Optical mark sense detector

Apparatus for detecting marks or other objects by comparing a decrease in reflected light while the mark or object is moving across optics, to the light reflected before the mark or object was encountered by the optics. A threshold reference voltage level is derived directly from the conductance level of a phototransistor prior to the introduction of the mark or object to be detected. This reference level is developed in such a manner as to cause it to be a dynamic voltage, varying percentage-wise as the conductance of the photo-transistor. The dynamic reference level so determined is thus always directly a portion of the level present before the mark or object to be sensed was introduced.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to optical mark detecting systems and more 
particularly to such systems as are used to sense pencil marks or objects 
on a document. 
2. Description of the Prior Art 
Optical mark sense detection systems are well-known in which data in the 
form of pencil marks on documents are recognized by a decrease in 
transmitted or reflected light when such a mark is encountered. In a 
typical prior art system, the method used to detect pencil marks on a 
document might be described as follows. Light from a high intensity source 
may be directed to the document by means of fiber optics. Light reflected 
from the document, such as a white or near-white paper material, may be 
received by other fiber optics and directed to the light-sensitive area of 
a photo-transistor circuit. A load resistor in that circuit will develop a 
voltage as a result of the conduction of the photo-transistor, which 
voltage is a function of the amount of light reflected from the document. 
If now a pencil mark is encountered by the optics, there will be a 
decrease in reflected light due to the decreased reflectance of the mark 
from the plain unmarked document. That is to say, a pencil mark does not 
reflect light as well as a white or near-white unmarked document. In these 
prior art devices a voltage threshold was set on a comparator and if, upon 
comparison, the voltage developed by the photo-transistor decreased below 
that threshold, the change in the comparator's output would be an 
indication of a sensed mark. Thus, light directed to a sensor causes the 
circuit to conduct current as a function of the amount of light received 
by the sensor. A reference or threshold level is applied to one input of a 
comparator while the conduction level of the circuit is applied to its 
other input. When the circuit conduction level is high, the output of the 
comparator will indicate no mark or object is present. When an object or 
mark passes the sensor and decreases the circuit conduction to a point 
equal to or lower than the reference or threshold level, the output of the 
comparator will change, thus indicating that a mark or object was 
detected. 
One popular prior art system employs a fixed reference or threshold level. 
If, for example, this reference level is set at 6 volts when conduction in 
the circuit drops from, say, 10 volts to 6 volts, the system will indicate 
that a mark or object has been detected. Thus a 40% decrease in conduction 
has been necessary to cause this system to indicate detection of a mark or 
object. 
Another prior art system has employed a fixed amount of voltage drop from 
an original voltage level. If, for example, this system has been set to 
detect a 4 volts drop from an original conduction level of again, say, 10 
volts, the reference will again be set to 10 volts minus 4 volts= 6 volts, 
thus again, a 40% decrease in conduction is required to cause this system 
to indicate detection of a mark or object. 
It is interesting and instructive to examine both of these prior art 
systems under new conditions commonly encountered in the art so as to 
point up deficiencies and areas in need of improvement in such prior art 
systems. In the interest of specificity, assume that the intensity of the 
light source has decreased from any causes whatsoever such as, for 
example, decrease in supply voltage, aging, or the like. Assume further 
that this decrease in light intensity results in a decrease in conduction 
with no mark or object present from 10 volts down to, say, 8 volts. 
Our first prior art system will still have the fixed reference or threshold 
voltage of 6 volts. It now requires, however, only a 2 volt drop in 
conduction to cause that system to indicate detection of a mark or object. 
This is only a 25% decrease in conduction. This increase in sensitivity 
may be enough to cause the system to indicate the presence of a mark or 
object when, in fact, none is present. 
In the other prior art system considered, in which a specified voltage drop 
is required, we still require a 4 volt drop for the system to indicate 
detection of a mark or object. This requirement is now for a 50% drop in 
conduction from the 8 volts conduction with no mark or object present. 
This decrease in sensitivity may be intolerable in the system thus 
described. 
Some of these prior art devices have been described in the below listed 
patents that were brought to the attention of applicant's attorney. It is 
hoped that this information may be of some help to the Examiner: 
1. U.S. Pat. No. 3,600,558 -- "Coded Card and Reading Means" -- Grosbard 
2. U.S. Pat. No. 3,639,729 -- "Data Reading Apparatus" --Marshall 
3. U.S. Pat. No. 3,639,730 -- "Optical Reader System" -- Higginbotham et 
al. 
4. U.S. Pat. No. 3,820,068 -- "Background Reference Level System and Method 
for Document Scanners" -- McMillin 
5. U.S. Pat. No. 3,870,865 --"Method and Apparatus for Optical Reading of 
Recorded Data" -- Schneiderhan et al. 
6. U.S. Pat. No. 3,894,216 -- "Illumination and Sensor Arrangement for Card 
Reader" -- Bottles 
7. U.S. Pat. No. 3,896,294 -- "Plural Mode Card Reading Apparatus" -- 
Schisselbauer et al. 
8. U.S. Pat. No. 3,904,110 -- "Large Mark Tolerance Card Reader" -- Bottles 
It would thus be a great advantage to the art to provide a system in which 
the reference threshold level is dynamically varied so as to maintain a 
constant sensitivity under some adverse conditions commonly encountered in 
such optical mark sense detection systems. 
Another great advantage to the art would be the provision of a method of 
optical detecting by an optical system wherein the reference level is 
always directly a portion of the level before the object or mark to be 
sensed was present. 
A further desirable advantage to the art would be the provision of a system 
in which the conduction level in the absence of marks or objects is 
constantly monitored and stored in a manner by which the stored level 
increases immediately with any increase in conduction but decreases very 
slowly when conduction decreases. 
Yet another advantage to the art would be a system wherein a preset portion 
or percentage of the stored level of conduction is used as a reference or 
threshold to compare to a sensed level of conduction that is not being 
stored. 
SUMMARY OF THE INVENTION 
It is thus an object of the present invention to provide a system in which 
the reference threshold level is dynamically varied so as to maintain a 
constant sensitivity under adverse conditions commonly encountered in 
optical systems of mark sense detection devices. 
It is still another object of the present invention to provide a method of 
optical detection wherein a reference level is always directly a portion 
of a level set before the object or mark to be sensed was present. 
An additional object of the present invention is the provision of a system 
in which the conduction level in the absence of marks or objects is 
constantly monitored and stored in a manner by which the stored level 
increases immediately with any increase in conduction, but decreases very 
slowly when conduction decreases. 
A still further object of the present invention is the provision of a 
system wherein a preset portion or percentage of the stored level of 
conduction may be used as a reference or threshold to compare to a sensed 
level of conduction that is not being stored. 
In the accomplishment of these and other objects, an optical mark sense 
detector is provided in which a conduction level is constantly monitored 
and is stored in a manner by which the stored level increases immediately 
with any increase in conduction but decreases very slowly with any actual 
conduction decreases. This means that during times of decreased conduction 
due to passing by the sensor of objects or marks to be detected, the 
stored level of conduction will remain at whatever level it was just prior 
to that period of time. A preset portion or percentage of the stored level 
of conduction is thus used as a reference or threshold to compare to the 
actual level of conduction that is not being stored. The reference or 
threshold level is applied to one input of a comparator while the instant 
conduction level is applied to the other input of the comparator. When the 
instant conduction level is high, the output of the comparator will 
indicate no object or mark is present. When an object or mark passes the 
sensor and decreases the actual conduction to a point equal to or lower 
than the threshold reference, the output of the comparator will change to 
indicate that a mark or object has been detected. The methods of the 
invention provide the ability to dynamically vary the threshold level so 
that the same percent of decrease in conduction is always required in 
order to indicate detection of a mark or object.

DETAILED DESCRIPTION 
Although specific embodiment of the invention will now be described with 
reference to the drawings, it should be understood that such embodiments 
are by way of example only and merely illustrative of but a small number 
of the many possible specific embodiments which can represent applications 
of the principles of the invention. Various changes and modifications 
obvious to one skilled in the art to which the invention pertains are 
deemed to be within the spirit, scope, and contemplation of the invention 
as further defined in the appended claims. 
Referring to FIG. 1 with greater particularity, there is shown a prior art 
device as has been described in U.S. Pat. No. 3,820,068 to McMillin of 
June 25, 1974. 
Three channels of an N-channel matrix are shown in which each channel is 
identical to the others. Our description will thus be limited to one 
channel thereof. Light incident upon photo-transistor 10 causes conduction 
therein. The output from transistor 10 causes a voltage to be developed 
across potentiometer 14 and thence to operational amplifier 12. It is a 
feature of this prior art device that amplifiers 12 are the linear gain 
type and the potentiometer 14 in each channel is adjusted so that the 
input to each amplifier 12 is the same when the photo-transistors 10 sense 
the same amount of light intensity. Light detected by photo-transistor 10 
causes the output of linear amplifier 12 to go negative to a voltage level 
which is a function of the amount of reflectance from a document. A mark 
may reduce the light intensity to a point which will cause 
photo-transistor 10 to conduct at a level allowing the output of amplifier 
12 to go positive. Signal output from amplifiers 12 is fed to resistors 18 
and to mark detection amplifiers 16. Resistors 18 average the outputs of 
all channels and are fed to summing resistor 20. During a sampling period 
a holding capacitor 24 is charged to the average of the sum of the outputs 
of amplifiers 12. At the appropriate time this level is fed to sample and 
hold amplifier 30 whose output appears at one input of all mark detection 
amplifiers 16. If all channels have been properly tuned, a mark of a given 
density incident at one channel will produce the same output signal as a 
mark of the same density detected by the photo-transistor 10 in any other 
channel. It is emphasized here that in this prior art device the outputs 
of the channels are averaged to arrive at a background level and this 
background level is then used as a reference level in the system. 
Immediately following the algebraic combination of the reflectance level 
signals for all channels into an average reflectance level signal, each 
channel is then compared with this average reflectance level signal. This 
prior art system works very well but still leaves room for improvement 
such as contemplated by the present invention. 
Referring now to FIG. 2, the essential concepts of the invention will be 
more fully described. When a document to be evaluated enters under the 
primary sensing optics in a head receiver board, reflected electromagnetic 
radiation therefrom causes photo-transistor 9 to conduct and develop a 
voltage across transistor load resistor 15 connected to its emitter. There 
will thus be developed a positive voltage at the top end of transistor 
load resistor 15 which will be applied at junction 27 by means of a lead 
41. This developed positive voltage will be applied to storage capacitor 
19 through diode 17, thus charging capacitor 19 at junction 38 to the 
voltage appearing at junction 27 minus the diode drop of diode 17 (0.6 
volts approximately). This voltage at junction 38 will be the 
representative "stored white card" voltage of this channel. This "stored 
white card" voltage is thus representative of signal output that is a 
function of incident electromagnetic radiation upon a background field in 
the absence of marks or other objects to be detected. Whenever a mark or 
object is encountered, the photo-transistor conduction will decrease, 
however, the storage capacitor 19 will not discharge because of the 
blocking action of diode 17. This "stored white card" voltage will be an 
input signal for follower 11 which has been chosen for its high input 
impedance, so as not to discharge capacitor 19 by any noticeable amount 
during the time that the conduction of the photo-transistor is decreased 
by reason of its encountering a mark or object. Input load resistor 21 has 
been chosen large enough so as to have negligibly small discharging effect 
on capacitor 19 while the sensor transistor 9 is under the influence of a 
mark or object, however, resistor 21 will act as a slight load on the 
input of follower 11 and will permit discharge of capacitor 19 after 
removal of the card or document thus stopping photo-transistor conduction. 
Due to the feedback arrangement accomplished through lead 42, the output 
signal from follower 11 will track its input signal, thus permitting a 
load to be driven without affecting the input. This output signal will 
appear at junction 31 and be fed back to the other input of follower 11 by 
means of the lead 42 thus locking the output signal to the input. This 
signal is also applied to a threshold adjusting potentiometer 32 which, 
for the moment, we will assume to have its lower (CCW) end grounded 
instead of connected to junction 43. If the wiper 33 of the potentiometer 
32 is set at the midpoint thereof, the wiper voltage would be half of the 
"stored white card" voltage and would be applied to the inverting input of 
comparator 13 by means of lead 35. The voltage applied to the 
non-inverting input of comparator 13 by means of lead 44 from junction 27 
will be whatever voltage has been developed by photo-transistor 9 across 
transistor load resistor 15. At this point, assuming no mark has been 
encountered, this would be the original white card voltage as developed 
before the diode drop of diode 17. There will thus be developed a positive 
output from comparator 13 indicating no data encountered. If now a mark is 
encountered, the conduction of photo-transistor 9 will decrease causing a 
decrease in the voltage applied to the non-inverting input of comparator 
13. If the conduction of photo-transistor 9 decreases to the point that 
the voltage developed at junction 27 is less than that appearing on 
potentiometer wiper 33, then the voltage as applied by way of lead 44 from 
junction 27 to the non-inverting input of comparator 13 when compared 
therein with the voltage applied by way of lead 35 from wiper 33 to the 
inverting input of comparator 13 will cause the output of comparator 13 to 
swing negative indicating detection of data. Before proceeding to a 
specific example, it is to be noted that the connection of the CCW end of 
potentiometer 32 has been shown connected to junction 43 of resistor 25 
and compensating diode 23 in order to compensate for the diode drop caused 
by diode 17. This will be more fully explained in the specific example 
explored below. 
Still referring to FIG. 2, let us assume a current of 0.5 milliamperes 
conduction when a document to be evaluated enters the sensing area thus 
causing that conduction through photo-transistor 9. If transistor load 
resistor 15 has a value of 10,000 ohms, a voltage of +5 volts will be 
created across it. This +5 volts will be effective to charge storage 
capacitor 19, which we will assume to have a value of 0.022 microfarads in 
our example, through blocking diode 17. Since 0.022 microfarads is a very 
small capacitance, it will be fully charged very quickly. An estimate of 
this charging time may be derived as follows. 
Charging time as mentioned is not the time needed to charge the capacitor 
as limited by an RC time constant. It is, rather, the observed rise time 
of the charging voltage resulting from the conduction of photo-transistor 
9 across transistor load resistor 15. If we multiply the impedance of 
blocking diode 17 by the capacitance of storage capacitor 19, we would 
obtain a very low time constant. For example, a quite high estimate of the 
impedance of blocking diode 17 would be 1000 ohms. One time constant, 
therefore, is: 
EQU 1.times. 10.sup.3 ohms.times. 2.2.times. 10.sup.-8 farads= 2.2.times. 
10.sup.-5 seconds 
One time constant will allow a capacitor to charge to within 63.2% of its 
total capacity and it is generally assumed that a capacitor will be fully 
charged in five time constants. Thus: 
EQU 2.2.times. 10.sup.-5 seconds.times. 5= 1.1.times. 10.sup.-4 = 0.11.times. 
10.sup.-3, 
that is, 0.11 millisecond, and we have the result that the storage 
capacitor 19 has the capability of charging up fully in 0.11 millisecond 
or less. The charging voltage from the photo-transistor 9, however, may 
take up to 2 milliseconds to rise to the full "actual white card" voltage 
after the leading edge of the document moves under the read head of the 
sensing optics. It has been determined that this 2 milliseconds is a 
worst-case situation. A reader selected for its fast rate, used in testing 
the operation of the invention causes a document to move past the read 
head such that the first data mark box will reach the read head 3 
milliseconds after the leading edge of the document. Therefore, we 
conclude that the limiting factor in charging the storage capacitor 19 is 
the rise time of the source voltage developed across transistor load 
resistor 15 as a result of conduction of photo-transistor 9, and not the 
RC time constant. That is, the storage capacitor 19 will charge as rapidly 
as the voltage at the emitter of photo-transistor 9 rises. These times and 
reactions thereto have been verified as observed by means of a dual trace 
oscilloscope with storage capabilities. Thus, the capacitor charging time 
is not a limiting factor and the reader will be ready to sense data well 
within the required 3 milliseconds. 
The storage capacitor 19 will not, however, charge up to the full +5 volts 
due to the diode drop attributable to blocking diode 17 of about 0.6 
volts. The charge appearing on storage capacitor 19 will be about +5 volts 
minus 0.6 volts= +4.4 volts. This +4.4 volts will be our representative 
"stored white card" voltage. As a mark or object is encountered, the 
current in photo-transistor 9 will decrease due to the decrease in 
electromagnetic radiation or light reflected to its sensitive surface. The 
resultant drop in voltage across transistor load resistor 15 will not 
cause 0.022 microfarads storage capacitor 19 to discharge due to the 
blocking action of diode 17. This "stored white card" voltage of +4.4 
volts will be sensed by signal follower 11 which, because of its very high 
input impedance, will not discharge storage capacitor 19 by any measurable 
amount during the time that conduction of photo-transistor 9 is decreased 
due to crossing a mark on the document or card. Input load resistor 21, 
say about 1 megohm, has been chosen large enough so as not to have any 
discharging effect on storage capacitor 19 due to crossing a mark, 
however, it will act as a slight load on the input of signal follower 11 
and will discharge the 0.022 microfarads storage capacitor 19 after a 
document or card has left the sensing head and photo-transistor conduction 
has stopped for a relatively long period of time. Since the output of 
signal follower 11 will track its input, a load may be driven by means of 
this output without affecting the input thereof. Thus, the output of 
signal follower 11 is the same as the charge on storage capacitor 19. In a 
representative test system, it was determined that a conventional 80 
column card takes about 90 milliseconds to pass under the read head and 
that the first data box is encountered about 3 milliseconds after the card 
enters under the head. Thus, as has been shown, storage capacitor 19 will 
be fully charged to the "stored white card" voltage well before the first 
data box is encountered. An observance of the output of signal follower 11 
illustrates that after the card leaves the read head, it requires about 15 
milliseconds for storage capacitor 19 to become halfway discharged. This 
is further verified by a consideration of the RC time constant involved in 
the circuit consisting of 0.022 microfarads storage capacitor 19 and 1 
megohm input load resistor 21. 
EQU 1.times. 10.sup.6 ohms.times. 2.2.times. 10.sup.-8 farads= 2.2.times. 
10.sup.-2 = 22.times. 10.sup.-3 sec. 
which is 22 milliseconds. Thus, 22 milliseconds is the time constant in 
question. Reasoning that if one time constant of 22 milliseconds permits 
discharge of the capacitor by 63.2%, then a 50% discharge should occur in 
about 15 milliseconds as was observed. The discharge is thus slow enough 
so that the decrease in photo-transistor conduction due to crossing marks 
will not cause a measurable discharge in a 0.022 microfarads capacitor. As 
a result, the "stored white card" voltage, as measured at the output of 
signal follower 11, will be steady. It is also to be noted that 
representative card speed is such that a typical mark on these 
conventional cards is about 0.5 milliseconds or less in width. 
Continuing with the example, at the point of the output of signal follower 
11 at +4.4 volts for "stored white card" level, it is instructive to 
illustrate detection of data. If the assumption is also made that the CCW 
end of threshold adjusting potentiometer 32 is grounded, then, if the 
wiper 33 is set at midpoint, the wiper voltage of half the "stored white 
card" voltage, or +2.2 volts, will be applied by means of lead 35 to the 
inverting input of comparator 13. The voltage applied to the non-inverting 
input of comparator 13 will be the voltage appearing at junction 27 or +5 
volts as applied by means of lead 44. Thus, there will be a positive 
output on lead 45 from comparator 13 indicating no data encountered by 
this channel in the head receiver board. If now the conduction of 
photo-transistor 9 decreases as it crosses a mark, the voltage seen on the 
non-inverting input of comparator 13 from junction 27 by means of lead 44 
will also decrease as the mark is encountered, however, the voltage on the 
inverting input will remain steady at +2.2 volts. If the conduction of 
photo-transistor 9 through transistor load resistor 15 decreases to the 
point that less than +2.2 volts is present on the non-inverting input of 
comparator 13, the output will swing negative indicating detection of 
data. About 0.5 milliseconds later we will have passed over the mark and 
the voltage seen at the non-inverting input of comparator 13 will return 
to greater than +2.2 volts and cause the output on lead 45 to swing back 
positive, indicating "white card output" condition again. 
During the very brief period of time of about 0.5 milliseconds or less, 
that the photo-transistor conduction decreased due to encountering the 
mark, the voltage on the inverting input of comparator 13 remained at a 
steady +2.2 volts. In our example, we detected data when the "actual white 
card" voltage dropped from +5 volts to about +2.2 volts, a drop of 2.8 
volts or, in other words, there was a 56% decrease in conduction due to 
the decreased reflectivity of the mark. 
EQU (5-2.2)/5.times. 100= 56% 
If sometime later, because of head wear, difference in lamp intensity or 
other degradation of performance, this same photo-transistor only conducts 
0.4 milliamperes under "white card" condition, the output of signal 
follower 11 would be: 
EQU 0.4.times. 10.sup.-3 .times. 10.times. 10.sup.3 = +4 volts- 0.6 volts= +3.4 
volts 
and the voltage at wiper 33 of threshold adjusting potentiometer 32 will 
be: 
EQU 3.4/2= +1.7 volts 
With +1.7 volts on the inverting input to comparator 13 and +4 volts on the 
non-inverting input, the photo-transistor 9 will have to decrease in 
conduction by 57.5%, that is, 
EQU (4-1.7/4.times. 100.times. 57.5% 
in order to force the junction 27 to drop to +1.7 volts to be applied by 
means of lead 44 to the non-inverting input of comparator 13 and thus 
indicate detection of data. It would appear that now, a slightly darker 
mark would be required since more of a decrease in conduction was 
necessary for data to be detected. This apparent infirmity has been 
overcome by attaching the CCW end of threshold adjusting potentiometer to 
a +0.6 volt reference line. This reference is derived, as shown, by 
attaching a +5 volt source through resistor 25 and compensating diode 23 
to ground thus placing the top of compensating diode 23 at junction 43 at 
+0.6 volts above ground. If now the wiper 33 of threshold adjusting 
potentiometer 32 is set at midpoint, it will equal one-half of the "actual 
white card" voltage generated across transistor load resistor 15. We have 
seen that the voltage at the output of signal follower 11 applied to the 
CW end of threshold adjusting potentiometer 32 will be 0.6 volts below the 
"actual white card" voltage as seen at junction 27 and applied to the 
non-inverting input of comparator 13 because of the diode drop of blocking 
diode 17. By raising the CCW end of the potentiometer 32 by +0.6 volts, we 
nullify the 0.6 volt drop caused by diode 17. In this configuration of the 
circuit, photo-transistor conduction can drop under "actual white card" 
condition without causing any effect on the percentage of decrease needed 
to indicate detection of data. That, the CW end of the potentiometer will 
be at +4.4 volts while the CCW end will be at +0.6 volts, showing a total 
drop of: 
EQU 4.4- 0.6= 3.8 volts 
Now, 
EQU 3.8/2= 1.9, 
half that drop, and thus the voltage at the wiper 33 will be: 
EQU 4.4- 1.9= 2.5 
which is half of the +5 volts appearing at junction 27. 
With this configuration under the decreased conduction conditions 
considered in which the photo-transistor only conducts 0.4 milliamperes 
under "white card" conditions, the output of signal follower 11 was seen 
to be: 
EQU 0.4.times. 10.sup.-3 .times. 10.times. 10.sup.3 = +4 volts- 0.6 volts= 3.4 
volts 
but now the voltage at the wiper 33 of the potentiometer 32 will be 
determined as: 
EQU (3.4-0.6)/2= 1.4, 
half of the drop across the potentiometer, and thus the voltage at the 
wiper will be: 
EQU 3.4- 1.4= 2 
which is half of the +4 volts appearing at junction 27, that is, 50% of 
"actual white card" voltage. 
To consider an example at the other extreme, assume the photo-transistor 
conducted 1 milliampere under "white card" conditions, thus developing 10 
volts across 10,000 ohm resistor 15. The CW end of threshold adjusting 
potentiometer 32 will now have +9.4 volts and the CCW end will have +0.6 
volts. If the wiper 33 is again set at midpoint, we have: 
9.4- 0.6 = 8.8 volts across potentiometer 32, 
8.8/2= 4.4, that is, half that voltage, and 
9.4- 4.4= 5 volts at wiper 33, 
that is, half of the "actual white card" voltage appearing at junction 27; 
still half the "stored white card" voltage. 
In the present configuration, even if photo-transistor conduction changes 
the "stored white card" reference, the wiper 33 of threshold adjusting 
potentiometer 32 will remain at the same percentage of "actual white card" 
voltage. With wiper 33 set at the midpoint of potentiometer 32, an 
indication of data detection will be realized every time a mark is 
encountered which decreases conduction from "actual white card" conditions 
by 50%. If lighter marks are required to be detected, wiper 33 should be 
moved toward the CW end of potentiometer 32 so that less percentage of 
decrease in conduction will be required for comparator 13 in order to 
indicate detection of data. If darker marks only are to be detected, wiper 
33 should be set closer to the CCW end so that a greater decrease in 
conduction will be necessary before the voltage on the non-inverting input 
of comparator 13 drops below the voltage on the inverting input as set by 
potentiometer 32. 
Diode 23 and resistor 25, nominally about 220 ohms, also have an additional 
important function. As the card leaves the read head and photo-transistor 
conduction drops to zero, the output of comparator 13 will go negative, 
indicating a black condition. This results because the voltage on the 
inverting input of comparator 13 still remains. Since photo-transistor 
conduction has ceased and therefore, there is no voltage by which the 
storage capacitor 19 may be charged, it will discharge through the 
resistor 21 and the output of signal follower 11 will also drop toward 
zero volts as capacitor 19 loses its charge. However, the CCW end of 
threshold adjusting potentiometer 32 has +0.6 volts on it. There will thus 
remain about +0.3 volts on the wiper 33 if it is still set at midpoint of 
potentiometer 32. The inverting input of comparator 13 will be positive by 
that amount greater than its non-inverting input and therefore its output 
will remain in the "black" state, or negative, when no card is under the 
read head. 
It has been necessary to refine the circuit still further in order to 
compensate for the range of conduction of photo-transistors. A numerical 
example may be in order so as to explain this further refinement. It has 
been found that a very badly worn head may cause a photo-transistor to 
conduct only about 0.2 milliamperes under "white card" conditions. No 
channel has yet been encountered in practice in which the photo-transistor 
conducted less than 0.2 milliamperes. A conduction of 0.2 milliamperes 
will derive a voltage of 2 volts across a 10,000 ohm load resistor. It has 
been found that this is a sufficient voltage with which to work. The 
problem is found at the other end of the spectrum. That is, a problem 
presents itself upon the use of a 10,000 ohm load resistor on the emitter 
of a photo-transistor when the circuit is used in a new reader with very 
good photo-transistor conduction. Some of these new channels may conduct 
as much as 1.8 milliamperes under "white card" conditions. Such a result 
requires 18 volts to be dropped across the 10,000 ohm emitter resistor and 
this is impossible with a +12 volt supply. The photo-transistor in such a 
situation is in saturation and can thus pass over a mark without even 
decreasing the voltage on the 10,000 ohm load resistor. If it is attempted 
to use a 5,000 ohm load resistor to thus generate +9 volts when 1.8 
milliamperes is conducted, the transistor is not saturated under "white 
card" condition on new readers and the solution is workable under these 
conditions. However, if the 5,000 ohm load resistor were to be used with a 
very worn head that only caused 0.2 milliamperes conduction, only 1 volt 
would be generated to operate the circuit and that voltage is too low to 
be reliable in operating the circuit. 
Referring now to FIG. 3, a solution to the problem posed in the above 
paragraph is illustrated. Instead of utilizing a +12 volt-to-ground power 
supply, a +12 volt to -12 volt source has been shown. A 3.3 volt Zener 
diode 37 creates a reference voltage at -8.7 volts on lead 46 and the top 
of diode 23 is now shown at -8.1 volts on lead 47 while resistor 25 is 
connected to ground instead of to a +5 volt source. With this circuit 
configuration, the photo-transistor 9 will not be in saturation when 
conducting 1.8 milliamperes into a 10,000 ohm load for an 18 volt drop. 
The lower end of transistor load resistor 15 is held at -8.7 volts by the 
supply voltage Zener, therefore, 
EQU -8.7+ 18= 9.3 volts 
is the voltage that will be developed at the other end of resistor 15 and 
is the voltage at the emitter of photo-transistor 9 under these 
conditions. 
For the remainder of our discussion, all voltage measurements will be 
considered as made in respect to the -8.7 volt line, which shall be 
referred to as "common." This line has been denoted by the numeral 46. As 
a warning, in making measurements on a circuit connected in this 
configuration, if the common lead of an oscilloscope is to be placed on 
this -8.7 volt line, first it should be verified that the instrument is 
floating and not grounded to equipment ground. The 3.3 volt Zener diode 37 
has been used to place the common line 46, 3.3 volts above the -12 volt 
supply in order that the operational amplifiers 11 and 13 do not ever have 
input voltages or output voltages that meet the negative power supply 
voltage. If the circuit was referenced to -12 volts as common, the output 
of signal follower 11 would be required to come very close to the negative 
12 volt supply when the 0.022 capacitor 19 was completely discharged, thus 
causing saturation of the output of the circuit. Additionally, it has been 
found that operational amplifiers may have a tendency to give false 
outputs when their inputs are brought this close to supply voltages. By 
referencing the circuit to -8.7 volts, neither the positive nor negative 
supply voltages is approached. 
In its present configuration the circuit now sees a larger voltage range 
over which to operate. For all intents and purposes, the supply voltages 
are 20.7 volts (8.7+ 12) instead of 12. The photo-transistors 9 will not 
be harmed as these components are operational for up to 40 volts. 
A second major problem was encountered with respect to the 0.6 volts 
applied to the CCW end of the threshold adjusting potentiometer 32. That 
voltage was employed in order to keep the wiper of potentiometer 32 at a 
desired percentage of "actual white card" voltage as that voltage changed. 
In theory, diode 23 compensated for the fact that the output of signal 
follower 11 was 0.6 volts lower than "actual white card" voltage because 
of the 0.6 volt diode drop across diode 17. In actual operation, the 
voltage drop across diode 17 is not 0.6 volts, but is only about 0.2 
volts. Diode 17 is a silicon diode which is usually considered to have a 
0.6 volt junction voltage drop but such a condition obtains only when 
sufficient current flows. Examination of the voltage versus current plot 
of FIG. 6 illustrates this point. Only when the current reaches a certain 
level is it great enough to reach what is called the "knee" of the curve 
and thus cause the voltage across it to remain at approximately 0.6 volts. 
From this point, as current increases, only a slight increase in voltage 
is noted due to the internal forward resistance of the diode. However, in 
our application, we are passing very little current through the diode. The 
only loading factors on that diode are the input impedance of signal 
follower 11 which, as we have said, is very high, the leakage of the 0.022 
microfarads capacitor 19, which is very low and therefore offers a very 
high impedance, and the 1 megohm load resistor 21. This 1 megohm load 
resistor 21 draws most of the current from diode 17 but is still so small 
that the diode only drops the voltage about 0.2 volts. This has been 
verified by comparing the "actual white card" voltage on the emitter of 
photo-transistor 9 to the voltage on the output of signal follower 11. The 
voltage on the output of signal follower 11 has been observed to be 0.2 
volts lower rather than the 0.6 volts as had been previously described. 
This result indicates a lack of linearity in the circuit since white card 
voltages will change unless we now establish the CCW end of potentiometer 
32 at 0.2 volts instead of 0.6 volts. 
In our test circuit, as shown in FIG. 4, a 100 ohm diode compensating 
potentiometer 34 was connected to the -8.1 volts junction between the 
resistor 25 and diode 23 and the -8.7 volts common line of our circuit. 
This potentiometer 34 can then be adjusted so that its wiper voltage is 
about 0.2 volts above the -8.7 volts. Once the required resistances are 
determined, potentiometer 34 may be replaced by fixed resistors since once 
set it will not require changing. Since there now appears only 0.2 volts 
with respect to -8.7 common at the CCW end of threshold adjustment 
potentiometer 32 when a card or document leaves the read head and the 
0.022 microfarads capacitor 19 discharges thus making the output of signal 
follower 11 and the CW end of potentiometer 32 at zero volts, there will 
only be about 0.1 volt on wiper 33 to be applied by means of lead 35 to 
the inverting input of comparator 13. A practical operational amplifier 
may have more than 0.1 volt input offset voltage and, under that 
condition, the output of comparator 13 will not be negative as it should 
be but rather, it will be positive. This condition may be further 
aggravated by the failure of the photo-transistor to drop to zero 
milliamperes conduction. There may be a slight residual conduction due to 
internal leakage or some conduction may remain due to light being 
reflected from card dust lying under the head or even from a slight 
scratch at the end of the optics. In any case, very little conduction of 
the photo-transistor would be needed to apply the slightly over 0.1 volt 
on the non-inverting input of comparator 13 so as to result in an 
erroneous output. 
Referring now to FIG. 5, it will be noted that signal follower 11 has been 
biased up so that it will not drop below 1.8 volts on its output. It was 
determined that no channel gets lower than 0.18 milliamperes conduction 
under white card conditions. If it should, by that time heads are long 
overdue for replacement. It was thus decided to bias up signal follower 11 
so that it never drops below 1.8 volts on its output as shown. In this 
configuration under the black, or no card, condition, the output of 
comparator 13 always remains correct since there will be maintained about 
+0.9 volts on the inverting input of comparator 13. If the voltage on the 
non-inverting input of comparator 13 does not drop all the way down to 
zero volts with no card, the circuit will still operate correctly as long 
as the voltage goes below 0.9 volts. Holding the output of signal follower 
11 at 1.8 volts has been accomplished by adding two more diodes 39 and 40 
in series with diode 23. The total voltage across these three diodes is 
1.8 volts. In this case, each diode will have a 0.6 volt drop across it 
since the 220 ohm resistor 25 will allow about 30 milliamperes conduction 
through the diodes. The 1 megohm resistor 21 and the 0.022 microfarads 
capacitor 19 will then be connected to this 1.8 volt line at junction 29 
to establish a minimum discharge voltage on a lead 48 and allow signal 
follower 11 to go no lower than 1.8 volts at its output. In normal 
operation the output of signal follower 11 will still follow at 0.2 volts 
below the white card voltage generated by conduction of photo-transistor 
9. The only requirement is that photo-transistor 9 must conduct at least 
0.2 milliamperes with a white card under the head and, as has been noted, 
none of the most worn heads in the most used readers has been found to 
conduct lower than this. 
Thus there has been described an improvement to optical mark sense readers 
showing constant percentage sensitivity under conditions of varying 
conduction of the photo-transistor. Great improvements in reliability, 
flexibility, maintainability, and operability have been provided through 
the novel advantages of the invention. 
It is pointed out that although the present invention has been shown and 
described with reference to particular embodiment, nevertheless various 
changes and modifications obvious to one skilled in the art to which the 
invention pertains are deemed to lie within the purview of the invention.