution in setting the photomultiplier tube operating characteristics 
particularly cooperates with the improved relationship between the 
photomultiplier tube and the exponential conversion circuitry to produce 
an optical densitometer having improved scale resolution. 
It is an advantage of the present invention that an increased production 
yield of photomultiplier tubes can be accommodated. 
It is another advantage that large current swings through the 
photomultiplier tube are minimized and system components are not subjected 
to extreme high currents on a continuous basis. 
Still another advantage is improved stability of the optical density output 
signal. 
Yet another advantage is an increased lifetime for the photomultiplier tube 
from the constant anode current operation. 
Still another advantage of the present invention is that only a single 
adjustment is required to select a suitable exponential decay 
characteristic. 
It is a feature of the present invention to derive a reference exponential 
signal from a predetermined envelope of exponential decay curves having a 
common crossing point, the reference exponential signal cooperating with 
the operating characteristics of a preselected photomultiplier tube to 
obtain a plurality of predetermined outputs when the photomultiplier tube 
is exposed to a corresponding plurality of known reference optical 
densities. 
Yet another feature of the present invention is a circuit for generating a 
reference exponential output having a decay characteristic continuously 
variable within a preselected envelope, the boundaries of the envelope 
being selected from the range of photomultiplier tube characteristics to 
be accommodated, and all of the decay characteristics cross at a common 
point. 
One other feature of the present invention is deriving a reference anode 
signal during a "zeroing" cycle for controlling anode current during a 
measurement cycle by deriving adjustments to an analog signal, the 
adjustments being exponentially related to the difference between the 
analog signal and the anode current produced by a known reference optical 
density. 
Yet another feature of the present invention is a circuit for providing an 
analog output signal which is exponentially related to the digital output 
from a counter. 
These and other features and advantages of the present invention will 
become apparent from the following detailed description, whereas reference 
is made to the Figures in the accompanying drawings.

DETAILED DESCRIPTION 
Referring first to FIG. 1, there may be seen a diagram of one embodiment of 
the optical densitometer which is the subject of the present invention. 
Photomultiplier tube 10 serves as the basic input device, and may 
conveniently receive light from direct exposure or from a light conveyance 
means such as a fiber optics probe which detects reflected light from an 
optical density sample to be measured and conveys that light to the 
photomultiplier tube 10. As hereinafter explained, photomultiplier tube 10 
is connected to obtain a constant anode 14 current and the dynode 12 
voltage levels are varied to provide an output signal related to the 
optical density being measured. 
In a first mode of operation for setting photomultiplier tube 10 operating 
parameters, the dynode 12 voltage levels are set to a reference value 
corresponding to a "white" sample. A white optical density sample is 
placed beneath the light conveyance means, switch S4 is closed, which may 
cooperate with a monostable multivibrator 54 to produce control signal 53 
which places switch S3 and switch S2 in the "zero-mode" position for 
adjusting the operating parameters of tube 10. In this condition, switch 
S3 is connected to receive signal 28 which is the output from a first 
comparator 27. Switch S2 connects to receive signal 22 which is a first 
reference voltage V1 and switch S1 is connected to receive the output from 
amplifier 18. 
In the automatic zero-mode condition, the circuit acts to establish 
operating parameters for photomultiplier tube 10, adjusting voltage V3 to 
equal reference voltage V1. Reference voltage VI is selected to maintain 
the dynode 12 collection efficiency over the expected operating range for 
the dynode 12 voltages. It may be seen from FIG. 1 that V3 is derived from 
the dynode 12 voltages and is proportional to the dynode voltages. The 
zener to-first dynode voltage (about 100 V) and the anode-to-last dynode 
voltage (about 62 V), are selected, to maintain dynamic operation in a 
linear range. The photomultiplier tube 10 anode current is then linearly 
related to the dynode 12 voltages for a given light intensity. As 
hereinafter explained, the circuit will automatically zero with respect to 
the white sample to produce voltage V3 equal to the reference voltage V1. 
Accordingly, voltage V3 and voltage V1 are first applied to comparator 1 
which produces an output 28 when V3 and V1 are not equal. Signal 28 is 
connected by switch S3 to amplifier 55 which has capacitor 56 in a feed 
back loop, forming an integrating amplifier circuit. Output signal 58 from 
amplifier 55 then connects to voltage converter 68 which sets the dynode 
12 voltages. Thus, integrating amplifier 55 provides an increasing output 
signal 58 until voltage V3 is equal to V1, thereby maintaining signal 28 
at a zero level. Output signal 58 controls voltage converter 68, which 
provides the high voltage output for the dynodes 12. 
While V3 is maintained equal to V1, as hereinabove discussed, a second set 
of circuits derives an analog signal functionally representing the 
reference anode operating parameters. The anode 14 current produced by the 
reference sample is interconnected with voltage source 46, which provides 
a second reference voltage V2. Voltage V2 may be selected to obtain an 
increasing anode voltage signal 47 as anode 14 current decreases and may 
be selected as ground potential. Resistor R2 is selected to provide a 
convenient voltage level to comparator 48. Thus, a voltage corresponding 
to the reference anode 14 current is presented to second comparator 48. 
Control output 49 from comparator 48 is presented to up/down counter 60, 
which is counting the input pulses from clock 62. A 12 bit digital output 
64 is functionally related to the anode 14 current produced by the 
reference sample. 
Converter 66 output signal 67 is then returned to second comparator 48. The 
up/down counter 60 is controlled by control signal 49 until signal 67 
equals signal 47. Thus, comparator 48 controls counter 60 to establish a 
binary word output from counter 60, which may conveniently be presented in 
12 bits, to represent the anode 14 current of the photomultiplier tube 10 
at the selected "zero" sample light intensity. 
At the completion of the zero-mode operation, the monostable multivibrator 
54, if provided in the circuit of the zero-mode selection switch S4, may 
return to its stable condition, returning switch S2 to conection with 
signal 26 and switch S3 to connection with signal 49, the output signal 
from comparator 48. In addition, binary word output 64 of counter 60 is 
latched to maintain output 67 from converter 66 at a level corresponding 
to the reference anode 14 current. 
The densitometer is now in the measurement mode and a light intensity 
representing the optical density of a sample to be measured is presented 
to photomultiplier tube 10. The new light intensity causes a change in 
anode 14 current. This current change is represented as a change in the 
voltage 47 presented to comparator 48, which is now comparing anode 14 
voltage with reference signal 67, the analog reference anode voltage 
derived during the auto-zero cycle. Comparator 48 produces an output 
signal 49 as anode 14 current deviates from the reference condition. 
Signal 49 is presented to integrator 55 to obtain an integrated output 
signal 58 for controlling voltage convertor 68. Thus, if a dark sample is 
being measured, anode 14 current is trying to decrease, resulting in an 
increased output 58 from integrator 55. Voltage converter 68 acts to 
increase the voltage across dynodes 12, thereby increasing the current of 
photomultiplier tube 10 until anode 14 current is returned to a level to 
produce voltage 47 equal to the reference voltage 67 at comparator 48. 
Thus, it may be seen that the optical density sample light intensity 
manifests itself as an increased current through dynode resistors 13 to 
obtain increased dynode 12 voltages. The increased current is presented at 
the input of amplifier 18 and converted to a proportionate increased 
voltage through resistor R4. Amplifier 18 produces an output signal 20 
which is the actual signal voltge V3. Voltage V3 is functionally related 
to the opacity of the sample and is presented through switch S1 to the 
first comparator 27. Voltage V3 is in a linear relationship with the input 
light intensity to photomultiplier tube 10 and the system must now convert 
this signal to the conventional scale for photographic density 
measurements. As hereinabove explained, the reference measurement system 
must compress the input signal logarithmically to obtain an output signal 
on an optical density scale of 0.000 to 4.000, corresponding to a range of 
relative light intensities from 1 to 1.times.10.sup.-4 (relative opacity 
from 1 to 1.times.10.sup.4). 
It is conventional to obtain this scale conversion using a resistance 
capacitance (RC) circuit to obtain an exponential reference signal 
characteristic for comparing against the unknown signal. As hereinbelow 
explained, a preferred embodiment of the subject densitometer incorporates 
a new RC circuit 24 which provides discharge characteristics continuously 
selectable from within an envelope defined by preselected discharge 
curves. Although the RC circuits are preferred, other electronic circuitry 
having exponential output characteristics may be used. As further 
hereinafter explained, all of the exponential reference signal 
characteristics cross at a common point, which may be conveniently 
selected to approximate the reference voltage V1. 
The function and operation of the dual RC circuit 24 may be better 
understood by reference to FIG. 2. The multiplication factor of a given 
photomultiplier tube 10 is a function of the number of effective dynodes 
12 in photomultiplier tube 10. The effective number of dynodes 12 is 
generally less than the actual number as a result of many variables in 
tube manufacture. The effect of such changes in tube amplification is to 
require that a variable logarithmic base be available to convert the light 
intensity to standard densitometer readings. It can be shown that the 
effect of the variable exponential output characteristics is to provide a 
variable base logarithmic-type function to provide an output to the 
standard industry scale or to other log-base scales, if desired. 
As hereinabove explained for the prior art, photomultiplier tubes were 
selected to obtain a log base which matched the discharge characteristics 
of a single RC circuit. In the present invention, the exponential 
characteristics can be widely varied to accommodate a range of 
multiplication factors which are obtained in a large percentage of a 
production lot of photomultiplier tubes. Accordingly, changing tube 
characteristics due to aging effects can be readily accommodated and 
photomultiplier tubes can be selected without rigid performance 
specifications. 
FIG. 2 generally illustrates the principles hereinabove described. Voltage 
V3 is functionally related to the dynode voltage which is derived to 
maintain the anode current at a predetermined reference level when 
measuring an optical density sample providing light intensity input to a 
photomultiplier tube having a given amplification factor. Voltage V1 is a 
reference voltage, hereinbelow discussed, and corresponds to the voltage 
V3 which would be produced by a white sample, i.e., relative optical 
density of 0.000. Two RC discharge circuits may be provided, RC1 and RC2, 
having relative exponential characteristics as shown in FIG. 2. The upper 
envelope boundary RC1 charges to a voltage V1 and has a decay constant T1. 
The lower envelope boundary RC2 charges to a voltage V2, which is less 
than V1, and has a decay constant T2 greater than T1. 
The exponential decay characterisitcs of RC1 and RC2 are determined by the 
values for (V1, T1) and (V2, T2), respectively. These values are selected 
to accommodate a selected range of amplification factors obtained from an 
acceptable yield of photomultiplier tubes as manufactured. As hereinbelow 
described, a potentiometer interconnecting the two RC circuits permits an 
output exponential characteristic to be selected which falls anywhere 
within the envelope determined by RC1 and RC2. 
Another design constraint for the RC1 and RC2 circuits is that the envelope 
boundary characteristics cross at a level which approximates the reference 
voltage V1. Where a separate voltage source is used to obtain V1, circuit 
components may have to be adjusted to obtain this capability. Alternately, 
the actual crossing point may be measured and used directly for the 
reference voltage. 
The exponential characteristic appropriate for a given photomultiplier tube 
is set initially using industry standard optical density samples. This is 
done by exposing the standard sample to the input probe and adjusting the 
exponential characteristics until the desired output reading is obtained. 
Adjustment need only be done infrequently, since tube characteristics 
change only slowly, or if the photomultiplier tube must be replaced. 
Thus, in operation, the capacitors in the RC circuits are charged to V1 and 
V2, as hereinabove discussed. As the sample measurement cycle begins, 
voltge V3 is compared with the selected exponential characteristic output 
voltage and counter 38 is enabled to begin counting when the two voltages 
become equal. The counter continues to count as the RC circuit voltages 
decay, the selected exponential characteristic voltage now being compared 
with the reference voltage V1. When the reference voltage V1 and the 
exponential characteristic voltage become equal, counter 38 is disenabled 
and the results may be displayed. Thus, it may be seen that the displayed 
results correspond to the measured optical density and converted to a 
standard densitometer scale output. 
Referring now to FIG. 3, there is more particularly depicted a preferred 
embodiment of an actual circuit for the dual RC circuit 24 shown in FIG. 
1. Each RC circuit, RC1 and RC2, operates in a substantially identical 
manner, and the operation of RC1 will be briefly described. A voltage 
source V+ is provided to charge RC1 to V1 during the display clock cycle 
determined by clock 3 (see FIG. 1). Amplifier AR4 acts to enable rapid 
charging of the capacitor during the display cycle. The application of 
control signal 25 from clock 3 (FIG. 1) actuates switch S6 to disconnect 
the circuit from AR4 causing discharge of capacitor C6 through resistor 
R9. The decaying voltage is presented through amplifier AR7 to a 
potentiometer P1 which, in conjunction with the decaying voltage from 
circuit RC2 determines the voltage decay curve presented to amplifier AR8 
as the selected exponential characteristic output signal 26. Only a single 
adjustment, potentiometer P1 is adjusted to obtain the desired exponential 
characteristic, as hereinabove discussed. Resistor R9 in RC1 and resistor 
R10 in RC2 may be selected to obtain the decay curve envelope hereinabove 
discussed. Typical component values for the circuit depicted in FIG. 3 are 
presented in Table 1. 
TABLE 1 
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V+ 11.7 V C7 1.mu.F 
R8 1 M C8 .01 
R9 40.2 K 1% P1 100K 
R11 1 M AR5 LM358 
R12 26.1 K 1% AR7 CA3140S 
R13 10.0 K 1% AR7 CA3140S 
C5 .01 AR8 CA3140S 
C6 1 .mu.F S5 CD4016 
S6 CD4016 
______________________________________ 
Referring now to FIG. 4, there may be seen a logical diagram for circuitry 
to determine a reference voltage V1 from the actual crossing of the two 
exponential characteristic envelope boundary curves. It is desirable to 
use the actual crossing point for improved accuracy since slight drifts in 
circuit parameters occur and some difference may develop between the 
actual crossing point and a fixed V1 reference voltage. A logical signal 
may be obtained when the decay voltages are equal and such a logical 
signal applied to disenable counter 38 at the time of the crossing, which 
may conveniently be done by blocking the input 32 from clock 1 (see FIG. 
1). It is more difficult to integrate the automatic zero feature with this 
feature since an actual voltage output must be derived for comparison with 
input voltage V3. 
FIG. 4 depicts one embodiment of logic circuitry for obtaining an 
internally generated reference voltage V1. In order to obtain the signal 
needed to disenable counter 38, the voltage outputs from RC1 70 and RC2 71 
are separately presented to comparator 73. An output from comparator 73 is 
obtained when the two inputs are equal, i.e., the time the decay curves 
intersect, and this output signal 74 is presented to flip-flop 78. A high 
Q output is obtained which may be applied as signal 28 to flip-flop 30 
(see FIG. 1) to reset the output of flip-flop 30 to a low Q signal 31. As 
shown in FIG. 1, a low Q signal 31 disenables AND gate 34 and interrupts 
the pulse train to counter 38. Low Q output 31 may also be applied through 
NOT gate 81 to reset flip-flop 78 for the next crossing signal. Signal 28 
thus indicates the event of decay curve crossing and not the actual value 
of voltage at the crossing in order to disenable counter 38 and display 
the contents of counter 38, representing the optical density of the 
measured sample. 
For use in an automatic-zero mode, signal 53 from the automatic-zero mode 
switch S4 is provided to cause RC1 70 and RC2 71 to begin a discharge 
cycle. The voltage outputs from each RC circuit are separately presented 
to a voltage follower and latch circuit 76 which determines the voltage at 
the crossing point and displays that voltage as signal 22, which is now 
reference voltage V1 at switch S2 in FIG. 1. The reference voltage V1 thus 
generated is used as described in FIG. 1 to set the reference system 
operating parameters. 
The voltage follower and latch circuit 76 as depicted in FIG. 4 may be 
formed in a variety of ways. One convenient circuit design would be the 
circuit hereinbelow discussed in the description of FIG. 5 for the analog 
reference anode signal generator. Converting the voltage outputs from RC1 
70 and RC2 72 to digital outputs for direct comparison may be easily 
accomplished using conventional integrated circuitry. The digital signal 
at the time of signal equality may be detected and transferred to a 
register for retention and conversion to a suitable analog signal 
representing reference voltage V1 throughout the automatic-zero cycle. 
Referring now to FIG. 5, there may be seen a schematic of an analog 
reference anode signal generator, used to generate the reference anode 
voltage and depicted in FIG. 1 as counter 60, clock 62, and 
digital-to-analog converter 66. An input signal 49 is provided to 
flip-flop 84, signal 49 representing the relative magnitudes of output 67 
and the reference anode 14 voltage, as shown in FIG. 1. Signal 49 will 
result in a high Q output from flip-flop 84 on the occurrence of a clock 
pulse from clock 86. For example, if the anode voltage 47 is greater than 
output 67 from the analog reference anode signal generator, comparator 48 
(FIG. 1) will produce an output signal 49 resulting in a high Q output 
from flip-flop 84 which is applied as control signal 49 to the up/down 
control of counter 88. The presence of a signal 49 may typically cause 
counter 84 to count up. 
Pulses from clock 86 are transferred to counter 88 through AND gate 87 
causing a 12 bit binary word to appear at the output of counter 88. This 
12 bit binary word 90 will move up and down about some mean value as 
analog reference anode signal generator output 67 is continuously compared 
with the anode voltage 47. 
Binary word 90 is then presented to digital-to-analog converter 92 to 
obtain an analog output functionally related to the actual anode current 
produced during the zeroing cycle. In general, the reference condition 
uses a white sample as the reference optical density. To accommodate a 
linear scale factor of 1:1.times.10.sup.4, however, it is very desirable 
to have expanded scale resolution about the reference, or "1", point. 
Accordingly, the output from converter 92 is presented to antilogarithmic 
amplifier 94. Now, output 67 is functionally related to the antilogarithm 
of counter 88 output and the improved "zero" resolution has been obtained. 
Suitable circuit components are shown in TABLE 2. 
TABLE 2 
______________________________________ 
FF84 CD4013 
Counter 88 CD4029 
D/A Converter 92 Burr-Brown DAC-80 
Anti Log.Amp. 94 Analog Devices No. 755 
______________________________________ 
Thus, a closed loop is presented, continuously comparing the analog 
reference anode signal output 67 with the reference anode 14 voltage. A 
steady state condition is never quite achieved, but output 67 will 
oscillate slightly about the desired reference voltage. When clock 86 is 
disenabled, counter 88 retains the last 12 bit binary word as output 
signal 90, thereby latching output signal 67 at the desired analog 
reference anode signal level for use during the measurement portion of the 
instrument cycle. 
As hereinabove explained, the anode current is maintained at a reference 
level throughout the measurement cycle by controlling dynode voltage until 
the anode current returns to its reference value. A photomultiplier tube 
10 requires high voltage to operate, while the various system logic 
components use relatively low DC voltage. Accordingly, low DC control 
voltages must be converted to high voltage for photomultiplier tube 10 
operation, and FIG. 6 depicts a preferred embodiment of a DC voltage 
controlled DC voltage converter 68, as depicted in FIG. 1. 
Basically, the voltage converter circuit 68 is composed of a clock 110 
which triggers flip-flop 112 to drive a switching circuit 114. Switching 
circuit 114 controls transistors Q116 and Q117 to drive transformer 120. 
Thus, an oscillating voltage is produced across the primary coils of 
transformer 120. The magnitude of this oscillating voltage is determined 
by the input of transformer 120. Thus, the primary voltage swing, and 
hence the secondary voltage swing, is determined by input control signal 
58. 
In one embodiment, the primary-to-secondary turns ratio of transformer 120 
is 520/14,000, whereby a high voltage output is produced. The secondary of 
transformer 120 is connected to a conventional voltage doubler and filter 
circuit comprised of diodes D130 and D131, and capacitors C132 and C133. 
Thus, a DC output voltage is obtained at a level sufficient to operate 
photomultiplier tube 10 and controlled by low level DC voltage 58. 
Typically, output voltages in the range of 360-1400 VDC may be controlled 
by voltages in the range 1-14 VDC. Typical circuit components are listed 
in TABLE 3. 
TABLE 3 
______________________________________ 
FF112 CD4013 C126 2.2/20.mu.F 
SW 114 CD4016 C132 .01.mu.F/1KV 
Q116 2N1711 C133 .01.mu.F/1KV 
Q117 2N1711 T120 520/14,000 
Q121 2N1711 L125 27.mu.H 
C119 .01.mu.F R128 10K 
C122 .01.mu.F D130 MR250-2 
C123 2.2/20.mu.F 
D131 MR250-2 
______________________________________ 
Referring again to FIG. 1, it may be seen that decoder/driver 38 presents 
the outpout to a resolution of 0.001. Prior art optical densitometers have 
heretofore resolved the measurement to only 0.01. Improved resolution is 
provided according to one embodiment of the present invention because of 
the combination of the selectable exponential characteristic and the high 
resolution from the automatic zero setting circuit. The automatic zero 
circuit resolution obtains a stable and relatively insensitive analog 
reference anode signal for use during subsequent sample measurements. 
Then, the matching capability provided by the variable exponential 
characteristic obtains increased accuracy using the 0.001 resolution at 
the output. 
It is therefore apparent that the present invention is one well adapted to 
attain all of the features and advantages hereinabove set forth, together 
with other advantages which will become obvious and inherent from a 
description of the preferred embodiment. It will be understood that 
certain combinations and subcombinations are of utility and may be 
employed without reference to other features and subcombinations. This is 
contemplated by and is within the scope of the present invention.