Arrangement for detecting the radiant energy of light-emitting semiconductor elements and its use in an electrophotographic printer

To detect the light energy of the light-emitting elements of a character generator of an electrophotographic printer via a photoelement (FE), the light-emitting elements are activated during the measuring process in such a manner that each element emits a sequence, associated with the element, of light pulses, having the same radiant energy and a predetermined pulse frequency. The output signals of the photoelement (FE) are supplied to an active low-pass filter (V3) which forms from these a mean value which is used as a measure of the radiant energy of the light-emitting elements during the calibration process. To reduce the influence of low-frequency interference (mains voltage), a gated potential control unit is provided. The arrangement is used in an exposure energy/correction device for the optical character generator of an electrophotographic printer.

BACKGROUND OF THE INVENTION 
In non-mechanical printers represented, for example, by electrophotographic 
printers, an image composed of individual dots is generated with the aid 
of a plurality of light elements of an optical character generator. The 
light elements can be, for example, light-emitting diodes which are 
brightly gated on by means of pulse-shaped signals of a particular 
frequency for generating the image. Since, in the case of a plurality of 
LEDs on an optical character generator, it cannot be expected that the 
light energy emitted by the LEDs is always the same over the operating 
time and from LED to LED because of the component spread and aging, it is 
necessary to calibrate optical character generators and during this 
process to measure the radiant energy, that is to say the radiant power 
multiplied by the duration of each individual light-emitting diode and to 
calibrate it in dependence on this measuring process, that is to say to 
adapt it with respect to the light energy. To automate this calibration, 
each individual LED is activated with a variable pulse-shaped signal in 
dependence on the desired radiant energy. In this process, the possible 
variations are the magnitude of the activating pulse-shaped signal and the 
variation of the duty ratio of the activating current, which is used with 
preference. 
Before, for example, the duty ratio can be correctly adjusted, the 
emittable light energy must be determined at a predetermined duty ratio 
for each individual LED by means of a measuring arrangement so that the 
necessary variation of the duty ratio can then be carried out by means of 
a correction device. The light energy in each case emitted for each 
individual LED is measured at a constant duty ratio by means of a moving 
measuring arrangement in which a photoelement is conducted by means of a 
motor-driven slider over the strip of light-emitting diodes, that is to 
say over each individual LED. Such an automatic calibration arrangement 
for the character generator of an electrophotographic printer is known 
from U.S. Pat. No. 4,780,731. 
The electrophotographic printer described there contains an exposure energy 
correction device for the optical character generator exhibiting a strip 
of light-emitting diodes. When a calibration routine is called up, the 
light-emitting elements are automatically calibrated by the fact that a 
photoelement detects the radiant power emitted by the light-emitting 
element and supplies it in the form of electrical signals to a control 
device coupled to the light-emitting element. The program-controlled 
device then allocates to each light-emitting element an individual 
operating time and stores this in a switching-time memory. As a result, 
each light-emitting element later supplies this radiant energy when 
operated. 
To measure the radiant energy, an extremely flat photoelement component is 
used which, in the form of a slider, moves past each individual LED during 
the measuring process. Since it must be reliably encountered by the 
radiation of each light-emitting diode, it also has a very large area 
(approximately 1 cm.times.0.5 cm). However, very flat photoelements which, 
nevertheless, have a large area, have the disadvantage that, on the one 
hand, they only have low sensitivity and, on the other hand, have 
relatively large unwanted electrical capacity which makes it more 
difficult to measure short light flashes. In addition, the conditions can 
vary significantly when the photoelements are changed. 
If rectangular pulses are used as light pulses, the electrical response 
pulses emanating from the photoelement are greatly changed. 
If the radiant power or the radiant energy of light-emitting semiconductor 
elements such as light-emitting diodes or the like are determined by means 
of large-area photoelements via individual light pulses or light flashes, 
the results achieved are inaccurate and in some circumstances may be 
afflicted with interference. 
SUMMARY OF THE INVENTION 
It is therefore the object of the invention to develop an arrangement of 
the type initially mentioned in such a manner that the light power or the 
light energy of light-emitting semiconductor elements can be reliably 
detected. 
It is another aim of the invention to provide an arrangement for 
calibrating the radiant energy of the light-emitting diodes of an optical 
character generator of an electrophotographic printer by means of which a 
reliable interference-free calibration of the optical character generator 
is possible. 
In arrangements of the type initially mentioned, this object is achieved by 
an arrangement for detecting the radiant energy of light-emitting 
semiconductor elements by using a photoelement arranged within the area of 
radiation of the semiconductor elements, which receives the light emitted 
by the semiconductor elements and generates electrical output signals as a 
function of this light. The semiconductor elements to be measured are 
activated in such a manner that they output a sequence of light pulses of 
the same radiant energy and predetermined repetition rate. Means are 
provided which detect the output signals, associated with the respective 
semiconductor element, of the photoelement and form from these a mean 
value which is used as a measure of the radiant energy of the respective 
semiconductor element. 
The arrangement can be used for calibrating the radiant energy of 
light-emitting elements of an optical character generator of an 
electrophotographic printer, using the photoelement which detects the 
light of the light-emitting elements in a measuring process required for 
the calibration and generates the electrical output signals as a function 
of this light. The light-emitting elements are activated during the 
measuring process in such a manner that each element emits a sequence, 
associated with the element, of light pulses of in each case the same 
radiant energy at a predetermined repetition rate. Means are provided 
which detect the output signals, associated with an element, of the 
photoelement and form from these a mean value which is used as a measure 
of the radiant energy of the elements during the calibration process. 
The means for detecting and forming is constructed as a low-pass filter and 
the low-pass filter is constructed as an active low-pass filter. The 
low-pass filter is constructed in such a manner that it suppresses the 
repetition rate of the light pulses and only passes the low-frequency 
signal components, used as a measure of the radiant energy of the 
elements, of the output signals of the photoelement, produced during a 
continuous scanning movement of the photoelement over the light-emitting 
elements. 
A four-terminal circuit arrangement containing a differential amplifier is 
provided for suppressing in-phase line interference on the signal lines. 
The inputs of the differential amplifier are logically combined to first 
and second inputs and its output is logically combined to a first output 
of the four-terminal circuit arrangement. Load resistors are connected to 
the differential amplifier in such a manner that, referred to a reference 
potential present at a second output of the four-terminal circuit 
arrangement, the inputs of the differential amplifier exhibit the same 
load resistance. A first load resistor of defined magnitude is arranged 
between the first input and the output of the differential amplifier and a 
second load resistor of the same defined magnitude is arranged between the 
second input of the differential amplifier and the reference potential. 
A gated potential control arrangement, which defines a predeterminable 
reference potential of the output signals, is provided for suppressing 
low-frequency interference in the electrical output signals. 
The gating frequency of the potential control arrangement is selected in 
such a manner that it corresponds to the light pulse frequency or to a 
subharmonic of the light pulse frequency. 
A reliable measurement of the radiant power or of the radiant energy is 
possible by means of the arrangement of electric components which detect 
an electrical output signal sequence generated by the photoelement in 
dependence on the received light pulses and form from this a 
time-dependent mean value which is then used as a measure of the radiant 
energy of the semiconductor element. 
For this arrangement, it must be initially assumed that, in order to 
measure the radiant energy of a light pulse, not only this one light pulse 
is used but the semiconductor element (light-emitting diode) to be 
measured is made to output several light pulses of the same radiant energy 
in each case and with a particular predetermined pulse frequency. It is 
only then that the next light-emitting diode is measured. 
Compared with measuring a single light pulse, a greatly increased 
protection against interference peaks on the measurement signal can also 
be expected in this manner since a single interference can be distributed 
over several flashes. 
The signal supplied by the light sensor (photoelement) in the arrangement 
according to the invention is normally very small. It can therefore be 
falsified by so-called hum interference which is caused by the line 
voltage, and by various types of other low-frequency interference which 
impairs the measurement accuracy and thus the quality of the printed 
image. 
In an advantageous embodiment of the invention, a gated potential control 
arrangement, which defines a predeterminable stable (that is to say 
undisturbed) reference potential of the individual signals of the output 
signal sequence, is therefore provided for suppressing low-frequency 
interference in the electrical output signal sequence. 
The gating frequency of the potential control arrangement is advantageously 
selected in such a manner that it corresponds to the flash frequency or to 
a subharmonic of the flash frequency.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The exposure energy correction device for the optical character generator 
of an electrophotographic printer, shown schematically in FIG. 1, is 
described in greater detail in U.S. Pat. No. 4,780,731 (hereby 
incorporated by reference). 
In this arrangement, the excitation of the individual light-emitting diodes 
LED of a character generator ZG on a photoconductor drum FL revolving at 
the speed v generates by self-focusing optics OP on a photoconductor layer 
of the photoconductor drum FL image characters which are then developed, 
in a manner not shown here, in a developer station and are transferred to 
continuous stationary in a reprinting station. The text to be represented 
is supplied from a central data processing system DVA of the printer 
control unit DS in this arrangement. 
Since the light power emitted by an LED depends not only on the electrical 
activating values of current and voltage but also on the component spread 
and the aging condition of the LED, it is necessary to correct the light 
power of the individual LEDs with respect to one another. 
In this arrangement, the correction device contains a photoconductor 
element FE which is arranged in the vicinity of the surface of the 
photoconductor drum behind the focusing optics OP. This photoelement FE is 
coupled to an electric motor M which moves the photoelement FE line by 
line over the focusing optics OP when a calibration routine is called up, 
for example via the printer control unit DS or via the central data 
processing system DVA. At the same time, the individual light-emitting 
diodes LED of the character generator ZG are excited via the central 
printer control unit DS. The light power emanating from the individual 
light-emitting diodes, taking into consideration the total transmission 
path including the focusing optics OP, generates at the output of the 
photoelement FE a corresponding electrical signal which is supplied to a 
program-controlled device PS coupled to the photoelement FE. The 
program-controlled device contains an amplifier V with subsequent 
analog/digital converter AD for converting the analog signal corresponding 
to the light energy into a digital signal. A microprocessor PR contains a 
central processing unit ZPU with associated memory unit SP. The 
microprocessor PR acquires the signals corresponding to the radiant power 
of the LEDs via the central processing unit CPU and stores them in the 
memory SP in the order in which they have been scanned. From the radiant 
power of the individual LEDs thus measured, a standard radiant energy, 
which is uniform for all LEDs, can then be generated by allocating an 
operating time adapted to the individual radiant powers of the individual 
LEDs. For this purpose, the microprocessor PR allocates individual 
operating times per LED to the individual powers stored in the memory SP 
and stores these operating times in the switching-time memories SCH1 to 
SCHN associated with the printer control unit DS and individually 
connected to the LEDs. Furthermore, the correction device exhibits a 
compensation arrangement K which detects via corresponding sensing 
elements for the operating parameters of the printer device, for example 
speed V of the recording medium FL, operating temperature CF of the 
light-emitting elements LED and, depending on these, uniformly determines 
the standard electrical operating parameters such as voltage and current 
intensity for all light-emitting elements LED and supplies these to the 
LEDs via driver stages T1 to TN. To detect the speed V of the 
photoconductor drum FL, a scanning device A constructed in familiar manner 
is used and for detecting the temperature, a temperature sensing element 
TF on the carrier accommodating the LEDs is used. Both the scanner A and 
the temperature sensing element TF supply a voltage corresponding to the 
measured quantities to the compensation arrangement, which voltage is 
compared with an adjustable standard direct voltage NG, supplied from the 
outside, at the comparators K1 and K2 and, depending on this comparison 
process, then determines the activation current and the activation voltage 
for the driver stages T1 to TN. The compensation arrangement K can also be 
separately adjusted, for example by varying the standard direct voltage NG 
and independently of its operating parameters. The scanning signal 
proportional to the speed of rotation V, supplied by the scanner, is also 
supplied to a clock device CL at the same time. The central printer 
control unit DS activates the LEDs of the character generator ZG microline 
by microline in conjunction with a clock signal supplied by this clock 
device CL. 
As stated in the introduction, the radiant energy of each individual LED is 
detected before the actual calibration process. For this purpose, the 
light-emitting diode LED1, the radiant energy of which is to be measured, 
is activated by means of a current pulse sequence of rectangular signals I 
of the current intensity i in dependence on the time t via the printer 
control unit DS in accordance with the representation of FIG. 5a. If a 
character generator ZG with approximately 11000 individual LEDs is used 
which are to be calibrated in a single calibration process which should be 
concluded within a time of approximately 30 sec, a light pulse frequency 
of 11.4 kHz is necessary, that is to say about 31 pulses are used for 
scanning one LED. This current pulse sequence I generates, in accordance 
with the representation of FIG. 5b, a light pulse sequence LI of 
individual pulses having the same radiant energy and a constant pulse 
repetition rate. 
If a photoelement FE with ideal electrical characteristics were present, 
this photoelement FE would convert the light pulses LI into corresponding 
electrical signals ES (FIG. 5c) which could then be detected by the 
program control unit PS. However, the electrical output signals ES of the 
photoelement have a sawtooth-like variation, shown in accordance with FIG. 
5d, due to the electrical characteristic of the photoelement. 
If the photoelement FE is represented as a four-terminal network in 
accordance with FIG. 2, the photoelement has an unwanted large 
barrier-layer capacitance CQ which is produced by the barrier layer 
between the positive and negative side of the photoelement. In this 
connection, resistors R designate damping resistors. These are necessary 
for preventing an oscillation of the current/voltage converter V2 
(four-terminal network 2), which follows the photoelement FE, due to the 
barrier-layer capacitance CQ. 
The barrier-layer capacitance CQ in conjunction with other interfering 
influences then has the consequence that the deformed output signal AS 
shown in FIG. 5d is present at the output of the photoelement FE. Such 
output signals AS are hard to process and can be used only to a limited 
extent for detecting the light energy of the light pulses received by the 
photoelement FE. 
In order to provide the possibility of optimum evaluation of the 
photoelement signal, nevertheless, an arrangement AA (FIG. 1) is used 
which is arranged between the program-controlled device PS and the 
photoelement FE. The arrangement AA is represented as a sequence of two 
four-terminal networks V2 (current/voltage converter) and a four-terminal 
network V3 which is constructed as active low-pass filter. The electrical 
characteristic of the photoelement FE is represented as four-terminal 
network V1. 
As already stated, the light pulses of the individual LEDs are to be 
detected, namely the radiant energy of an individual pulse, that is to say 
its radiant power multiplied by the pulse duration (t.times.J). In this 
connection, the minimum pulse duration is 24 .mu.s. Calculating the above 
time-constant of the four-terminal network V1, a time constant of 60 .mu.s 
is obtained with a damping resistance value R of 5 kiloohms. This is 
distinctly more than the pulse duration. Thus, the current at the output 
of the four-terminal network V1 is still rising when the pulse time of an 
individual pulse is already at its end. No constant settled measurement 
signal is available. 
Although it would be conceivable to scan the signal at a particular point, 
the full deviation of the signal, which is very small in any case, would 
not be used, on the one hand, because the measurement would be taken early 
before the final value is reached and, on the other hand, the capacitance 
CQ has a fairly large spread from photoelement to photoelement (.+-.30%) 
so that the measurement signal can rise at different rates and is then at 
an amplitude which depends on the respective photoelement at a given 
scanning time. In addition, it is not possible to measure the pulse 
duration in this manner. 
To be able to correctly measure the exposure energy of the light-emitting 
diode LED, the arrangement AA described in the text which follows is 
necessary. 
It exhibits a four-terminal network V2, which consists of a current/voltage 
converter with an operational amplifier OP. 
In the case shown, the space conditions around the photoelement FE are 
extremely constricted so that the evaluation electronics cannot be 
accommodated in the immediate vicinity. The photoelement FE is therefore 
connected to the evaluation electronics by means of a line which is also 
extremely thin because of the space conditions. In addition, this line has 
sufficient flexibility in order to follow the movements of the slider on 
which the photoelement FE is located and which must be able to move past 
the entire LED strip for measuring all LEDs. 
The line can have a length of up to 70 cm. The current range supplied by 
the photoelement is between approximately 0 and 0.1 .mu.A. A current value 
within this range must be converted by the evaluation electronics into a 
voltage U (FIG. 2) between 0 and 1 V at the output of the four-terminal 
network V3, the value of which must only have an error of less than .+-.1% 
from its possible maximum value. With such extreme requirements, it is 
appropriate to additionally suppress any interferences on the cable by 
special electronic measures. 
In the amplifier arrangement (V2) shown in FIG. 3, two resistors RG are 
arranged which are identical with respect to one another. In this 
arrangement, the output of the operational amplifier OP is fed back to the 
negative input via one resistor. The positive input of the operational 
amplifier OP is connected to ground via a resistor RG of the same 
magnitude The arrangement of this resistor results in an especially 
noise-suppressing characteristic of the amplifier. 
Interferences coupled on the two feed lines to the photoelement generally 
have the characteristic that they are present as a voltage having the same 
sign and initially the same magnitude at the two inputs since the two 
lines run in parallel. An operational amplifier is largely insensitive to 
identical voltages at its inputs, that is to say this does not create any 
voltage at its output. However, the noise voltages present only remain 
identical if they encounter the same load resistor with respect to ground. 
This is the one resistor RG at the negative input and it is also the 
resistor RG at the positive input, the values of the two resistors RG must 
be identical. 
To evaluate the pulse voltage U of the four-terminal network V2, the 
four-terminal network V2 is followed by an active low-pass filter V3 
having a configuration according to FIG. 4. In this arrangement, the 
four-terminal network V3 solves the problem of the inertia of the 
photoelement which is here caused by the capacitance CQ. 
In this connection, it must be assumed that, in order to measure the 
radiant energy of a pulse, it is not only this one pulse which is used but 
the LED1 to be measured (FIGS. 5a-5e) is allowed to output several pulses 
(LI) of the same radiant energy in each case at a particular predetermined 
pulse frequency. It is only then that the next LED2 is measured with new 
light pulses of a different radiant energy, which, however, is the same 
for all these pulses, and a predetermined pulse frequency. 
Compared with measuring a single light pulse, a greatly increased 
protection against noise peaks on the measurement signal can be expected 
in this manner since individual interferences can be distributed over 
several flashes. 
Initially, an LED is investigated which continuously outputs light pulses 
without changing to a different LED. In the present case, it is assumed to 
output light pulses at a frequency of 11.4 kHz and, for example, as in 
FIG. 5, with a duty ratio of 
##EQU1## 
light pulses, that is to say it is switched on for 25% of the duration of 
the period T0 and switched off for 75%. The mean value U formed at the 
output of the active low-pass filter V3 is directly a measure of the 
radiant energy of an individual light pulse. This means that the mean 
value signal U, formed by the arrangement AA and shown abstracted in FIG. 
5e, is a direct measure of the radiant energy of a single light pulse. 
In the case where switching occurs from one LED1 to the next LED2 so that 
finally the entire row of LEDs can be measured, the condition that light 
pulses are emitted by one LED whilst the one following in each case does 
not operate at all, that is to say emits light pulses with a radiant 
energy of 0 as it were, is obtained in an unfavorable case. The low-pass 
filter is constructed in such a manner that the mean value function U can 
be obtained with the fewest possible single flashes per LED. An 
analog/digital converter component AD (FIG. 1) can then be used for each 
individual LED for detecting this voltage and forwarding it digitally to 
the microprocessor control PR during the time in which a constant voltage 
U is present. 
The frequency response, and thus the cut-off frequencies, of the low-pass 
filter will be adapted to the light pulse frequency. In this connection, 
active and passive low-pass filters can be used. However, an active 
low-pass filter is of advantage compared with a passive RC low-pass 
filter, shown diagrammatically in FIG. 4, which has unnecessarily severe 
roundings in its frequency response. 
An active low-pass filter of the order n=2 has been found to be 
advantageous. With a higher-order low-pass filter, no improvement 
justified in relation to the increased expenditure (more OP chips, 
problems with component tolerances, development cost) can be expected 
since, although an ideal low-pass filter has a short settling time because 
it is possible to increase the cut-off frequency, it tends to have 
overshoot. This opinion has been confirmed by practical tests with 
low-pass filters of the order n=4. During the implementation of the active 
low-pass filter, it has been found to be most advantageous to implement a 
so-called Butterworth low-pass filter, to observe the settling and then to 
change the low-pass filter in the direction of a Bessel low-pass filter 
until a minimum time for reaching the settled state is reached. 
Assuming a light pulse frequency of 11.4 kHz and stipulating 0.6 ms as time 
for evaluating the measurement value, the value of 2.7 ms is obtained as 
time necessary for measuring an LED in a preferred illustrative embodiment 
of the invention. This results in a frequency of the mean value of the 
sensor signal of 185 Hz. 
The signal supplied by the light sensor FE is normally very small. It can 
therefore be falsified by so-called hum interference according to the 
representation of FIG. 6a BS, which is caused by the mains voltage, and by 
various types of other low-frequency interference which impairs the 
measurement accuracy and thus the printed image. This interference can be 
quite considerably reduced by means of a gated potential control GP 
corresponding to that shown in FIG. 4. In this arrangement, the signal 
from the photoelement FE is conducted via a capacitance C and then onto 
the low-pass filter TP. The capacitor C is followed by an electronic 
switch ES which is logically combined to it and which, controlled by a 
clock source TQ, forces the output of the capacitor C to a particular 
potential, which can be arbitrarily set, at particular times, namely at 
times T1 to TN (FIG. 5d). This potential can also be, for example, ground 
potential. 
The closing times of the switch ES must be selected in such a manner that 
they occur at the times at which the sensor signal has decayed or is at a 
fixed signal-independent potential. The preferred position of the switch 
closing point is indicated in FIG. 5d, T1 to TN. This shows that the 
preferred frequency of closing is equal to the light pulse frequency. If 
necessary, a subharmonic of the light pulse frequency can be used for the 
switch. FIG. 6 shows the effect of the potential control. At the times T1 
to TN drawn in, the switch is closed and a particular potential is forced 
at the output of the capacitor C in FIG. 4. In the ideal case, the 
sinusoidal noise variation of FIG. 6a is then reduced to the noise signal 
variation of FIG. 6b. In FIG. 6b t designates time and i designates level 
in this connection. The effect can be demonstrated by means of a numerical 
example. The sinusoidal interference is assumed to have the value 2 from 
peak to peak. The frequency is 50 Hz. The switch closing frequency is 
10,000 Hz. The remaining noise then has a magnitude of 0.062 from peak to 
peak. The noise is therefore reduced by a factor of 31.8. 
The invention is not limited to the particular details of the apparatus 
depicted and other modifications and applications are contemplated. 
Certain other changes may be made in the above described apparatus without 
departing from the true spirit and scope of the invention herein involved. 
It is intended, therefore, that the subject matter in the above depiction 
shall be interpreted as illustrative and not in a limiting sense.