DE3133168C2 - Device for keeping constant the luminous flux supplied by a fluorescent tube of a fiber optic for a copier - Google Patents

Abstract

An exposure value control system for a copier with a narrow exposure slot, in which a fluorescent lamp and a fiber lens arrangement are used as a light source or as an optical imaging system, has a lighting device for the fluorescent lamp, a manually adjustable DC voltage generator and a control device for electrically controlling a lamp current , which is fed from the lighting device of the fluorescent lamp as a function of the DC voltage of the generator.

Description

a) an HF voltage source (20) is provided for feeding the fluorescent tube (2, 3),

b) the control circuit contains a comparator (22) which compares the electrical signal of the photoelectric converter (9) with one of several selectable reference values (V _SEF ) and generates an output signal corresponding to their difference,

c) the control circuit contains a clock pulse generator (14),

d) the control circuit one of the clock pulse generator (14) and the output signal of the comparator (12) controlled pulse width modulator (13) contains, and

e) in the HF supply circuit (26) of the fluorescent tube (2, 3) one from the pulse width modulator (13) controlled switch circuit (27) is provided.

2. Device according to claim 1, characterized in that that the RF supply circuit has an alternating current source (21), one of these downstream Rectifier circuit (22) and an oscillator connected to the rectifier circuit (22) (23), the latter forming the input variable of the switch circuit (27).

3. Device according to claim 2, characterized in that between the oscillator (23) and the Switching circuit (27) a transformer (24) is connected.

4. Device according to claim 1, characterized in that the comparator (12) consists of an operational amplifier consists.

5. Einrihtung according to one of claims 1 to 4, characterized by a further comparison stage (16) which receives the output signal of a sensor device (9, 15) as an input variable and compares this with a predetermined adjustable reference value (V ,,, V _1. ) , and by a warning device (17, 18) controlled by the comparison stage (16).

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The invention relates to a device for keeping constant the value of a fluorescent tube of a fiber optic supplied luminous flux for a copier, with a «> photoeiekirischeii Wanuici £ ui Ei diffraction of a dem Luminous flux of the fluorescent tube corresponding electrical signal, and with a control circuit that picks up electrical signal of the photoelectric converter and a constant control of the luminous flux performs.

Such a device for keeping constant the amount fed from a fluorescent tube to a fiber optic Luminous flux for a copier is known from DK-OS 29 26 360. This known device includes In addition to the fiber optics for the transmission of a luminous flux, there is also a photoelectric converter for Generation of an electrical signal corresponding to the luminous flux of a fluorescent tube and a Control circuit that receives the electrical signal from the photoelectric converter and a constant control of the luminous flux, but this regulation is a phase regulation in the usual way is.

DE-OS 26 00 933 discloses a method for regulating light emission, in particular for photocopying systems known, wherein the light is guided by an optical system along a light path in order to hit a desired object. According to this known method, a light detector element arrives for use which receives the light emitted from the light source, the light detector element is arranged outside the optical system and deliver a signal proportional to the incident light can. A control device is also used, which controls the actuation of the input to the Controls the light source as a function of the output signal supplied by the light detector element. According to this known method, the regulation is likewise carried out in the usual manner by means of phase control performed.

From US-PS 41 53 364 an optical focusing system in connection with a photocopier is known to transfer the image of an original via the optical focusing system onto a rotating photoconductive Projecting drum. This known system is equipped with a circuit arrangement for control and Control of the lighting intensity, whereby the conventional phase control is also implemented here will. According to one embodiment, the circuit arrangement comprises a comparison stage, which as a the first input variable receives a signal reproducing the light value and a second input variable Receives equivalent voltage. The difference signal generated in the comparison stage is sent to a level modulator, which converts the difference signal into a corresponding level signal. It is also an AC power source provided to which the lamp to be controlled is connected. To the AC power source is a Zero-crossing detector switched on, which emits a pulse when the alternating voltage crosses zero generated. This pulse triggers a pulse width modulator, which then has its own Impulse generated. The width of the generated pulse is determined by the level signal of the level modulator certainly. The pulse width modulation signal thus obtained is sent to a trigger generator and is used in converted to a trigger signal. This trigger signal is sent to an electronic switch as a trigger variable, which is switched on in the circuit of the lamp. With the help of the trigger pulses, the part the initial phase of each alternating current half-wave, so that the classic phase

However, this known principle of phase control is not particularly suitable for keeping constant of the luminous flux emitted by a fluorescent tube, since the control characteristic is not linear.

DE-PS 7 05 959 is an execution for automatically keeping the effective luminous flux constant for copier machines by means of a photocell

known, which is supplied with a proportion proportional to the effective luminous flux. The essence of this known device consists in that the voltage supplied by the photocell over two input side amplifier tubes connected in parallel two in different Controls the conduction direction against each other connected current regulating tubes, through which the the alternating current network The feed current drawn from the copier light source flows.

From DE-OS 23 50 149 a device for controlling the exposure of a playback device is especially known for recording microfilms. It should be achieved that a original to be recorded, for example a specific individual image of a microfilm a precisely defined one Receives amount of light. For this reason, the amount of light generated during exposure is integrated to thereby to get an exact measure of the total exposure.

From US-PS 30 02 099 a control circuit for controlling the amount of light emitted by a light source is known, wherein the reflected from an illuminated object light from a photoelectric The transducer is captured and converted into a corresponding electrical signal. With the help of this obtained electrical signal is activated after its amplification a controlled rectifier, the feeds the light source.

From DE-OS 26 21 378 an exposure device is finally known for a mercury vapor lamp, which is also regulated, d. H. there is also a photoelectric converter that measures the intensity of the exposure during the Exposure time recorded. In this case, however, the exposure time is controlled in which the emitted by the light source Light with a given intensity reaches the exposure plane.

The object underlying the invention is to provide a device for keeping the of Luminous flux supplied to a fluorescent tube of a fiber optic for a copier of the type indicated at the beginning Type to be improved in such a way that the luminous flux is independent of the desired value set in each case can be kept constant very precisely.

This object is achieved by the features specified in the characterizing part of claim 1.

The use of an HF voltage source with which the fluorescent tube is fed results a much more favorable basis to be able to carry out a very precise regulation. According to the invention is used to achieve a very precise control of the fluorescent tube supplied RF energy with the help of a Chopper controlled in the form of the switch circuit.

The device according to the invention is comparatively simple and can be made commercially available prefabricated assemblies can be produced. It also has the advantage of being independent of the desired and set light value or luminous flux a very precise maintenance of the Luminous flux is made possible.

advantageous refinements and developments of the invention emerge from the subclaims 2 to 4.

In the following, the invention will be described in greater detail on the basis of exemplary embodiments with reference to the drawing explained. It shows

Fig. 1 (a) shows an example of a conventional exposure device with a focusing fiber lens assembly and fluorescent tube;

Fig. 1 (b) is a schematic illustration for explanation an opening angle in the case of a fluorescent tube;

2 shows a schematic representation of one of the embodiments of the device mi '. Characteristics according to the invention;

Fig. 3 is a block diagram of a control circuit;

Fig. 4 shows waveforms of control pulses supplied from the control circuit in Fig. 3, and the corresponding lamp current;

Fi ^. 5 (a) and 5 (b) graphs in which the characteristic values of the change in luminous efficiency are shown over time when the fluorescent tubes are switched on will;

6 and 7 further embodiments of circuits for a warning generator.

In Fig. 1 (a), an exposure device for a copier is shown in which a fiber lens array is used as an imaging optical system. A template arranged on a glass plate 1 becomes illuminated by two fluorescent tubes 2 and 3, and the image to be copied of the original is through a fiber lens array 6 is projected onto a photosensitive member 8. As the surface of the photosensitive part is electrostatically charged beforehand with a corona charger, a corresponding to the image of the original, creates an electrostatic latent image on its surface. The electrostatic, latent image is developed transferred to a duplex paper and fixed on it, thereby corresponding to a conventional Copying process the requested copy is received. In addition, lamp housings 4 are also shown in FIG. 1 (a) and 5 and a slot 7 for protection against stray light.

As described above, when such a fiber lens assembly 6 is used, the development latitude becomes W is less and is only about a quarter of the width of the conventional optical Lens system, so that the selectable mechanical adjustment of the exposure value becomes rather difficult. .

As is known, the choice or spread of an exposure value can be obtained by mechanically selecting or adjusting the width of a slit which is arranged in a beam path from the original to a photosensitive part; however, the tolerance requirement with regard to the shape of the slot and with regard to its attachment is very strict, since the width W provided for an exposure is relatively narrow. As a result, it is difficult in practice to provide such a slot in a place where there is not enough space. In addition, a change in the exposure value with respect to the slit width is not uniform. In other words, a change in the light output when the slit width is widened from zero in steps of 1 mm becomes non-linear or results in very small and large parts in the change quotient, which then makes the exposure control uneven.

In addition, it is known to those skilled in the art that the control of the exposure value can be carried out by rotating a fluorescent tube provided with an opening, with which a narrow exposure width W can be effectively illuminated. As shown in Fig. 1 (b), the fluorescent tube has an opening part IA or 3/4 which is formed in such a way that the part corresponding to an opening angle A inside the tube is not provided with or coated with fluorescent material

would. As a result, this tube has a light distribution characteristic which can be represented by a directional effect, as indicated by a dashed circle is. A method of adjusting exposure by rotating or panning those with a Apertured fluorescent tubes as described above, however, have various disadvantages since the mechanism for rotating and pivoting the tube is complicated, it is easy to have contact difficulties the connection pin of the tube as the tube is rotated and the tube itself gets quite expensive.

However, in view of the durability and the uniformity of exposure, it should be noted that the luminous efficacy of a light source in general with a change in ambient temperature, a Change in voltage of an energy source or deterioration of a lamp due to aging and the like changes. Although different lamps can be used as a light source in an exposure device, the ambient temperature of a fluorescent tube has a very strong influence on a fluctuating light output. In the case of a fluorescent tube, the luminous efficiency drops considerably at a low ambient temperature from, and the light output immediately after switching on is only compared to continuous operation about 30%. If in an exposure device, i. H. in an image-forming facility one Copier, the light output of a light source, namely changes its exposure value, has this one uneven density on each copy or a different degree of density from copy to copy and the required copy quality cannot be obtained. As exposure value stabilization device an indirect exposure value stabilization system is known to those skilled in the art in which the light output of the lamp is kept constant that the lamp voltage, the lamp current or the energy source potential is determined and then fed back to a lighting device. This However, the system is only effective for one lamp, its electrical characteristics or light properties as with tungsten lamps are fixed from the start, and consequently this system is for a fluorescent tube not suitable, as the light output of the fluorescent tube changes significantly with its ambient temperature. With In other words, no satisfactory stabilization can be expected from a fluorescent tube will. The aforementioned exposure value stabilizing device is already used to solve this problem has been proposed in which the light output of a lamp is thereby stabilized and constant is held that the lamp light received by means of light-receiving elements and to a lighting device is fed back.

Although it makes a difference whether the stabilization is achieved indirectly by determining the energy source potential, the ambient temperature, etc., or directly by sensing the light outputs, consists of this no difference that the supply voltage at the light source in the lighting device as a function is regulated by the size of the determined values. Hence, it is with this type of exposure value stabilizing device It is common in practice that the lighting device of a light source has a higher Power consumption has as the rated power of the lamp, and that this difference as a margin or Safety margin to absorb the changes in light output caused by various factors is used. In order to accommodate all the changes in the light output of a lamp, which in a Fluorescent tubes have very strong fluctuations in the light output, the performance must the lighting device can be made large enough to meet this requirement. Such a one However, lighting equipment must be of very high quality to operate under normal operating conditions to be able to absorb the changes; the stabilization device itself must be sized accordingly thereby increasing the manufacturing cost and having adverse effects on the service life surrender to the lamp.

In the case of a fluorescent tube, it is therefore inevitable that the luminous efficiency of the fluorescent tube in particular at low ambient temperatures is considerably lower, and the light output is immediate after switching on is only about 30% compared to continuous operation. However, it comes to such big changes only when the ambient temperature is lower than 10 "C, and when switching on, so that this condition rarely occurs. Consequently, it is not economical to have such a powerful one Provide lighting equipment so that it can accommodate all changes that occur so infrequently. The above conclusions are entirely correct in one respect; however, if a Relatively small lighting device is used in terms of their performance, in which the possibility of large changes in light output occurring at the time of power-on is neglected, an idle or idle period is created during which fuzzy copies can be generated by the copier since the change in light output cannot be regulated by regulating the performance of the stabilization device can be compensated for during this idle period.

Although it is known to preheat the tube wall of the fluorescent tube in order to avoid the change in the rise in the light output at the time of To reduce switching on, this has the disadvantage that a heating device, which is often a safety problem can represent how a surface heater and the like is required that by the complicated Execution, the manufacturing cost can be increased, and that the preheating is maintained even during a waiting period must be or when the exposure device is not in operation.

As can be seen from the above, such countermeasures should preferably be used be provided that the stabilization device is designed according to exactly the predetermined To cover a range of, for example, ± 30% with regard to a change in the light output of the lamp, and that the dealer or manufacturer recommends that the user only close the exposure device after it has been put into continuous operation, as the device itself is not designed to do so is that it can absorb larger changes that go beyond ± 30% in order to prevent that the lighting device must be of too high quality.

The device with features according to the invention will be explained below on the basis of various embodiments described. In Fig. 2 is the overall structure of a device with features according to schematically the invention shown, which the already based

from l ^? ig. 1 has the exposure device described. As already stated at the outset, an original is fed one after the other over the platen 1, and a part of the original surface, which is arranged above the glass plate 1, is uniformly exposed from the underside by means of the two fluorescent tubes 2 and 3, which are used as exposure lamps with a corresponding Opening are provided. Apart from the opening areas IA and 3/4, the remaining area of the lamps is covered by the lamp housings 4 and 5 (see FIG. 1). The light reflected from the illuminated original is guided via the focusing fiber lens arrangement 6 (FIG. 1) to a photosensitive part 8 (FIG. 1) which is previously uniformly electrostatically charged and imagewise exposed to the image of the original in order to be shown on the Photosensitive part to create an electrostatic, latent image, which is then developed by means of toner. The toner image is then transferred to duplex paper and finally fused to obtain the required copy. A photoelectric converter 9 is arranged at a point from which either one or both fluorescent tubes can be detected in order to determine the light output or power of the lamps. In the embodiment of FIG. 2, a photodiode is arranged on the rear side of the fluorescent tube 3. The photocurrent obtained by the photodiode 9 and corresponding to the light output is applied to a control circuit 10. An HF voltage source 20 is activated by control pulses generated by the control circuit 10. The lamp current I _L from the RF voltage source 20 is controlled so that the light output of the fluorescent tube 8 as a whole is made constant.

According to FIG. 3, the HF voltage source 20 has an oscillator 23 which is excited by a rectifier circuit 22 which is connected to a conventional energy source 21. The fluorescent tubes 2 and 3 are connected to the secondary side of an output transformer 24 of the oscillator 23. A line 25, which leads from the secondary side of the transformer 24 to the fluorescent tubes 2 and 3, represents a heating power supply circuit, and a line 26 represents an RF supply circuit. A switch circuit 27 is in the RF supply circuit 26 for switching on and off of the lamp current I _L and is controlled by the control circuit 10.

The reason why the high frequency stabilizer is used as the lighting device is mainly that the stabilizer can be made compact and is lightweight. In a lighting system, a problem due to a fluctuation in a lamp power source is inevitable. If such fluctuations occur, a copy may have an irregularity in darkness in the form of a stripe pattern, and consequently the quality of the copy will be markedly inferior. The magnitude of the influence due to such fluctuation generally depends on the photosensitive member linear speed V , the power source frequency / for the light source, and the exposure width W , and its effect is remarkable as the linear speed is higher, the frequency / is lower and the Width W is narrower. Since the linear velocity V and the exposure width W are determined by other factors required of a copier, the power source frequency of the light source must be increased in order to reduce the effects of fluctuation. In addition, in a discharge tube such as a fluorescent tube, the luminous efficiency is improved as the frequency becomes higher.
In the control circuit 10, an operational amplifier 11 is provided which works as a photocurrent-voltage converter, its non-inverting input being connected to the anode of the photodiode 9 and its inverting input being connected to its cathode. As a result, the output voltage V _{L of} the opera-

tion amplifier 11 higher when the amount of light from the fluorescent tubes 2 and 3 and thus the photocurrent of the photodiode 9 is greater. The output voltage V _{L of} the operational amplifier 11 is fed to the inverting input of an operational amplifier 12.

rather operates as a comparator, is applied and compared to a reference voltage V _{REF applied} to the non-inverting input of operational amplifier 12. The operational amplifier 12 compares the DC voltage V _L with the reference voltage

V _REF and at its output there is a voltage V ₀ which, with respect to the reference voltage VW, is proportional to a deflection value (V _REF -V _L ) . The voltage K ₀ is applied to the input terminal CV of a pulse width modulator 13, which has a monostable MuI-

having tivibrator. At the other input of the pulse width modulator 13, a pulse train from a pulse generator 14 is always applied, which has a freely oscillating multivibrator. A duty cycle D of the pulse train is shown as 7177 ", where T

represents a repetition period and 71 represents a pulse width. The pulse width modulator 13 changes the duty cycle D of the pulse train of the pulse generator 14 in such a way that the duty cycle increases when the DC voltage applied to the input CV is greater.

ßer. As a result, the output of the pulse width modulator 13, that is, the output pulse train V _{P of} the control circuit 10 becomes a rectangular waveform of the duty ratio which changes according to the output voltage of the operational amplifier 12 as by

dashed lines in Fig. 4 is shown. As a result, the excitation period of the switching arrangement 27, which carries the lamp current I _L , is also changed.

The output pulse train V _{1 of} the control circuit 10 is fed to the switch circuit 27. The switch circuit 27 is turned on only for a period of time corresponding to a pulse width in each subsequent period or pulse train V _P , and causes the lamp current to flow therethrough. The greater the value of the duty cycle D of the pulse train V _P , the longer the section

during which the lamp current flows, or the greater is the average lamp power. in the The difference is the smaller the value of the taste ratio the lower the average lamp power will be.

If the amount of light from the fluorescent tube 2 or 3 becomes smaller for any reason with respect to a given set point, the photocurrent of the photodiode 9 becomes smaller and the output V _{L of} the operational amplifier 11 also becomes smaller. Since the Opera-

tion amplifier 12 of the subsequent stage generates a voltage _{as output V 0} that is proportional to the difference (V _REF -V _L ) between the reference voltage V _REF and the output voltage V _L , the output voltage V _{0 of} the operational amplifier 12 and increases

consequently the input to the pulse width modulator 13 on when the output voltage V _L becomes low, and the duty cycle D of the output pulse train V _P from the pulse width modulator 13 becomes larger. Consequently

the switch-on duration of the switching arrangement 27, d. H. the continuity time for the lamp current is longer, and the average lamp power becomes larger, thereby increasing the light output.

In contrast to this, if the light yield at the photodiode 9 is higher, the duty cycle D of the output pulse train V _P becomes smaller and the average lamp power becomes lower, as a result of which the light yield also decreases. Since this system as a whole operates so that the input voltage V _{1 of} the operational amplifier 12 used as a comparison amplifier becomes equal to the reference voltage V _RFF , the luminous efficiency is kept at a certain constant value. As a result, the amount of light with which the surface of the original is illuminated can thereby be kept constant in order to reduce the unevenness in the concentration and hence the blackening on the copy and to ensure that excellent copies are obtained. Similar effects can be obtained in that the photodiode 9 is arranged on the substrate of the control circuit 10 and the light output of the fluorescent tube is guided to the photodiode 9 with the aid of a light guide in order to detect it indirectly.

The exposure value can be set to any value by changing the reference voltage V _R1 : _{f of} the operational amplifier 12, thereby adjusting the light output of the fluorescent tube. Consequently, according to the invention, the exposure value in the exposure device can be kept constant in an advantageous manner in that a rapid change in the light output at the time of switching on the fluorescent tube, a voltage fluctuation of the energy source and a change in the luminous efficiency are fully compensated for, which are due to a decrease in the luminous efficiency due to deterioration of the lamp and the like, and thus exposure value control becomes possible.

Since the above-described device according to the invention is designed so that the exposure value can be changed by electrically adjusting the luminous efficiency or output of the fluorescent tube, the conventional mechanical adjustment of the exposure value does not pose any difficulty in implementation, and there are any restrictions, such as the space for accommodation etc. are eliminated. In addition, it has a uniform characteristic with respect to the change in light output or efficiency at the time when the reference voltage V _{RLy is} changed, so that precise smooth exposure value control is possible, and the controllability of an exposure control lever is greatly improved.

In FIGS. 5 (a) and 5 (b) it is shown by a curve A how the luminous efficiency (in%) increases with time (in minutes) from the time the lighting is switched on, provided that the fluorescent tubes 2 and 3 have a rated current of 0.8 A and a rated power of 60 W and the lighting device has a rated current of 1.2 A and a rated power of 85 W, whereas a curve B is obtained when the control circuit 10 is not used.

Specifically, FIG. 5 (a) shows a characteristic curve for light output values when the ambient temperature T _{11 is} equal to or higher than 15 ° C. It can be seen from this curve that the change in the luminous efficiency or power can be expressed as ± 3% with a change of ± 30% in the light output of the fluorescent tube. Consequently, with this exposure value control system, the disturbance of the luminous efficiency can be absorbed.

generated when the fluorescent tube is switched on when the ambient temperature T _{a is} equal to or higher than 15 ° C. If, however, the ambient temperature T " becomes lower than 10 ° C, the system can reduce the fluctuation in light output when the light source is switched on.

Do not catch the tube any more, as this change is much larger than before, as shown in Fig. 5 (b). In other words, when the temperature T _{11 is} below K) "C, the light yield drops sharply, so that for the start-up section C, which corresponds to about 0.5 min

is an astable state.

Although it has been described that the in I-'ig. 3 embodiment shown the power supply to the fluorescent tubes by a pulse width modulation is set, the invention is Selbstverstäiul-Hch not limited to this, but any other corresponding method can also be used will. For example, a frequency modulation or a combination of both types of modulation can be used be applied.

As stated above, the execution will take place with the features according to the invention an excellent exposure control has become possible, since the lamp light yield with respect to the exposure device with a narrow exposure width elec- tric is adjusted when the focus fiber lens assembly is used. Also is the controllability greatly improved by an operator because it has a uniform characteristic with a fluctuation the light output can be obtained if the adjusting device is moved. Using the fluorescent tube may cause deterioration the characteristics of the focusing fiber lens arrangement due to the effect of heat are avoided, because the amount of heat developed is very small.

4 «If a stabilizer is also provided to automatically compensate for the light yield or output values, irregularities in concentration or blackening are prevented, and the use of a high-frequency stabilizer or stabilizer in the lighting device can exclude effects due to fluctuations in the energy source so that good copies are always made.
The unstable condition represented by the section "('" in Fig. 5 can be eliminated with the aid of a lighting device with a power of more than 120 W instead of only 85 W (at 1.2 A), but how As stated above, the use of a lighting device with such a high output leads to excessively good conditions under normal operating conditions and creates adverse effects on the life of the lamp. In addition, it is very rare that the temperature around the lamp is lower than 10 ° C For this reason, it is not economical to increase the power of the lighting device to cope with such a rare occurrence.

In order to cope with this rare case described above as well, the exposure value control system is provided with a warning signal generator, which indicates to the user of the copier, daü the change in the light output exceeds a range of ± 30%, which is automatically not slabili-

can be ized.

In Kig. 6 shows one of the embodiments of a warning signal generator. Here, a thermistor 15 is used as a sensing device in order to convert the illumination energy of the fluorescent tube 2 or the tubes 2 and 3 into "electrical signals. The thermistor 15 is arranged near the tube wall of the fluorescent tube 2 in order to determine the temperature on the tube wall. A resistor K ₁ is connected in series with the thermistor 15 to a direct voltage source V _{DC , and a voltage V 0} at the thermistor 15, which is divided by the resistor R ₁ and the thermistor 15, is applied to the non-inverting input of an operational amplifier 16.

CdS element and the like are used as a device, to determine the light output of the lamp.

In Fig. 7, a second embodiment of the warning generator is shown. A difference in that

Fig. 6 is that the photoelectric converter 9 (Fig. 3) for adjusting and stabilizing the exposure value is usually used as a means for detecting the lighting state, whereby the apparatus itself is simplified. In this arrangement, the inverting input of the operational amplifier 16 is connected to the output of the operational amplifier 11 in the control circuit of FIG. 3, so that the amplifier 16 has the output V ₁

which is used as a comparator. At the 15 received from the operational amplifier 11. The non-

inverting input of operational amplifier 16 is inverting input of operational amplifier 16 is

a reference voltage V _{b is} provided, which is determined by a voltage divider consisting of resistors R _s and A ₆

Voltage divider from a series circuit of resistor connected to which the reference potential V _REF for

would stand R, and R _{2 is} obtained. The output of the operational amplifier 12 in the control circuit

Operational amplifier 16 is applied across a resistor R _t 20, so that it em through the voltage divider

connected to the base of a switching transistor 17, the reference potential V ₁ . receives. This reference potential V _c

sen emitter is grounded. An indicator lamp 18 is in the will by appropriately selecting the values of the

Collector circuit of the transistor is provided and is used as resistors A ₅ and /? "To about 90% of the reference

Warning device used. potentials V _REF set. For the period of time

Bci of a fluorescent tube there is a corresponding 25 rend to which the DC voltage V _L , which is proportional

Relationship between the temperature at the tube wall and the change in light output. When the temperature at the tube wall of the fluorescent tube is low, the thermistor 15 has a high resistance and provides a high terminal voltage V _n .

When the light output of the fluorescent tube and the temperature on the tube wall rise, the Resistance of the thermistor and consequently its

the light output of the fluorescent tube 2 is lower than the reference potential V _c , the operational amplifier 16 generates the warning signal A1 with a high level, so that the lamp 18 lights up. When the potential V _L exceeds the reference potential V _c , the lamp 18 goes out. This mode of operation corresponds to the mode of operation of FIG. 8. For the period of time until the output power of the fluorescent tube reaches the output power which is approximately 90% of the reference potential V _REF

Connection voltage. If the values correspond to the resistors, that is, during the time during which R ₂ and R _{3 are} correctly selected, the - ^; - ™ —w <"'""- ^ wrc-ht ν .., - htPt fnlolirh Hip operational amplifier 16 a warning signal for the in
Fig. 5 (b) denoted, unstable time period. In
In this embodiment, the reference voltage V _{6 is} so

40

an astable condition prevails, the lamp 18 lights up to indicate to the user that the device has not yet been set to its steady state. Of course, the determined light output on the fluorescent tube can be compared with the output V _{L of} the operational amplifier 11 in the control circuit with a certain reference potential which has been set by means of the output K _{0 of} the operational amplifier of the next stage

is determined to be equal to the terminal voltage V _{0 of} the thermistor obtained when the luminous power of the fluorescent tube reaches about 90% of the steady state thereof.

During the period of time during which the light output is 45 immediately after switching on.

Fluorescent tube, d. H. During a rise time of As described above, with that in FIG

30 s, which is represented by the area "C" in FIG. 5 (b) or the circuit represented by the lighting up, is in a range from 0 to 90% of the stable state of the lamp to the user an astable period of time is due to low temperatures at the indicated, during which the light output does not reach the tube wall the connection voltage V ₀ at the 50 predetermined value, and after the lamp thermistor is higher than the reference potential V ₆ . As a result, the light power is extinguished by the control, the output of the operational amplifier high (H) circuit of Fig. 3 is kept exactly at the required value or it generates a warning signal A1, whereby it is kept. When the light output or power of the transistor 17 is switched on, so that the warning has stabilized, the exposure device can be used lamp 18 lights up continuously. When the light output becomes about 55 without the lighting unit's output exceeding 90% of its steady state and the direction having to be increased. This prevents the temperature on the tube wall from falling to the corresponding level

corresponding value comes, the connection voltage V _{0 is} lower than the reference potential V ₆ , whereby the voltage

hlß d Oi

the lighting fixture becomes an excessively high quality fixture, and the overall size of the fixture can be sized accordingly, and

voltage level at the output terminal of the operational equipment 60 its production costs are therefore reasonable,

amplifier 16 is switched to low (L). At the same time "'. *.. -. Α ... ei. The warning signal Al and the warning lamp 18 disappear
goes out.

Accordingly, if the user only uses the exposure device when the lamp 18 is not lit, 65 can be secured to a copying machine, the signal not only to the Al that excellent copies receive advertising warning, but also to be used to can Verden. Of course, other uses of the copier can also be forbidden if it is in an unstable state in light-receiving elements, such as a photodiode, ie the copier

In the above-described embodiments, an indicator lamp is used as a warning device; however, an acoustic signal generator such as a buzzer can also be used. aside from that

13th

drove to lock during such periods. This ^;

can be achieved by conventional means

Copying processes during the so-called »Aufwänn- fl

time of the fuser «and» when the paper closes. Φ

The end goes «to be prevented. A copying process 5 can therefore only be performed if the light output is kept constant or performance, so that thereby any erroneous copying are avoided. Although the exposure value control system provided with a warning io facility is equipped. is effective in the light output of a light source, which sharp rise characteristics, such as, for example, a fluorescent tube, the system can of course also with other discharge lamps, such as tungsten lamps, can be used.

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Claims (1)

Claim: 1. Device to keep the of one Fluorescent tube of a fiber optic supplied luminous flux for a copier, with a photoelectric Converter for generating an electrical signal corresponding to the luminous flux of the fluorescent tube, and with a control circuit, which picks up the electrical signal of the photoelectric to converter and a constant control of the Performs LichtslToms, characterized in that