WO2000046639A1

WO2000046639A1 - Device and method for calibrating a light modulator

Info

Publication number: WO2000046639A1
Application number: PCT/EP2000/000878
Authority: WO
Inventors: Bernhard Lorenz; Matthias MÄNDL
Original assignee: Agfa-Gevaert Aktiengesellschaft
Priority date: 1999-02-05
Filing date: 2000-02-03
Publication date: 2000-08-10
Also published as: DE19904749A1

Abstract

The invention relates to a device and to a method for calibrating a light modulator (6). Said light modulator (6) is irradiated by a light source (1) and is controlled pixel by pixel by a control device (14). A reflector (10) can be inserted into or is arranged in the imaging beam path and reflects a part of the modulated light onto a photoelectric sensor (12) which is linked with the control device.

Description

Device and method for calibrating a light modulator

The present invention is based on a device and a method for calibrating a light modulator which can be controlled pixel by pixel according to the preamble of claims 1 and 17.

Light modulators are used in photographic copiers, as are described, for example, in DE 195 45 626 C1. In this example, index prints with an LED array, an LCD or a PLZT modulator are exposed on photographic negative copying material.

In DE 195 45 625 C1 the point-wise exposure of electronic image signals on photo paper is carried out on the basis of the modulation of the light by means of DMDs (Digital Micromirror Device).

These pixel modulatable light modulators have the disadvantage that individual pixels fail or their properties can change. In particular when using PLZT and LCD modulators, it has been shown that the state fluctuations of the pixels are so strong that the amount of light transmitted can vary every hour. As a result, structures may appear on the exposed image that are not visible in the original. This is intolerable with today's image quality. Accordingly, it is an object of the invention to develop a calibration device for checking the transmission or reflection properties of the pixels of light modulators, in order to be able to make corrections when driving individual pixels, if necessary. The check must be carried out quickly, in short time intervals and without greatly impairing the exposure process, so that the copying performance of the device in which the light modulator is used is not unnecessarily reduced.

This object is achieved by the characterizing features of the device according to claim 1 and the method according to claim 17. Advantageous embodiments of the invention are the subject of the dependent claims.

According to the invention, the amount of light modulated by a line of the light modulator is reflected and directed to a sensor.

The amount of light received at the sensor is then analyzed on an analyzer. In the event of a discrepancy between the measured actual value and the target value, which should be transmitted in the case of optimal light transmission on the basis of the selected control, a signal is sent to the modulator control, which causes the pixels to be controlled so that the discrepancy between the actual and Setpoint is compensated.

The invention ensures that the current value of the amount of light is determined, which, for. B. from a pixel line of the light modulator (LCD, LED array,

PLZT modulator or other light modulator with a transparent ceramic) is modulated. If this value deviates from a given setpoint, the light modulator must be recalibrated.

Since there is not much space in the device for contact imagesetters between the light modulator and the transport device for the light-sensitive material, it is special advantageous to reflect the modulated amount of light back into the beam path on which it has stepped through the light modulator.

With some contact imagesetters, however, there is a little more space in the device and the modulated light can be reflected directly out of the beam path to a sensor. This avoids further light scattering that occurs when reflecting into the incident beam path and thus running through the entire beam path twice.

If the modulated beam is reflected back into the beam path, it is necessary to separate the incident and reflected beam in order to direct the reflected beam onto the sensor. This can be done by a partially reflecting mirror, which only breaks the reflected beam away from the beam path, while the incident beam, which is guided to the light modulator, passes through the partially reflecting mirror.

Various arrangements can be implemented to reflect the light back into the beam path. Arrangements with transmissive light modulators generally have light guides and self-octave arrays which guide the light to one line of the light modulator in each case. Partially reflecting surfaces can be provided on these Selfociens arrays. These surfaces reflect a small part of the radiation without an additional reflecting component having to be introduced into the beam path. With this arrangement, only a small proportion of light is coupled out for calibration, the exposure is hardly impaired and is not interrupted. This arrangement therefore makes it possible to continuously calibrate the light modulator even during normal operation of the device; there are no calibration breaks to take.

Another way of reflecting the light back into the beam path is to briefly insert a mirror into the beam path between the exposure of two light-sensitive materials instead of the transport device. bring to. If the transport device consists of a transport roller, the mirror can be attached to the suspension of the roller in such a way that it is initially on the roller side facing away from the light modulator. The suspension, which supports the roller and mirror, can be rotated around a pivot point below the roller center. The rotation removes the transport roller from the beam path and replaces it with the mirror. It would also be conceivable to provide a mirrored surface on the transport roller itself, which reflects the modulated light beam as soon as there is no more light-sensitive material on the transport roller. A partially reflecting surface could also be attached directly to the side of the light modulator that faces the light-sensitive material. Any number of further variation options for the reflector arrangement are open to the person skilled in the art here.

The reflected light modulated by the light modulator always relates to a line of the light modulator in a line imagesetter. However, since each individual pixel of the light modulator is controlled during exposure due to different data information, each individual pixel must also be calibrated. For this purpose, the light-sensitive material is removed from the beam path or covered in a calibration phase and the amount of light passing through the light modulator is measured. This amount of light, which is transmitted by a light modulator line, is assigned to the current value of a pixel. For this purpose, all pixels of a line are first switched dark so that no light passes through. This sets the zero point. The pixels are then switched on and off at constant time intervals and the total intensity measured in each case. Characteristic curves are created with these values and compared with characteristic curves of the target state. After this comparison, the pixel control is redefined.

A fixed time interval is assigned to each pixel of the light modulator. A pixel then corresponds to 1 ns, for example. Then after 7 ns the seventh pixel of the line is switched on and the intensity measured at the sensor speaks of the intensity passed on from the seventh pixel. This is then compared with the target value of the intensity of a pixel, and the seventh pixel is controlled anew in accordance with the difference between the actual value and the target value.

To speed up the process, areas of pixels can also be switched on and checked together. For example, you can always switch on ten adjacent pixels on a line and check whether the transmitted intensity then corresponds to 10 times the target value of a pixel. If this is not the case, the pixels can be switched through individually or the ten pixels can be checked again in areas; z. B. you can switch the first five and then the next five pixels together and find out in which area the faulty pixel is located, etc.

As soon as the modulator has been calibrated, a further deviation of the transmission from the setpoint can be continuously adjusted during the operating phase. Because different amounts of light are to be continuously transmitted from all pixels with continuous image lines, each pixel in different lines makes a different contribution to the sum of the total amount of light transmitted per line. If exactly one pixel in its transmission rate deviates from the target value, the deviation in the total amount of light is proportional to the amount of light to be transmitted by this pixel. From the change in the sum of the deviations from the target value from line to line, it is therefore possible to infer the different pixel and to control it differently. Since the changes in the pixel transmission rates generally occur over a longer period (at least every hour), it can be assumed that generally only one pixel changes noticeably during the confession of a few lines, so that no more complex control algorithms are necessary. If several pixels suddenly change, a new calibration cycle must be initiated outside the exposure phase. If the light that has passed through the modulator is to be directed away from the beam path directly to a sensor, this can be done either with a partially transparent or a fully reflecting mirror. The advantage of using a partially transparent mirror is that it can also be calibrated during the exposure. The disadvantage, however, is that part of the

Light intensity is lost through the mirror. If a fully reflecting mirror is used, the mirror would have to be introduced into the beam path between the exposure phases and the exposure process would thus have to be interrupted in order to carry out a calibration. However, since the mirror should advantageously be inserted between the light modulator and the paper plane, no lengthy interruption is necessary, since nothing needs to be changed in the exposure setup; the paper can remain in the plane of the paper. The advantage here is that a very short, but independent calibration phase must be initiated so that the light modulator can be switched pixel by pixel and a change in each pixel of the

Modulators can be determined directly without making complicated calculations, as is necessary for the reflection back into the incident beam path.

Further details and advantages of the invention are explained in more detail below on the basis of specific exemplary embodiments and the subclaims. Show it:

Fig. 1 shows the schematic structure of a device for exposing an image to light-sensitive material, Fig. 2 shows a section of the schematic structure of an exposure device with a partially reflecting surface on the Selfoclens array, Fig. 3 shows a section of the schematic structure of an exposure device with a reflective 4 a schematic representation of a transmitting light modulator (e.g. PLZT), 5 shows a characteristic curve of the intensity measured at the sensor as a function of the on-time of the pixels,

6 shows a section of the schematic structure of an exposure device in which a partially reflecting mirror reflects the light out of the beam path and

Fig. 7 shows a detail of the schematic structure of an exposure device, in which a reflector is arranged above the light-sensitive material.

1 shows the schematic structure of a device for exposing an image to light-sensitive material with a calibration device according to the invention. The light emitted by a lamp 1 is radiated onto a fiber optic 4 by a filter wheel 2 and a partially transparent mirror 3. The fiber optics 4 guides the light onto a first self-clip array 5, which focuses the light onto a line of the light modulator 6. Any self-illuminating light modulator (e.g. LED array) or transmissive light modulator (LCD, FLCD, PLZT) can be used as the light modulator. In the arrangement shown, the light modulated by one line of the light modulator is exposed onto the light-sensitive material 8 by a second selfoclens array 7. The light-sensitive material is moved in the transport direction T by means of a transport roller 9. A mirror 10 is connected to the transport roller 9, which is in position A during the exposure and - as soon as no light-sensitive material is transported into the beam path of the light - is brought into position B instead of the transport roller 9. This can be done, for example, by rotating the transport roller and the mirror around a common pivot point D, as indicated in FIG. 1. In position B, the mirror 10 reflects the modulated light back to the partially transparent mirror 3 via the same optical path that the light had taken to the mirror. At this mirror 3, the reflected light beam is faded out of the beam path and onto an optics 11 Sensor 12 steered. This passes the measured amount of light on to an analyzer 13, in which the Information is evaluated. The result of the evaluation is forwarded to the control 14 of the light modulator. The control controls each pixel depending on the value analyzed for each pixel.

Fig. 2 shows a section of the schematic structure of an exposure device. The section represents the area of exposure by means of the light modulator 6 controlled by the control 14. In the arrangement shown, there is a partially reflecting surface 101 on the second self-clip array 7 between the light modulator 6 and the light-sensitive material 8. This reflects a small portion of the light beam passing through the light modulator 6 back into the same beam path. In this exemplary embodiment, the partially reflecting surface assumes the function of the mirror 10 in FIG. 1. In contrast to the structure in FIG. 1, the overall structure of the arrangement does not have to be changed in this example to block out the light beam for calibration purposes. The calibration can therefore be carried out more frequently and more quickly and even during the exposure, since the light-sensitive material can remain in the beam path. If there is enough space between the light modulator and light-sensitive material 8, the partially reflecting surface 101 can also be arranged such that it reflects part of the light beam away from the beam path directly onto a sensor 12. This means that the reflected beam does not have to cover the entire optical path.

Fig. 3 shows a section of the schematic structure of an exposure device. Here is another way to reflect back the light modulated by the light modulator line. In this arrangement, a reflective surface 102 is attached to the transport roller 9. It is also advantageous to place a reflective ring around the entire roller so that regardless of the roller position, the light is immediately reflected as soon as there is no more light-sensitive material on the transport roller. Any number of possible variations are still open to the person skilled in the art, a partially reflecting surface between light-sensitive material and light modulator 6 or to attach a reflective surface after the light modulator 6, which either reflects the modulated light back into the beam path to a partially transparent mirror 3 or directly onto a sensor 12.

4 is intended to explain the calibration in detail using the PLZT modulator. A transmissive light modulator consists of a linear polarizer 61, a transparent substance 62 (here a PLZT ceramic), which rotates the direction of polarization of the light passing through when an electric field is applied, and an analyzer 63. The polarization planes of linear polarizer 61 and analyzer 63 are perpendicular to each other so that the light that is polarized at polarizer 61 cannot pass through analyzer 63. Even if a PLZT ceramic 62 is located between the two polarizers, the light initially maintains its direction of polarization, no light is let through by the polarizer and analyzer (I = 0). However, if a voltage U is applied to the PLZT ceramic, it becomes birefringent and changes the direction of polarization of the light. The rotation of the direction of polarization depends on the strength of the applied electric field and the Kerr constant. The Kerr constant can change under different conditions, such as temperature fluctuations. When exposing photosensitive material, a PLZT modulator is divided into individual pixels, which are controlled by electric fields so that the light either maintains its polarization or rotates through 90 ° as it passes through. It is thereby achieved that the light through a pixel of the light modulator is either not at all or completely transmitted. Since the Kerr constant and thus the strength of the electric field, which is necessary to turn the polarization direction by 90 °, but changes over time, it must be determined again and again what field strength is necessary to drive a pixel in this way that the maximum amount of light passes. For this purpose, the transmitted light is directed to a sensor 12, from which the current value of the transmitted intensity I is measured. This measured intensity is then compared by an analyzer 13 with the target value I _m a, so that the field strength can then be readjusted until I = I _max . There with In the proposed structure, only the light I _{g of} an entire line of the light modulator arrives at the sensor 12, the calibration must be carried out in such a way that the intensity can be assigned to each individual pixel. This can be achieved, for example, by switching off the electric field (U = 0), as a result of which the intensity at the sensor becomes l _g = 0. Then one pixel at a time is switched on and off at constant time intervals Δt, so that ideally the maximum intensity l _{ma ma is} always measured on the sensor after optimally adjusting the voltage after a specific time Δt.

As shown in FIG. 5, this can be transferred (for example for n = 5) into a characteristic curve. The intensity I is plotted against the time t, a time interval Δt always corresponding to one pixel. With the optimal setting of the light modulator, the dotted curve results; all pixels allow the maximum intensity to pass. The solid characteristic curve results in a situation in which the second pixel has to be recalibrated. These curves are created and evaluated on the analyzer 13. A signal is then transmitted to the controller 14 so that it is caused to change the pixel control.

6 shows a section of the schematic structure of an exposure device. The section represents the area of exposure by means of the light modulator 6. A partially reflecting surface 103 is attached in the self-clip array 7, which reflects part of the light used for the exposure onto a sensor 15. The sensor is either moved perpendicular to the drawing plane along the individual lenses of the Selfociens array 7 or it is designed as a line sensor. This construction, in which the light does not have to be reflected back through the entire beam path before it reaches the sensor, has the advantage that no unwanted scattered light can reach the sensor. Another advantage is that the sensor is not in the paper plane and therefore does not interfere with the exposure. If there is enough space between the selfoclens array 7 and light-sensitive material 8, the mirror 103 can also be arranged in this intermediate space or can be introduced if necessary. If the mirror 103 is only introduced when necessary in calibration phases, it can also be a fully reflecting mirror.

If calibration is only carried out in calibration phases, it would also be possible to move the sensor together with a mirror along the light modulator parts and to calibrate each pixel of the line one after the other.

Instead of the sensor 15, the light reflected from the self-reflecting array by the partially transparent mirror 103 can just as well be directed to a second mirror 16, from which it is reflected back into the beam path. A corresponding structure is shown schematically in FIG. 7.

4 sheets of drawings

Claims

Claims:

1. Device for calibrating a light modulator, which can be controlled pixel by pixel by a control device, with a light source for illuminating the light modulator and a photoelectric sensor which is connected to the control device, characterized in that at least one reflector is arranged in the imaging beam path of the light modulator or can be introduced into the beam path that modulated light from the light source can be reflected on the sensor.

2. Device according to claim 1, characterized in that the pixel-controllable light modulator has a PLZT ceramic.

3. Apparatus according to claim 1, characterized in that it is a DMD in the pixel-controllable light modulator.

4. The device according to claim 1, characterized in that the light modulator which can be controlled pixel by pixel is an LCD.

5. The device according to claim 1, characterized in that the reflector is arranged so that the modulated light or a part thereof is reflected back into the beam path.

6. The device according to claim 5, characterized in that a semitransparent mirror is arranged in the beam path between the lamp and light modulator, which directs the light reflected by the reflector onto the sensor.

7. The device according to claim 1, characterized in that the reflector is arranged so that the modulated light or a part thereof is reflected out of the beam path.

8. The device according to claim 1, characterized in that a mirror is attached below a carrier and / or transport device of light-sensitive material.

9. The device according to claim 8, characterized in that the carrier and / or transport device of the light-sensitive material is designed so that it can be moved away from the beam path.

10. The device according to claim 9, characterized in that the carrier and / or transport device is a transport roller, wherein the transport roller axis and a mirror rigidly connected and around a fixed

Are pivotable.

11. The device according to claim 1, characterized in that the carrier and / or transport device has a reflective surface.

12. The apparatus according to claim 1, characterized in that a partially reflecting mirror is arranged between the light modulator and the light-sensitive material.

13. The apparatus according to claim 2, characterized in that there is a Selfoclens array between the light modulator and light-sensitive material, which has a partially reflecting surface.

14. The apparatus according to claim 1, characterized in that the light modulator on the side which faces the light-sensitive material has a partially reflecting surface.

15. The apparatus according to claim 1, characterized in that a mirror between the light modulator and light-sensitive material is movable.

16. The apparatus according to claim 15, characterized in that a sensor is movable with the connected mirror.

17. A method for calibrating a light modulator which can be controlled pixel by pixel, in which the light from a light source illuminates the light modulator, is modulated by it and is passed to a photoelectric sensor and / or to light-sensitive recording material, characterized in that at least one reflector is inserted into the Beam path of the modulated light is introduced or arranged in the beam path of the light that part of the modulated light is reflected on the sensor.

18. The method according to claim 17, characterized in that the light received by the sensor is analyzed by an analyzer and compared with comparison values.

19. The method according to claim 18, characterized in that the pixel-wise controllable light modulator is controlled as a function of the analysis.

20. The method according to claim 19, characterized in that the light modulator is controlled in regions.

21. The method according to claim 19, characterized in that the light modulator is driven pixel by pixel in succession.

22. The method according to claim 21, characterized in that the analysis is carried out over a time assignment.