Method for measuring the quantity of light emergent from an optical tip array and image forming apparatus provided with an optical tip array

A method for measuring the quantity of light emitted from an optical write head which drives an array of light shutter elements extending in a main scanning direction individually in accordance with image data. In order to measure the quantity of light outputted from each of the light shutter elements, a photosensor is moved in the main scanning direction at a constant speed while the light shutter elements are driven in such a way that adjacent elements are not driven at a time. While light shutter elements which are in odd numbers in the array are driven, the photosensor is moved forward to measure the quantities of light outputted therefrom, and while light shutter elements which are in even numbers in the array are driven, the photosensor is moved backward to measure the quantities of light outputted therefrom. The optical write head is employed in a printer together with a light-quantity measuring unit for the optical write head, so that light-quantity measurement and production of light-quantity correction data are possible at real time.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a method for measuring the quantity of
 light emergent from a light shutter array made of PLZT or emitted from an
 LED array and an image forming apparatus which forms an image
 (electrostatic latent image) on a recording medium with such an optical
 tip array.
 2. Description of Related Art
 Conventionally, various kinds of optical write heads which turn on and off
 light for each pixel with use of a light shutter array made of PLZT or an
 LED array have been used to form images (electrostatic latent images) on a
 silver-salt print sheet or film or an electrophotographic photosensitive
 member. For formation of images without unevenness, such an optical write
 head needs to be subjected to measurement of the quantity of light
 outputted from each element and to correction in quantity of light
 according to the measurement result.
 Japanese Patent Laid Open Publication No. 61-150286 has disclosed a method
 of measuring the quantity of light outputted from each element. According
 to this publication, while LEDs are turned on one by one in order in a
 main scanning direction, a light-quantity sensor which faces the LEDs at a
 specified distance therefrom is moved in the main scanning direction. In
 such a one-by-one lighting method, it is necessary to synchronize the
 turning-on/off of the LEDs with the movement of the sensor accurately, and
 minute control of the start position of the sensor is necessary.
 Accordingly, an encoder and an accurate shifting mechanism need to be
 provided. Also, from the result of measurement in this method, influence
 of light leaking from adjacent elements cannot be recognized, and in
 forming a thick image, a problem of unevenness is left unsolved.
 Further, for formation of a multi-tone image, it is necessary to measure
 the output light-quantity characteristic of each element accurately. It is
 insufficient to measure the quantity of light outputted from each element
 only when it is on as disclosed by the method of Japanese Patent Laid Open
 Publication No. 61-150286.
 Also, conventionally, an optical write head is fitted in a jig before it is
 employed in an image forming apparatus, and the quantity of light
 outputted from each element is measured with a light-quantity measuring
 device comprising a photosensor. Then, correction data are produced based
 on the results of the measurement. However, these correction data are
 effective only in the early time of the optical write head and are not
 useful when the output characteristic of each light element changes
 because of aging and/or a change in environmental conditions.
 Meanwhile, Japanese Patent Laid Open Publication No. 6-347925 has suggested
 an image forming apparatus which is provided with three optical write
 heads so as to form an image at a high speed. For this type of apparatus,
 it is more important to measure the quantity of light outputted from each
 element and carry out correction at real time. However, this is left
 unachieved.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide a method of measuring
 the quantity of light outputted from each element of an optical write head
 accurately in simple control and an optical write head which is subjected
 to measurement in this method.
 It is another object of the present invention to provide a method of
 measuring the quantity of light outputted from each element of an optical
 write head accurately enough such that the optical write head can be used
 for formation of multi-tone images.
 Another object of the present invention is to provide a full-color optical
 write head which is subjected to accurate measurement of the quantity of
 light of each color outputted therefrom in simple control.
 Another object of the present invention is to provide an image forming
 apparatus wherein an optical write head employed therein can be subjected
 to measurement of the quantity of light outputted from each element
 without being detached from the apparatus so that the apparatus can
 produce light-quantity correction data at real time.
 Further, another object of the present invention is to provide an image
 forming apparatus wherein a light-quantity sensor for measuring the
 quantity of light outputted from each element of an optical write head to
 make light-quantity correction data is also used to monitor the quantity
 of light outputted from the optical write head during actual image
 formation.
 Furthermore, another object of the present invention is to provide an image
 forming apparatus which can measure the quantity of light outputted from
 each of a plurality of optical write heads employed therein with a simple
 mechanism and produce light-quantity correction data at real time so as to
 form quality images at all times.
 In order to attain the objects above, according to the present invention,
 an optical write head which drives an array of optical elements extending
 in a main scanning direction individually in accordance with image data
 comprises control means which commands thin-out driving of the optical
 elements wherein adjacent elements are not driven at a time. In order to
 measure the quantity of light outputted from each of the optical elements,
 a light-quantity sensor with a light-receiving slit is moved in the main
 scanning direction with the light-receiving slit facing the array of
 optical elements while the optical elements are thin-out driven. Then, the
 quantity of light outputted from each of the optical elements is
 calculated from the output of the light-quantity sensor.
 According to the present invention, because light-quantity measurement is
 carried out during thin-out driving of the optical elements, addressing of
 the optical elements and light-quantity measurement can be carried out
 merely by moving the light-quantity sensor at a constant speed. It is not
 necessary to synchronize the drive of the optical elements with the
 movement of the sensor, and the control is simple. Also, since adjacent
 optical elements are not driven at a time, by measuring the quantity of
 light outputted from an undriven element, the quantity of light leaking
 from an adjacent driven element and entering thereto can be recognized.
 Thus, the quantity of light actually outputted from each element can be
 calculated accurately, which is effective to solve the problem of density
 unevenness in forming a thick image.
 If each optical element is driven at a specified frequency and a specified
 duty for the measurement so that a plurality of samplings from each
 element can be carried out, the address of each element can be recognized
 from the maximum value, and the maximum value is determined as the
 quantity of light outputted from the element. In this way, during one
 scan, the addresses of the optical elements can be recognized, and the
 quantities of light outputted therefrom can be measured accurately.
 Further, toward a full-color type optical write head which drives optical
 elements individually while switching the light color at a high speed, the
 light-quantity measurement with respect each color is carried out while
 the optical elements is thin-out driven under conditions (duty, voltage,
 current, etc.) suitable for the light color. Thereby, accurate measurement
 can be carried out. In this case, the light color is switched at a lower
 speed during the light-quantity measurement than the speed during actual
 image formation so that the light of each color can be turned on and off a
 plurality of times while the light-quantity sensor is detecting the
 quantity of light outputted from each of the optical elements. Thereby,
 during one scan, the quantities of light of each color at different levels
 outputted from each element can be measured, and the output light-quantity
 characteristic of each element can be measured more accurately, which is
 effective for formation of multi-tone images.
 Further, the present invention relates to a method of measuring the
 quantity of light emitted from an optical write device which drives an
 array of optical elements individually in accordance with image data, and
 the method comprises the steps of: addressing each of the optical elements
 based on the output of a photosensor which is moved in the main scanning
 direction during thin-out driving of at least one optical element wherein
 optical elements adjacent thereto are not driven simultaneously with the
 drive of the optical element; measuring the quantity of light outputted
 from each of the optical elements by moving the photosensor in the main
 scanning direction during all driving of optical elements wherein a
 plurality of serial optical elements are driven at a time under a
 specified condition; and determining the quantity of light outputted from
 each of the optical elements based on the addresses of the optical
 elements which have been recognized from the output of the photosensor
 during the thin-out driving and measured values during the all driving.
 According to the method, the addresses of the optical elements can be
 recognized from the output of the photosensor during the thin-out driving
 of the optical elements. For example, if the photosensor is operated
 during driving of every other optical element (optical elements which are
 on odd numbers in the array or optical elements which are on even numbers
 in the array), peaks of the output waveform of the photosensor indicate
 the addresses of the driven elements, and the addresses of the undriven
 elements can be figured out by uniformly dividing the intervals among
 peaks. Also, if the arrangement of the optical elements is inputted
 accurately into the control section beforehand, the addresses of all the
 optical elements can be recognized based on the peak of the output
 waveform of the photosensor during thin-out driving of at least one
 element. Further, according to the present invention, in addition to
 addressing the optical elements based on the output waveform during the
 thin-out driving of the optical elements, the photosensor is operated
 during all driving of the optical elements, and the quantity of light
 outputted from each of the optical elements is determined based on the
 output waveform during the all driving in consideration of the addresses
 of the optical elements.
 An image forming apparatus according to the present invention comprises: an
 optical write head which drives an array of optical elements extending in
 a main scanning direction individually in accordance with image data;
 transporting means which transports a recording medium in a sub scanning
 direction on a focal position of light emergent from the optical write
 head; a photosensor which receives the light emergent from the optical
 write head; and moving means which moves the photosensor in the main
 scanning direction.
 In the image forming apparatus, while the photosensor is moving in the main
 scanning direction, the quantity of light outputted from each optical
 element of the optical write head is detected by the photosensor, and
 simultaneously, light-quantity correction data are produced. Then, at a
 time of image formation, the light-quantity correction data are used.
 Thus, since a light-quantity measuring device is employed in the
 apparatus, it is possible to carry out light-quantity measurement and
 renew light-quantity correction data periodically or at any time, and it
 becomes possible to carry out light-quantity correction which copes with a
 change in output characteristic of each optical element because of aging
 and/or a change in environmental conditions.
 If the optical write head, a guide member of the recording medium, the
 photosensor and the moving means are assembled into a unit, it is possible
 to subject the unit to adjustment of the position of the optical write
 head and to light-quantity measurement before fitting the unit in the
 apparatus. Thereby, the productivity is improved, and with a simple
 positioning mechanism, the unit can be fitted in the apparatus accurately.
 In an image forming apparatus provided with a plurality of optical write
 heads, preferably, a number of photosensors same as the number of the
 optical write heads are provided, and these photosensors and moving means
 are employed in the apparatus as a unit. While the unit is moved in the
 main scanning direction, the quantities of light emergent from the optical
 write heads are measured, and light-quantity correction data are produced.
 Then, at the time of image formation, the data are used.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Some exemplary methods of measuring the quantity of light emitted from an
 optical write head and exemplary image forming apparatuses according to
 the present invention are described with reference to the accompanying
 drawings.
 First Embodiment
 Optical Write Head
 FIG. 1 shows an optical write head 20 which is used to write full-color
 images on a silver-salt print sheet. This optical write head 20 mainly
 comprises a halogen lamp 21, a heat insulating filter 22, a color
 correction filter 23, a light dispersing cylinder 24, an RGB filter 25, an
 optical fiber array 26, a slit plate 27, a light shutter module 30, an
 imaging lens array 35 and a dust protective glass 36.
 The heat insulating filter 22 cuts the infrared component of light emitted
 from the halogen lamp 21, and the color correction filter 23 controls the
 light in quality so that the light will match the spectral sensitivity
 characteristic of the print sheet. The light dispersing cylinder 24 is to
 improve the efficiency of use of light and to suppress the unevenness in
 quantity of light. The RGB filter 25 is driven to rotate in
 synchronization with image writing by the light shutter module 30, which
 will be described later, and transmits different colors line by line.
 The optical fiber array 26 is composed of a large number of optical fibers.
 The optical fibers are bundled at one end 26a, and the end 26a faces the
 dispersing cylinder 24 with the RGB filter 25 in-between. The optical
 fibers are aligned to extend in a main scanning direction indicated by
 arrow X at the other end 26b, so that light is emergent from the optical
 fiber array 26 linearly. The slit plate 27 has mirror surfaces 27a so as
 to guide the light emergent from the optical fiber array 26 to the light
 shutter module 30 efficiently. Further, a heater (not shown) is provided
 to the slit plate 27. The heater is to maintain the temperature of PLZT
 shutter tips of the light shutter module 30, and the temperature control
 is carried out in accordance with a detection result of a thermometer (not
 shown) provided to the module 30.
 The light shutter module 30 has an array of PLZT light shutter tips, each
 of which has shutter elements, on a slit of a ceramic substrate or on a
 glass substrate. Further, an array of driver ICs are provided in parallel
 to the array of PLZT light shutter tips. The light shutter elements are
 driven by the driver ICs in such a way that only specified pixels will be
 printed. Also, a polarizer 33 and an analyzer 34 are provided before and
 after the module 30, respectively. As is well known, PLZT is a ceramic
 which has an electrooptic effect at a large Kerr constant and is
 light-transmitting. The light is linearly polarized by the polarizer 33,
 and the polarization plane of the light rotates in accordance with
 turning-on and turning-off of electric fields caused by supplies of
 voltages to the light shutter elements. Consequently, the light emergent
 from the analyzer 34 is turned on and off part by part.
 The light emergent from the analyzer 34 is focused on the print sheet via
 the imaging lens array 35 and the dust protective glass 36, and thus, a
 latent image is formed on the print sheet. The print sheet is transported
 in a direction (sub scanning direction) perpendicular to the main scanning
 direction X at a constant speed.
 Light-Quantity Measuring Device and Measuring Method
 FIG. 2 shows a measuring device 70 which measures the quantity of light
 outputted from each light shutter element of the optical write head 20.
 In the measuring device 70, a measuring unit 71 which has a photoelectric
 conversion sensor 72 and a tool maker's microscope 77 is fitted to guide
 shafts 76 to be slidable thereon. The guide shafts 76 extend in parallel
 to the main scanning direction X of the light shutter module 30, and the
 measuring unit 71 reciprocates in the direction X at a constant speed
 while the sensor 72 is right above the light shutter array. Referring to
 FIG. 11, one of the guide shafts (76a) has a male screw on its
 circumference, and a nut (not shown) provided in the measuring unit 71
 engages with the male screw. Accordingly, the measuring unit 71 moves
 reciprocally following the forward/reverse rotation of the guide shaft
 76a.
 By the light incidence side of the sensor 72, a slit plate 73 and a light
 dispersing plate 74 are provided. The slit plate 73 has a slit 73a with a
 width which is 25% to 400% (preferably 50% to 200%) of the width of a
 pixel and is located on a focal plane F of the imaging lens array 35 of
 the optical write head 20. The sensor 72 has a spectral sensitive
 characteristic substantially equal to or larger than that of the print
 sheet.
 The tool maker's microscope 77 is integrated with a CCD camera 78. Each of
 the light shutter elements is photographed by the CCD camera 78 via the
 microscope 77 and displayed on a monitor display 79. The person in charge
 of this measurement adjusts the position of the optical write head 20
 minutely so that the light shutter elements at both ends of the light
 shutter array will be positioned correctly while watching the screen of
 the monitor display 79. The optical write head 20 is so mounted on a table
 (not shown) that the height and the inclination with respect to the sensor
 72 and the distance from the sensor 72 are adjustable.
 The light-quantity measuring device 70 and the optical write head 20 are
 controlled by a sequencer so that the reciprocal movement of the measuring
 unit 71 can be timed to measurement of the quantity of light outputted
 from each light shutter element. The optical write head 20 is driven in a
 mode (driving frequency, duty, on/off data) which has been programmed
 beforehand. The measuring device 70 integrates measured values with
 respect to each light shutter element in synchronization with the
 programmed drive. Usually, in consideration of the relationship between
 the driving frequency of the optical write head 20 and the driving speed
 of the sensor 72, ten and several times of samplings/holds from each light
 shutter element are carried out. The output of the sensor 72 is subjected
 to A/D conversion and is transmitted to a control section for necessary
 processing.
 The driving mode of the optical write head 20 for the measurement is
 determined in accordance with driving conditions of an apparatus which the
 optical write head is to be employed in. Here, a case wherein the optical
 write head 20 has a printing density of 400 dpi and is to be employed in a
 printer which is driven at a frequency of 1 kHz, that is, which has a
 system speed of 63.5 mm/s is described.
 First, while light shutter elements which are in odd numbers in the light
 shutter array are driven (turned on and off repeatedly), the sensor 72 is
 moved forward from an initial position which is outside a scanning area.
 The outputs of the sensor 72 during an on-period (1 msec) are integrated,
 and the integrated value is sampled/held and subjected to A/D conversion
 and is sent to the control section. If the sensor 72 is moved at a speed
 of 1 mm/s, since the printing density of the optical write head 20 is 400
 dpi (63.5 .mu.m), 63.5 times of samplings/holds from each light shutter
 element are carried out. After moving the sensor 72 by a distance a little
 longer than the main scanning length, data sending to the control section
 is stopped, and the sensor 72 is returned to the initial position. Next,
 while light shutter elements which are in even numbers in the light
 shutter array are driven (turned on and off repeatedly), the
 light-quantity measurement and data input are carried out in the
 above-described manner. In this way, all the light shutter elements are
 subjected to the light-quantity measurement. It is possible to carry out
 the light quantity measurement of the light shutter elements which are in
 even numbers during the return movement of the sensor 72, and rather, this
 is more efficient.
 An effective measuring way for light-quantity correction is to collect
 values from each light shutter element in four different conditions. The
 optical write head 20 is driven at different duties (high, middle, low and
 off). In this case, approximately 16 times of samplings/holds from each
 light shutter element in each condition of the four levels are carried out
 during one scan (a one-way movement of the sensor 72). It is possible to
 carry out the four-level light-quantity measurement during two to four
 scans. Also, the number of levels of the duty is not necessarily to be
 four.
 The number of samplings/holds from each light shutter element can be
 increased by lowering the speed of the sensor 72 or by heightening the
 driving frequency of the optical write head 20. It cannot be said that
 there is no change in quantity of light outputted from each light shutter
 element with a change in driving frequency of the optical write head 20.
 However, the change is small enough to be allowable. If the correlation
 between the driving frequency and the quantity of light is made clear
 beforehand, the change in quantity of light with a change in driving
 frequency can be handled by using a correction coefficient.
 If the optical write head 20 is a type which can write full-color images,
 such measurement must be carried out with respect to each light color (R,
 G and B). Accordingly, in this case, the above-described measuring process
 is repeated three times while the light color is switched by the RGB
 filter 25.
 In the control section, with respect to each light shutter element, the
 maximum value and the minimum value are found out from the inputted data,
 and the address of the element is figured out from these values. Then,
 with respect to each light shutter element, measured quantities of light
 at the four levels are expressed by approximate cubic curves, and
 correction coefficients at respective levels (for example, 0 through 255
 tone levels) are determined.
 Next, referring to FIGS. 3 and 4, the principle of the light-quantity
 measurement is described.
 First, a driving signal A is applied to light shutter elements which are in
 odd numbers in the light shutter array. The driving signal A has a
 frequency and a duty which are equal or nearly equal to the driving
 conditions of an apparatus which the optical write head 20 is to be
 employed in. Each light shutter element makes an output B, and the sensor
 72 which is moving forward in the main scanning direction X outputs a
 waveform indicating the output B. Values detected by the sensor 72 during
 an on-period is integrated, and after the on-period, the integrated value
 is sampled/held and subjected to A/D conversion.
 In the measurement, since the slit 73a which has a width substantially
 equal to the width of each light shutter element is moved in the main
 scanning direction X at such a speed as to carry out a plurality of
 samplings from each light shutter element, the output after the A/D
 conversion is as shown in FIG. 4. When the sensor 72 comes to a position
 exactly opposite a driven light shutter element 31, the sensor 72 detects
 the maximum quantity of light, and when the sensor 72 comes to a position
 between adjacent driven light shutter elements 31, the sensor 72 detects
 the minimum quantity of light. Accordingly, from the position of a peak of
 the output waveform of the sensor 72, the address of a driven light
 shutter element can be recognized. The minimum quantity of light detected
 between the driven light shutter elements 31 depends on the MTF of the
 imaging lens, the width of the slit 73a, etc. Although the position of an
 undriven light shutter element can be recognized from the position of a
 through of the output waveform of the sensor 72, it is possible to take an
 exact midpoint between adjacent peaks as the address of an undriven light
 shutter element.
 Next, while the driving signal A is applied to light shutter elements which
 are in even numbers in the light shutter array, the sensor 72 is moved
 backward to detect the quantities of light outputted from the light
 shutter elements. By superimposing the results with respect to the light
 shutter elements which are in even numbers on the results with respect to
 the light shutter elements which are in odd numbers, the output
 light-quantity characteristics of all the light shutter elements can be
 recognized.
 In the above-described measurement, since the positions of the light
 shutter elements are found out based on the output of the sensor 72, it is
 not necessary to monitor the position of the sensor 72 by use of an
 encoder and a detector for the initial position of the sensor 72. In this
 embodiment, the driving signal A is to drive each light shutter element to
 come to an on-state and an off-state alternately, and the sensor 72
 detects the quantities of light both in periods of the on-state and in
 periods of the off-state.
 The quantity of light in an off-period (the quantity of leakage light which
 means the quantity of light leaking from the halogen lamp 21) is measured
 in the same manner as that in an on-period. Specifically, values detected
 by the sensor 72 during the off-period is integrated, and after the
 off-period, the integrated value is sampled/held and subjected to A/D
 conversion. It is possible to find out the address of a light shutter
 element based on the outputs of the sensor 72 during the off-periods.
 However, because the outputs of the sensor 72 during the off-periods are
 small, the address of a light shutter element is determined from the
 position of the maximum output of the sensor 72 during the on-periods of
 the light shutter element, and an output during an off-period adjacent to
 the on-period when the sensor 72 outputs the maximum value is taken as the
 quantity of light in its off-state. It is preferred to determine the
 minimum quantity of light during the on-periods by using the following
 expression:
EQU quantity of light outputted from a light shutter element=maximum quantity
 of light measured from the element+(quantity of entering light-quantity of
 leakage light).times.correction coefficient
 In the above expression, the quantity of entering light means the quantity
 of light leaking from adjacent elements.
 The correction coefficient, which depends on the driving pattern of the
 light shutter elements and the width of the slit 73a, is within a range
 from 0.2 to 1.0. When line/dot images (sharp images) are to be formed, it
 is sufficient to carry out a light-quantity correction to each light
 shutter element only in accordance with the maximum quantity of light
 measured from the element. Further, in a case of reproducing one dot (a
 pixel), the light-quantity correction only in accordance with the maximum
 quantity of light is preferred.
 Although a pattern of driving every other light shutter element is adopted
 in the above-described measuring method, various driving patterns are
 possible.
 FIG. 5 shows an integrating circuit 80, and FIG. 6 shows a timing chart.
 The integrating circuit 80 is to measure the quantities of light outputted
 from each light shutter element at different four levels, at a high duty,
 at a middle duty, at a low duty and at an off level, and the integrating
 circuit 80 is composed of four lines although FIG. 5 shows only two of the
 four. A photoelectric signal outputted from the sensor 72 is inputted to
 an integrator 82 in one of the four lines via a polarity inverting
 amplifier 81. Further, the signal is sent to a sample/hold amplifier 84
 and is subjected to A/D conversion. The photoelectric signal is subjected
 to integration in the integrator 82 while an analog switch connected
 thereto is kept on in response to the corresponding one of signals AS-1
 through AS-4. The integrated value is held in the sample/hold amplifier 84
 when the corresponding one of signals SH-1 through SH-4 drops and is
 converted into a digital signal by an A/D converter (not shown).
 With this integrating circuit 80, the quantities of light outputted from
 each light shutter element at four levels can be measured during one scan.
 From these measured values, approximate expressions to indicate the light
 quantity characteristics of the light shutter elements are calculated, and
 a light quantity correction table is made based on the expressions. Thus,
 a suitable light-quantity correction can be made to each light shutter
 element for formation of a quality multi-tone image.
 Structure and Operation of the Driver ICs
 Now, the driver ICs which drive the optical write head 20 and its operation
 for the light-quantity measurement by use of the light quantity measuring
 device 70 are described.
 The thin-out driving of the light shutter elements for the light-quantity
 measurement can be realized by transmitting necessary data from the CPU to
 drive the optical write head 20. However, this function can be imparted to
 the driver ICs.
 FIG. 7 shows the structure of a driver IC 60 which is a type for formation
 of two-value images. For practical use, a plurality of driver ICs 60 are
 connected to each other by a ladder chain to drive 1024 shutter elements.
 Each driver IC 60 is to drive 64 light shutter elements, and comprises a
 shift register 61, a latch circuit 62, a gate circuit 63, a level shift
 circuit 64 and a driver circuit 65.
 Image data DATA(A) and DATA(B) are shifted into the shift register 61 based
 on a shift signal R/L in synchronization with a shift clock signal S-CLK
 and are latched in the latch circuit 62 controlled by a strobe signal STB.
 When a gate signal GATE is inputted to the gate circuit 63, signals
 D.sub.1 through D.sub.64 are sent to the driver circuit 65 via the level
 shift circuit 64. A driving voltage Vd is applied to the driver circuit
 65, and the driver circuit 65 outputs HV.sub.1 through HV.sub.64 to the
 shutter elements. The pulse widths of HV.sub.1 through HV.sub.64 are set
 in accordance with the signals D.sub.1 through D.sub.64 sent from the
 level shift circuit 64.
 In the driver IC 60, when the light-quantity measurement is commanded, a
 data signal DATA is sent to the shift register 61 in synchronization with
 the shift clock signal S-CLK and is latched in the latch circuit 62
 controlled by the strobe signal STB. Then, by driving the gate signal GATE
 at a specified duty, the light shutter elements can be driven to output a
 specified quantity of light. The thin-out driving of the light shutter
 elements can be realized by dividing the shift clock signal S-CLK and by
 outputting the image data DATA associated with the divided shift clock
 signal by use of an AND gate. If the shift clock signal S-CLK is divided
 into two (the frequency is made a half), every other light shutter element
 can be driven. A repetitive signal is used for the thin-out driving, and
 by using a plurality of gate signals GATE with different duties, the
 above-described light-quantity measurement can be carried out without a
 printer controller. If the light shutter elements are arranged in two
 lines staggeringly, that is, in a line of odd numbers and a line of even
 numbers, at least a volume of image data DATA covering the light shutter
 elements in one line are set at "H" level, sent to the shift register 61
 and latched in synchronization with the strobe signal STB. Thereafter, the
 above-described control is carried out. In this way, the drive of every
 other light shutter element can be realized more easily. Also, by
 providing a test input terminal to the gate circuit 63 to turn on the gate
 circuit forcibly and by sending a duty signal to the terminal, the
 light-quantity measurement can be carried out easily.
 The operation of the driver ICs 60 for the light-quantity measurement is
 described in more detail referring to FIGS. 8 and 9. A basal clock signal
 CLK is divided by a divider 67 into a shift clock signal S-CLK and a
 counter signal. A pulse period counter 68 is to determine a pulse period
 and produces the strobe signal STB. When the counter 68 counts up a
 specified time, a one-shot multi-vibrator is driven to output the strobe
 signal STB, and at the same time, the counter 68 is reset. Thereby, the
 strobe signal STB is outputted periodically. By making the time to be
 counted by the counter 68 variable, the duty becomes variable. A shift
 volume counter 69 is to determine the volume of data to be transmitted to
 the shift register 61. While the counter 69 is counting, the counter 69
 makes an output. The output from the counter 69 and the clock signal CLK
 are inputted to an and gate, and then, the shift clock signal S-CLK is
 outputted.
 The data signal DATA can be made from the clock signal CLK into various
 patterns. A signal for a drive of every other light shutter element can be
 produced by using a signal into which the clock signal CLK is divided to
 have a half frequency. Other patterns can be made by use of a simple logic
 circuit. In the structure wherein light shutter elements are arranged in
 two lines staggeringly, in a line of odd numbers and in a line of even
 numbers, the shift volume counter 69 is not necessary, and the circuit is
 simpler. In this case, the data signal DATA is kept at the level of "H" at
 all times so that the shift clock signal S-CLK is outputted continuously.
 Color Printer
 FIG. 10 is a schematic view of a color photoprinter. The color printer
 comprises a print sheet containing station 1, an image forming station 2
 and a processing station 3. A print sheet 4 is contained in the station 1
 in the form of a roll. In the image forming station 2, the optical write
 head 20 shown by FIG. 1 and the measuring unit 71 shown in FIG. 2 (the
 microscope 77 and the CCD camera 78 are omitted) are provided. Further, in
 the station 2, pairs of transport rollers 5, 6 and 7, a cutter 8 and
 transport guide plates 11 and 12 which are used for handling of the print
 sheet 4 are provided.
 The print sheet 4 is guided into the image forming station 2 through the
 transport rollers 5 with its photosensitive side facing down. When a
 specified length comes into the station 2, the rollers 5 are stopped, and
 the cutter 8 is driven to cut the print sheet 4. The cut piece of print
 sheet 4 is transported by the rollers 6 and 7 at a constant velocity. When
 the print sheet 4 passes over the optical write head 4, it is exposed to
 light coming through an opening made in the guide plate 11, whereby a
 latent image is formed on the print sheet 4. After the exposure, the print
 sheet 4 is subjected to development, fixation and drying in the processing
 station 3, and then, the print sheet 4 is discharged onto a tray 15.
 The print sheet 4 is transported to the exposure position in such a way
 that writing of the optical write head 20 can start at a writing start
 point of the print sheet 4, and during the writing, the print sheet 4 is
 transported at a constant velocity. However, transportation of the print
 sheet 4 at the exposure position does not have to be continuous but may be
 intermittent at a pitch corresponding to the density in the sub scanning
 direction.
 If the print sheet 4 is cut while the print sheet 4 is passing the exposure
 position, the coincidence between the start of writing of the optical
 write head 20 and the writing start point of the print sheet 4 may be
 damaged. In order to avoid this trouble, the print sheet 4 is cut while
 the sheet 4 is bent, or the exposure is started after a cut of the print
 sheet 4 although this necessitates a long print sheet transport path. When
 a roll of print sheet is used in the structure wherein a cut of the print
 sheet is carried out before exposure, in order to prevent the print sheet
 4 from being exposed unnecessarily, the lamp 21 of the optical write head
 20 is turned off, the output of the lamp 21 is reduced to such an extent
 as not to cause exposure of the print sheet 4, or the optical write head
 20 is shut mechanically by use of a shutter.
 As shown in FIG. 11, the measuring unit 71 is located opposite the optical
 write head 20 and is capable of reciprocate in the main scanning direction
 X with forward/backward rotation of the driving guide shaft 76a. The
 measuring unit 71 is controlled by a control section 91 and a sequencer
 92. The guide shaft 76a is driven by a reversible motor 93. Prior to
 exposure of the print sheet 4, the measuring unit 71 measures the quantity
 of light outputted from each light shutter element of the optical write
 head 20 in the above-described manner. Then, the measuring unit 71
 retreats from the print sheet transport path so as not to interfere the
 transportation of the print sheet 4 (see the alternate short and long dash
 line in FIG. 11). The measuring unit 71 is in the retreating position at
 all times other than the time of the light-quantity measurement.
 The guide plate 11 is so located that its guide surface 11' is on the focal
 surface F (see FIG. 12a) of the optical write head 20, and a focal shift
 never occurs even when a print sheet with a different thickness is used.
 The pairs of transport rollers 6 and 7 are controlled by a pulse motor to
 rotate at a constant velocity, and thereby, the sub scanning speed is kept
 constant. An upper guide surface 12 is provided to prevent a float of the
 print sheet and is pressed onto the print sheet by its own weight or by a
 spring or the like.
 The slit plate 73 of the measuring unit 71 is on the focal surface F of the
 optical write head 20, but as mentioned, the measuring unit 71 retreats
 from the print sheet transport path at all times other than the time of
 the light-quantity measurement.
 During the light-quantity measurement, light emergent from the optical
 write head 20 is incident to the sensor 72 through openings made in the
 guide plates 11 and 12. If the whole body or the light passing portion of
 the guide plate 11 is made of a light transmitting material such as an
 acrylic material, the opening is not necessary. If the guide plates 11 and
 12 have no openings, the guiding functions of the guide plates 11 and 12
 are improved. With respect to the upper guide plate 12, it can be
 structured to retreat from the guiding position in the time of the
 light-quantity measurement, and in this case, the opening is not
 necessary.
 As shown in FIG. 12b, it is possible to provide a lens 75 between the
 imaging lens array 35 and the slit plate 73. With the lens 75, the
 measuring unit 71 can be located apart from the focal surface F, and it
 becomes no longer necessary to make the measuring unit 71 retreat from the
 print sheet transport path in the time of exposure, thereby reducing the
 size of the apparatus. In this case, the upper guide plate 12 must be made
 of a light transmitting material.
 In this color printer, the RGB filter 25 of the optical write head 20 is
 rotated to switch the color of the light at a high speed, and line by
 line, images of R, B and G are written while the light shutter elements
 are turned on and off. This printer is usually powered on by a timer to
 carry out temperature control of the developer, etc. In this warm-up
 operation, the light-quantity measurement and the light-quantity
 correction (calibration) are carried out. The calibration, as described
 above, is a process to make correction to the light shutter elements of
 the optical write head 20 in quantity of light in accordance with the
 results of measurement under conditions of substantially the same as
 actual exposure, and thereby, quality images can be obtained.
 In a case of a full-color printer, first, only light shutter elements which
 are on odd numbers in the light shutter array are driven at a specified
 frequency (depending on the image density in the sub scanning direction)
 to output a specified quantity of light (duty or intensity), and the color
 of the light is switched in synchronization with the drive. Meanwhile, the
 measuring unit 71 is moved forward to measure the quantities of light
 outputted from the light shutter elements at times of emitting RGB colors
 at different duties. The measuring unit 71, while moving backward,
 measures the quantities of light outputted from light shutter elements
 which are on even numbers at times of emitting RGB colors at different
 duties in the same manner.
 In order to make an accurate light-quantity correction to each light
 shutter element, it is effective to measure the quantities of light at
 four levels including the quantity of light in an off-state (quantity of
 leakage light). During the measurement, the color switching speed is
 reduced to one fourth of the speed for actual image formation, and with
 respect to each color, the quantities of light at four levels are
 measured. Twelve kinds of quantities of light (RGB.times.4) outputted from
 each light shutter element are measured during one scan. Integrated values
 of photoelectric outputs of the sensor 72 are sampled/held and subjected
 to A/D conversion, and in the control section, an approximate output
 light-quantity characteristic curve is made based on the values at the
 four levels. Then, the light-quantity correction is carried out referring
 to the curve. The light quantity correction is carried out based on the
 light shutter element which has a minimum measured value. Data for the
 correction are stored in a memory for a look-up table (for example, a
 flash ROM).
 Further, the color switching speed during the measurement may be equal to
 that for actual image formation. In this case, for measurement of the
 quantities of light at a plurality of levels with respect to each color,
 the driving frequency is heightened. Furthermore, if both the color
 switching speed and the driving frequency during the measurement are equal
 to those for actual image formation, the quantities of light at a
 plurality of levels are measured during a plurality of scans.
 In the first embodiment, during one reciprocate scan, all the light shutter
 elements are subjected to the light-quantity measurement. However, it is
 possible to divide the measurement according to levels and colors. In this
 case, the number of scans for the measurement is increased, thereby
 consuming time, but it has an advantage that the integrating circuit can
 be simplified.
 The number of levels of the quantity of light to be measured depends on the
 output characteristics of the light shutter elements. If the light shutter
 elements have output characteristics of good linearity, measurement of the
 quantities of light at two levels is practical. Further, if the quantity
 of leakage light is zero, the quantity of light at one level is practical.
 However, the outputs of light shutter elements generally do not have ideal
 linearity, and measurement of the quantities of light at four levels is
 practical to any element. With respect to the light colors, if the light
 shutter elements have the same output characteristic in outputting light
 of any of the colors, measurement with respect to only one color is enough
 for correction. Also, if there is such small differences in output
 characteristic among the light colors as to be allowable, only measurement
 with respect to green or white is sufficient.
 Light shutter elements made of PLZT change their light transmitting
 characteristics according to the driving voltage applied thereto.
 Therefore, preferably, during the measurement, a driving voltage with the
 same waveform as that of the driving voltage for actual image formation is
 applied to the light shutter elements. A specific way is to apply a
 voltage which is optimal for blue exposure to the light shutter elements
 for the light-quantity measurement with respect to blue and the other
 colors (red and green). Another way is to apply voltages which are optimal
 for exposures of the colors to the light shutter elements for the
 measurement with respect to the respective colors. In the first
 embodiment, the driving voltage must be changed at a high speed, thereby
 causing rounding of the waveform of the driving voltage. Therefore, in the
 first embodiment, it is preferred that identical power sources or a single
 power source are/is used for the measurement and for actual image
 formation.
 In the above-described light-quantity measuring method, the address of each
 light shutter element is determined based on the output of the sensor 72
 without using any special devices for determination on the address.
 Therefore, when the measuring device 70 is used to test an optical write
 head, by counting the number of samplings between peaks of the output
 waveform, trouble (pitch error, errors in alignment of the light shutter
 elements, etc.) of the optical write head can be detected. Also, when the
 measuring unit 71 is employed in a printer provided with an optical write
 head, by counting the number of samplings between peaks, abnormal movement
 of the measuring unit 71 can be detected. In case of abnormal movement of
 the measuring unit 71, the abnormality is displayed and warned, and the
 printer is stopped. Further, when the measuring unit 71 is employed in a
 printer, light-quantity correction which copes with aging of the light
 shutter elements becomes possible.
 Image data read by a film scanner are unfolded on a bit map memory of the
 image memory. Corrections are made to the data on the bit map memory
 referring to the look-up table which is stored with light-quantity
 correction data, and the corrected data are transmitted to the driver of
 the light shutter module 30. Thus, an image with a density equal to that
 of the original image can be reproduced while the light color is switched
 at a specified speed.
 Further, it is possible to carry out the light-quantity measurement and the
 light-quantity correction at any time as well as the time of warm-up
 operation of a printer.
 Furthermore, the first embodiment can be so modified that light emergent
 from the optical write head 20 is partly directed to the sensor 72 of the
 measuring unit 71 which is in the retreating position indicated by the
 alternate long and short dash line in FIG. 11 while the optical write head
 20 is operating for actual image formation. More specifically, a dummy
 light shutter element is additionally provided to the light shutter module
 30, and the output light of the dummy light shutter element which is
 driven under specified conditions (duty and frequency) is directed to the
 sensor 72. It is also possible to direct light emitted from the halogen
 lamp 21 to the sensor 72 via an optical guide fiber (not shown). In such a
 structure, the quantity of light outputted from the optical write head 20
 during image formation can be monitored, and by carrying out correction or
 raising an alarm comparing the monitored value with a reference value,
 image formation can be stabilized.
 In the printer, some kinds of print sheets with various widths can be set,
 and the optical write head 20 is capable of writing an image on a print
 sheet with the maximum width. Accordingly, the light-quantity measurement
 is not always carried out toward all the light shutter elements but is
 carried out toward only the light shutter elements which are to be used
 for image formation on the print sheet set in the sheet containing
 section. More specifically, the length of the movement of the sensor 72 in
 the main scanning direction is controlled in accordance with the width of
 the print sheet. With this arrangement, the measurement time and the
 calculation time can be shortened.
 Structure and Operation of the Driver ICs
 FIGS. 13 and 14 show the structure of a driver IC 40 for multi-tone image
 formation and the timing chart of its operation. For practical use, a
 plurality of driver ICs 40 are connected to each other by a ladder chain
 to drive 1024 shutter elements. Each driver IC 40 is to drive 64 light
 shutter elements, and comprises a six-bit shift register 41, a six-bit
 latch circuit 42, a six-bit comparator 43, a six-bit counter 44, a gate
 circuit 45 and a driver circuit 46.
 Image data DATA(A) and DATA(B) are shifted into the shift register 41 based
 on a shift signal R/L in synchronization with a shift clock signal S-CLK
 and are latched in the latch circuit 42 controlled by a strobe signal STB.
 Thereby, the tone level of each pixel is set. The counter 44 counts the
 clock signal C-CLK, and the comparator 43 compares the counter value with
 the latched value. When the both values become equal, the gate circuits 45
 stops the output. The counter 44 is cleared on receiving a clear signal
 CL.
 A driving voltage Vd is applied to the driver circuit 46, and the driver
 circuit 46 outputs HV.sub.1 through HV.sub.64 to the shutter elements. The
 pulse widths of HV.sub.1 through HV.sub.64 are set in accordance with
 signals D.sub.1 through D.sub.64 sent from the gate circuit 45. Thus, each
 light shutter element is turned on for a time (pulse width) in accordance
 with image data DATA for the corresponding pixel.
 Control for the light-quantity measurement toward an optical write head
 with the multi-tone driver ICs 40 is basically similar to the control of
 the two-value driver ICs 60. A specified quantity of light to be outputted
 from each light shutter element is commanded by a data signal DATA by use
 of a dip switch or the like. The data signal DATA is sent to the shift
 register 41 and latched controlled by the strobe signal STB, and a duty in
 accordance with the data signal DATA is produced in the comparator 43.
 Then, specified light shutter elements are driven to output the specified
 quantity of light controlled by a gate signal GATE. Such signals for
 thin-out driving are repetitious signals and are produced in a
 comparatively simple circuit.
 In the structure wherein the light shutter elements are arranged in two
 lines staggeringly, that is, in a line of odd numbers and in a line of
 even numbers, the thin-out driving can be carried out by setting the data
 signal DATA to be sent to one of the lines at "H" level, which is simpler
 control. In order to vary the quantity of light to be outputted from each
 light shutter element, the setting of the dip switch is changed.
 Second Embodiment
 The second embodiment embodies a method of finding out the address of each
 light shutter element and measuring the quantity of light outputted
 therefrom. In the second embodiment, the optical write head and the
 measuring unit shown by FIGS. 1 and 11 are used. Therefore, in the
 following description, the members of the optical write head and the
 measuring unit are provided with the same reference symbols as those in
 the first embodiment.
 For accurate light-quantity measurement, it is better to measure the
 quantity of light outputted from each light shutter element of an optical
 write head while the element and nearby elements are driven at the same
 time (all driving) than to measure the same while the elements are driven
 one by one in order because the measurement result in the former includes
 the quantity of light leaking and entering from the nearby elements.
 However, in the former (all driving), it is difficult to find out the
 address of the element which is subjected to the measurement, and
 therefore, in the second embodiment, thin-out driving is also adopted to
 address each element.
 FIGS. 15a and 15b show a first example of the addressing and measuring
 method. FIG. 15a shows an output waveform of the sensor 72 while only the
 light shutter elements which are on odd numbers in the light shutter array
 are driven (thin-out driving), and based on the output waveform, the
 respective addresses of all the light shutter elements are determined.
 FIG. 15b shows the output waveform of the sensor 72 while all the light
 shutter elements are driven (all driving). The position of the first light
 shutter element 31.sub.1 is apparent from the rising characteristic or the
 first peak of the output waveform at the time of thin-out driving, and
 accordingly, the addresses of the other elements are determined. Then, the
 quantity of light outputted from each element is determined based on the
 output waveform of the sensor 72 at the time of all driving in
 consideration of the address recognized from the output waveform at the
 time of thin-out driving.
 FIG. 16 shows a second example of the addressing and measuring method. In
 the second example, during one scan of the sensor 72, both the addressing
 and the light-quantity measurement are carried out. The sensor 72 is moved
 in the main scanning direction X for the measurement while all the light
 shutter elements other than the second element 31.sub.2 are driven. The
 lower part of FIG. 16 shows the output waveform of the sensor 72 at this
 time. In this case, the first element 31.sub.1 is in the state of thin-out
 driving, and the position of the first light shutter element 31.sub.1 is
 apparent from the rising characteristic or the peak of the output
 waveform, and the quantity of light of each of the third and succeeding
 elements is determined based on the output waveform. In this example, only
 the third and succeeding elements are used for actual image formation.
 This second example is usable only when information on the positions of
 the respective light shutter elements is available from an output waveform
 of the sensor 72 at the time of thin-out driving which has been obtained
 beforehand or from designed values.
 FIGS. 17a and 17b show a third example of the addressing and measuring
 method. In the third example, color sensors 72R, 72G and 72B are used to
 measure the quantity of red light, the quantity of green light and the
 quantity of blue light, respectively, outputted from each light shutter
 element. Thin-out driving and all driving of the light shutter elements
 are carried out in the same way as the first example shown by FIGS. 15a
 and 15b. Each of the sensors 72R, 72G and 72B outputs waveforms as shown
 by 17a and 17b at the time of thin-out driving and at the time of all
 driving, respectively. From the measurement results with respect to each
 of the colors, the address of each light shutter element and the quantity
 of light outputted therefrom are determined. In this example, white light
 is incident to the light shutter elements, and a color correction filter
 (not shown) is provided to each of the sensors 72R, 72G and 72B. It is
 possible to determine the address of each element from the measurement
 results by use of either one of the sensors 72R, 72G and 72B.
 FIG. 18 shows a fourth example of the addressing and measuring method. In
 the fourth example, six serial light shutter elements 31.sub.1 through
 31.sub.6 and the tenth elements 31.sub.10 are driven at a time, and all
 the elements are driven in the same pattern in order. While the elements
 are driven in this way, two sensors 72a and 72b which are located apart
 from each other at a distance of five elements are moved together in the
 main scanning direction X to measure the quantity of light outputted from
 each element. The lower part of FIG. 18 shows the output waveforms of the
 sensors 72a and 72b. In this case, the elements 31.sub.1 through 31.sub.5
 are in the state of all driving, and the tenth element 31.sub.10 is in the
 state of thin-out driving. The position of the tenth element 31.sub.10 is
 apparent from the rising characteristic or the peak of the output waveform
 of the sensor 72a, and the addresses of the elements 31.sub.1 through
 31.sub.5 can be figured out from the position of the tenth element
 31.sub.10. Then, the quantity of light outputted from each of the elements
 31.sub.1 through 31.sub.5 is determined based on the output waveform of
 the sensor 72b. Next, while the elements 31.sub.5 through 31.sub.10 and
 31.sub.14 are driven, the measurement is carried out by use of the sensors
 72a and 72b, and in the same way, the address of each of the elements
 31.sub.6 through 31.sub.9 and the quantity of light outputted therefrom
 are determined. Then, while the sensors 72a and 72b are moved in the main
 scanning direction X, the quantities of light of all the light shutter
 elements are determined in the same manner. Needless to say, various
 driving patterns (combination patterns of thin-out driving and all
 driving) are possible as well as the pattern shown by FIG. 18.
 FIG. 19 shows a fifth example of the addressing measuring method. In this
 fifth example, an addressing sensor 72a and color sensors 72R, 72G and 72B
 are used. The light shutter elements are driven in the same pattern as
 described in the fourth example, and the sensors 72a, 72R, 72G and 72B are
 moved in the main scanning direction X to measure the quantity of light
 outputted from each element. As in the third example, white light is
 incident to the light shutter elements, and a color correction filter (not
 shown) is provided to each of the color sensors 72R, 72G and 72B. The
 address of each element is determined based on the output waveform of the
 sensor 72a, and the quantity of light outputted from each element is
 determined based on the output waveforms of the sensors 72R, 72G and 72B.
 Third Embodiment
 FIGS. 20 and 21 show the main part of the third embodiment. The third
 embodiment is a color printer which forms an image by use of a
 photosensitive drum 100 in an electrophotographic method. The
 photosensitive drum 100 is driven to rotate in a direction indicated by
 arrow "a", and the optical write head 20 shown by FIG. 1 is used to write
 an image. Around the photosensitive drum 100, there are provided an
 electric charger, a developing device, a transfer charger, etc. These
 devices are well known, and the descriptions thereof are omitted.
 The light-quantity measuring unit 71 adopted in the third embodiment is
 basically of the same structure as the one shown by FIG. 11. However, a
 prism 165 for deflecting light (a mirror or any deflecting member) is
 additionally provided. The measuring unit 71 is initially in a retreating
 position outside an image forming area D (see FIG. 21) of the
 photosensitive drum 100 and is capable of moving from the retreating
 position and reciprocating in the main scanning direction X with rotation
 of the guide shaft 76a driven by the motor 93 (see FIG. 11). For
 light-quantity measurement, light emergent from the optical write head 20
 is reflected by the prism 165 and incident to the sensor 72 through the
 slit plate 73 and the light dispersing plate 74. The light-quantity
 measurement by use of the sensor 72 is carried out in the same way as
 described referring to FIGS. 3, 4 and 5 or as described in the second
 embodiment. The drive of the optical write head 20 during the measurement
 is the same as described in the first embodiment.
 The printer of the third embodiment can be so structured that while the
 optical write head 20 is writing an image, the quantity of light emergent
 from the optical write head 20 can be monitored by the measuring unit 71
 which is in the retreating position. In this case, a dummy light shutter
 element which is additionally provided to the optical write head 20 is
 driven under specified conditions (duty and frequency), and the light
 outputted from the dummy element is directed to the sensor 72. It is also
 possible to direct light emitted from the halogen lamp 21 to the sensor 72
 via an optical guide fiber (not shown).
 In the third embodiment, the optical write head 20, the prism 165, the
 measuring unit 71 and its reciprocating mechanism form an exposure unit
 110. Like the first embodiment, by adjusting the focal position of the
 optical write head 20, etc. before fitting the exposure unit 110 in a
 frame 120 of the printer, adjustment after the fitting can be omitted.
 FIG. 22 shows a measuring device 130 for adjustment of the exposure unit
 110 before assembly, and the device 130 corresponds to the measuring
 device 70 shown in FIG. 2. In the measuring device 130, a tool maker's
 microscope 132 provided with a CCD camera 133 is disposed in an adjusting
 jig 131 in such a way to be capable of moving in the main scanning
 direction along a guide shaft 134, and images photographed by the CCD
 camera 133 are displayed on a monitor display 135. The adjusting jig 131
 has a fitting surface 131a which is identical to the fitting surface 120a
 of the frame 120 (see FIGS. 20 and 21), and the exposure unit 110 is
 fitted on the fitting surface 131a. The microscope 132 has a focal point
 132a at a position corresponding to the light receiving surface of the
 photosensitive drum 100, and when the optical write head 20 is set in a
 specified position, light emergent therefrom is imaged on the focal point
 132a.
 While the optical write head 20 is driven and the microscope 132 is moved
 in the main scanning direction, inclination is adjusted so that the light
 comes to the center of the field of view, and focusing of the optical
 write head 20 is carried out by moving the optical write head 20 in a
 direction indicated by arrow "b" in the exposure unit 110. For the
 focusing of the optical write head 20, it is possible that, during a
 thin-out drive of the light shutter elements, the optical write head 20 is
 automatically moved in accordance with the output of the CCD camera 133.
 Next, the position of the sensor 72 is adjusted by moving the whole
 measuring unit 71 in a direction indicated by arrow "c". This adjustment
 is carried out so that the output signal-to-noise ratio of the sensor 72
 will be a maximum. Generally, the position of the sensor 72 hardly shifts,
 and this adjustment can be omitted.
 Further, the measuring unit 71 which is additionally provided with a
 deflecting member can be employed in an apparatus which forms an image on
 a print sheet, a film or the like as well as a printer which forms an
 image on a photosensitive drum by an electrophotographic method.
 Fourth Embodiment
 FIG. 23 is a schematic view of a color photoprinter. The color printer
 comprises a print sheet containing station 201, an exposure station 202
 and a processing station 203. A print sheet 204 is contained in the
 station 201 in the form of a roll and is drawn to a loop forming stage 207
 guided by guide rollers 205 and 206. In the exposure station 202, three
 optical write heads 220 (220B, 220G and 220R) shown by FIG. 24 and a
 light-quantity measuring unit 271 shown by FIG. 25) are provided.
 The print sheet 204 is transported to the right in FIG. 23 by pairs of
 transport rollers 208 and 209 guided by a guide plate (not shown) with its
 photosensitive side facing down. The print sheet 204 is exposed to lights
 emergent from the optical write heads 220, and thus, a latent image is
 formed thereon. After the exposure, the print sheet 204 is subjected to
 development and is cut into a specified size in the processing station 203
 and is discharged onto a tray 210. Lights of the three primary colors,
 namely, blue (B), green (G) and red (R) are emergent from the three
 optical write heads 220B, 220G and 220R, respectively, in synchronization
 with the transport of the print sheet 204 to form a full-color image on
 the print sheet 204.
 Each of the optical write heads 220, as shown in FIG. 24, is basically of
 the same structure as the optical write head 20 shown by FIG. 1. The same
 parts and members are provided with the same reference symbols, and the
 descriptions thereof are omitted. What is different from the optical write
 head 20 is to omit the RGB filter 25 and to have a color separation filter
 28 instead of the color correction filter 23. In the optical write heads
 220, white light emitted from the halogen lamp 21 is separated into blue,
 green and red by the color separation filter 28. Further, a mechanical
 shutter (not shown) is provided to each of the optical heads 220 so as to
 prevent light leakage from the halogen lamp 21 in an undriven state. It is
 possible to provide a color correction filter or an ND filter at the light
 source section.
 Next, the light-quantity measuring unit 271 for measuring the quantity of
 light outputted from each light shutter element of the optical write heads
 220B, 220G and 220R is described.
 The measuring unit 271, as shown in FIG. 25, three photoelectric conversion
 sensors 272 (272B, 272G and 272B) in a casing 277, and the casing 277 is
 capable of sliding on guide shafts 276a and 276b. The guide shafts 276a
 and 276b extend in a main scanning direction, and the measuring unit 271
 reciprocates in the main scanning direction at a constant speed with the
 sensors 272B, 272G and 272R located right above the light shutter arrays
 of the optical write heads 220B, 220G and 220R respectively. The guide
 shaft 276a is connected to a motor 278. The guide shaft 276a has a male
 screw on the circumference, and a nut (not shown) provided to the casing
 277 is in engagement with the male screw.
 Usually, the measuring unit 271 is in a retreating position shown by the
 solid line in FIG. 25, that is, outside a main scanning area of the
 optical write heads 220. With forward/reverse rotation of the guide shaft
 276a driven by the motor 278, the measuring unit 271 reciprocates in the
 main scanning direction to measure the quantity of light emitted from the
 optical write heads 220B, 220G and 220R. During the light-quantity
 measurement, the print sheet 204 must not exist in the exposure station
 202. Therefore, the light-quantity measurement is carried out at a time of
 loading a new print sheet 204 or at a time of warm-up operation of the
 printer. Otherwise, prior to the light-quantity measurement, the transport
 rollers 208 are reversed to transport the print sheet 204 back to the loop
 forming stage 207.
 The light-quantity measurement toward the optical write heads 220 by use of
 the measuring unit 271 is carried out in the same way as described
 referring to FIGS. 3, 4 and 5 or as described in the second embodiment.
 In the fourth embodiment, the sensors 272B, 272G and 272R which are to
 measure the quantities of light emergent from the optical write heads
 220B, 220G and 220R respectively are encased in the casing 277 to form a
 unit 271, and the unit 271 is driven by the motor 278. Thus, the mechanism
 for the light-quantity measurement is simplified and can be employed in a
 printer at low cost. Also, the use of three sensors 272B, 272G and 272R
 enables the light-quantity measurement toward the three optical write
 heads 220B, 220G and 220R to be carried out simultaneously during one
 scan, whereby the time for the measurement and production of correction
 data can be shortened.
 Further, it is not always necessary to carry out the light-quantity
 measurement by use of the sensors 272B, 272G and 272R simultaneously. The
 fourth embodiment can be so structured that the sensors 272B, 272G and
 272R are operated one by one. In the structure, during one scan, the
 light-quantity measurement and production of light-quantity correction
 data are carried out with respect to only one of the optical write heads
 220B, 220G and 220R. In this case, although it takes more time for the
 measurement and the data production, only a single circuit is necessary
 for the measurement and the data production, thereby resulting in low
 cost.
 FIG. 26 shows a case wherein a single sensor 272 is used for the
 light-quantity measurement toward the three optical write heads 220B, 220G
 and 220R. The sensor 272 is fitted in the casing 277 in such a way to be
 capable of sliding in a sub scanning direction (indicated by arrow "Y"),
 so that the sensor 272 can be positioned above any of the light shutter
 arrays of the optical write heads 220B, 220G and 220R.
 Fifth Embodiment
 FIG. 27 shows the structure of the fifth embodiment. The fifth embodiment
 is a color photoprinter like the fourth embodiment shown in FIG. 23. The
 same members and parts as those in the fourth embodiment are provided with
 the same reference symbols, and the descriptions of these members are
 omitted. In the fifth embodiment, a guide drum 401 is provided in an
 exposure station 400.
 More specifically, a print sheet 204 is held on the circumference of the
 guide drum 401 pressed by pressing rollers 405 and 406, and the print
 sheet 204 is transported in a sub scanning direction in accordance with
 rotation of the guide drum 401 in a direction indicated by arrow "c". The
 optical write heads 220B, 220G and 220R are arranged along the curvature
 of guide drum 401.
 As shown in FIG. 28, a light-quantity measuring unit 720 is located inside
 the guide drum 401. In the measuring unit 720, sensors 722B, 722G and 722R
 are fitted in a casing 721 at positions opposite the optical write heads
 220B, 220G and 220R respectively, and the casing 721 is movable in a main
 scanning direction along guide shafts 723a and 723b. In the guide drum
 401, slits 402b, 402g and 402r for light transmission are made.
 Light-quantity measurement and production of light-quantity correction
 data are carried out in the same way as described in the first embodiment
 or as described in the second embodiment.
 The fifth embodiment has the advantages of the fourth embodiment.
 Additionally, because the print sheet 204 is transported around the
 circumference of the guide drum 401, the transportation of the print sheet
 204 during exposure is stabilized, thereby improving the accuracy of
 registration. Further, in the fifth embodiment, it is not necessary to
 retract the measuring unit 720 from a main scanning area of the optical
 write heads 220.
 FIG. 29 shows a case wherein a single sensor 722 is used for the
 light-quantity measurement toward the three optical write heads 220B, 220G
 and 220R. For the light-quantity measurement, the sensor 722 of the
 measuring unit 720 is positioned opposite a slit 402 made in the guide
 drum 401, and the measuring unit 720 and the guide drum 401 are driven to
 rotate so that the sensor 722 will face to directions B, G and R, that is,
 face the optical write heads 220B, 220G and 220R via the slit 402.
 A further possible structure is that a single sensor 722 is fixed inside
 the guide drum 401 whose inner surface is made as a light dispersing
 surface or a mirror surface so that the sensor 722 can measure the
 quantity of light which is emergent from each of the optical write heads
 220B, 220G and 220R and incident into the guide drum 401 through the slit
 402.
 Also, in the fifth embodiment, if the guide drum 401 is made of a light
 transmitting material, the slits 402, 402b, 402g and 402r are not
 necessary.
 Sixth Embodiment
 FIG. 30 shows the sixth embodiment. In the sixth embodiment, a
 light-quantity measuring unit 730 which has prisms 732B, 732G and 732R for
 deflecting light and sensors 733B, 733G and 733R is provided in the
 exposure station 400 of the fifth embodiment shown in FIG. 27. A casing
 731 which has the shape of a shadowed part in FIG. 30 is driven by a motor
 (not shown) to be reciprocally movable in a main scanning direction of the
 optical write heads 220 along guide shafts 734a and 734b. The casing 731
 is in a position outside a main scanning area at all times other than the
 time of light-quantity measurement.
 For the light-quantity measurement, the casing 731, the prisms 732 and the
 sensors 733 move in the main scanning direction. Because the prisms 732B,
 732G and 732R move along the light shutter arrays of the optical write
 heads 220B, 220G and 220R, the lights emergent from the light shutter
 arrays 220B, 220G and 220R are incident to the sensors 733B, 733G and 733R
 respectively.
 Seventh Embodiment
 FIG. 31 shows the seventh embodiment. The seventh embodiment is an
 electrophotographic printer which is provided with three optical write
 heads 220X, 220Y and 220Z. The optical write heads 220X, 220Y and 220Z are
 so arranged that the respective light shutter arrays of adjacent optical
 write heads shift from each other in a main scanning direction by an
 amount of 1/3 of a pixel. The electrophotographic printer forms a latent
 image on a photosensitive belt 301 with the optical write heads 220X, 220Y
 and 220Z, develops the latent image and transfers the image onto a sheet S
 transported in a direction indicated by arrow "d".
 Because of the shift of the optical write heads 220X, 220Y and 220Z in the
 main scanning direction, only with low-density optical tips, images with a
 high density, which can be realized by multiplying the performance of the
 optical tips by the number of optical write heads, can be formed. Based on
 the results of light-quantity measurement, suitable positioning of the
 optical write heads and suitable light-quantity correction are carried
 out.
 The photosensitive belt 301 is an endless belt held by guide rollers 302
 and 303 and is driven to rotate in a direction indicated by arrow "e".
 Around the photosensitive belt 301, there are provided an electric charger
 311, a developing device 312, a transfer roller 313 and a belt cleaner
 314. Like the sixth embodiment shown by FIG. 30, a light-quantity
 measuring unit 730 for measuring the quantities of light emitted from the
 optical write heads 220X, 220Y and 220Z in an exposure station 310. The
 measuring unit 730 is of the same structure as the one shown in FIG. 30,
 and the description thereof is omitted.
 Eighth Embodiment
 As FIG. 32 shows, the eighth embodiment is an electrophotographic printer
 which is similar to the seventh embodiment. In the eighth embodiment, the
 photosensitive belt 301 is held by three guide rollers 305, 306 and 307
 and is driven to rotate in a direction of arrow "e". Around the
 photosensitive belt 301, there are provided an electric charger 311, a
 developing device 312, a transfer roller 313, a belt cleaner 314. The
 exposure station 320 is located in a flat portion of the photosensitive
 belt 301, and the optical write heads 220X, 220Y and 220Z are located as
 the optical write heads 220B, 220G and 220 of the fourth embodiment are
 (see FIG. 23). Further, the optical write heads 220X, 220Y and 220Z are so
 arranged that the respective light shutter arrays of adjacent optical
 write heads shift from each other in a main scanning direction by an
 amount of 1/3 of a pixel as those of the seventh embodiment.
 Because of the shift of the optical write heads 220X, 220Y and 220Z in the
 main scanning direction, even with low-density light shutter tips,
 formation of high-density images becomes possible.
 As the light-quantity measuring unit 730 employed in the sixth and seventh
 embodiments does, a light-quantity measuring unit 740 has prisms 742X,
 742Y and 742Z for directing lights emergent from the optical write heads
 220X, 220Y and 220Z to sensors 743X, 743Y and 743Z respectively. A casing
 741 which has the shape of a shadowed part in FIG. 32 is driven by a motor
 (not shown) to be reciprocally movable in the main scanning direction of
 the optical write head 220 along guide shafts 744a and 744b. The casing
 741 is in a position outside a main scanning area at all times other than
 the time of light-quantity measurement.
 Other Embodiments
 As well as the PLZT light shutter array, LEDs (light emitting diodes), LCSs
 (liquid crystal shutters), a DMD (deformable mirror device), an FLD
 (fluorescent device), etc. can be used as the optical array of an optical
 write head.
 The modulation of the light shutter module for formation of a multi-tone
 image can be realized by pulse-intensity modulation as well as by
 pulse-width modulation.
 Also, the drive of a light-quantity measuring unit may be carried out by
 any mechanism such as one using a belt and a wire as well as the mechanism
 wherein a guide shaft is rotated by a motor.
 Further, the present invention is applicable to an image projector which
 projects an image onto a display as well as to an image forming apparatus
 which forms an image on a silver-salt print sheet and an image forming
 apparatus which forms an image on an electrophotographic photosensitive
 member.
 Although the present invention has been described in connection with the
 preferred embodiments above, it is to be noted that various changes and
 modifications are possible to those who are skilled in the art. Such
 changes and modifications are to be understood as being within the scope
 of the present invention.