Method and apparatus for exposing recording material having a temperature compensated optical isolator

A method and apparatus is provided for exposing the recording material. A light beam is generated by a light source. The light beam passes through an optical isolator, through a light modulator, and then exposes recording material point-by-point and line-by-line. The modulator is activated during exposing of each line in an exposing time span, the modulator being deactivated at least at times within a return time span for each line. The optical isolator is enabled during the exposing time span. To compensate for heating of the optical isolator, the optical isolator is also activated for at least a portion of the return time span. A sum of time extents of the exposing time span and the compensation time span are approximately a constant for each line. Activation of the isolator during the compensation time span compensates for thermal effects which occur in the optical isolator as a result of activation and de-activation thereof. Thus, the optical isolator is thermally stabilized and is held at a constant temperature in the exposure time spans and in the return time spans.

BACKGROUND OF THE INVENTION 
The invention is in the field of electronic reproduction technology and is 
directed to a method for the point-by-point and line-by-line exposing of 
recording material with a light beam, and is also directed to an 
electronic exposing unit, also called an exposer, recorder or image 
setter. 
In a recorder, a light beam modulated by a video signals is conducted 
point-by-point and line-by-line across a recording material to be exposed. 
The recording material is thereby fixed on a holder that moves relative to 
the light beam. Given an inside-the-drum device, the recording material is 
fixed on a stationary holder shaped like a cylindrical segment or on an 
exposure trough, and the light beam is conducted across the recording 
material point-by-point and line-by-line with a rotating light beam 
deflection means. However, the recorder can also be designed as a drum 
device or flat bed device. 
A laser is often employed as a light source for generating the light beam. 
The modulation of the light beam dependent on the video signal occurs with 
a modulator, for example with an acousto-optical modulator. In the 
traditional technology, the drive of the modulator with the video signal 
occurs such that the modulator is activated line-by-line within exposure 
time spans wherein the light beam sweeps the lines to be recorded on the 
recording material and is deactivated during return time spans wherein the 
light beam is conducted to the next line to be exposed. 
In a traditional recorder, back reflections from the modulator or the light 
beam deflection unit can disadvantageously occur in the direction toward 
the light source, and these must be prevented in order to achieve a good 
recording quality. Beyond this, a high efficiency of the modulator is 
required so that a light beam with high luminous power is available for 
exposing the recording medium. Further, the components that are employed 
must have optimally constant operating properties since, for example due 
to heating of the components, the optical properties of the light beam can 
vary, this in turn diminishing the recording quality. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to improve a method and 
an apparatus for exposing recording material such that an improved 
constancy of the operating properties is achieved while assuring a high 
efficiency. 
According to the invention, a method is provided for exposing recording 
material wherein a light beam is generated in a light source. The light 
beam passes through an optical isolator and then through a video modulator 
where the light beam is modulated. The recording material is exposed 
point-by-point and line-by-line by the modulator light beam. The modulator 
is activated during each line of exposure in an exposing time span and is 
de-activated at least at times within a return time span for the light 
beam for each respective line. The optical isolator substantially prevents 
reflected light from reaching the light source. Internal heating in the 
optical isolator is compensated by activating the optical isolator not 
only during the exposing time span, but also during a compensation time 
span during at least a portion of the return time span. A sum of time 
extents of the exposing time span and the compensation time span is 
approximately constant for each line.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the basic structure of an electronic apparatus for the 
point-by-point and line-by-line exposing of recording material, referred 
to below as a recorder. A light source 1 generates a light beam 2 along an 
optical axis 3. The light beam 2 passes through a controllable optical 
isolator 4 and a controllable video modulator 5 that are arranged on the 
optical axis. A typical diameter of the light beam (2) in the region of 
the optical isolator amounts to 1 mm, and a focusing to a beam diameter 
of, for example, 70 .mu.m occurs in the region of the video modulator 5. 
The light beam 2 emerging from the video modulator 5 is incident onto a 
mirror 6 oriented obliquely relative to the optical axis 3 in a light beam 
deflection unit 7 that turns around the optical axis 3 by means of a 
rotatory drive 8. The light beam deflection unit 7 deflects the light beam 
2 reflected at the mirror 6 across a recording material 9 to be exposed 
point-by-point and line-by-line in a deflection plane that proceeds 
perpendicularly relative to the plane of the drawing. Given a flat bed 
apparatus, the recording material 9 is fixed on a planar holder and, given 
an inside drum apparatus as shown in greater detail in FIG. 2, it is fixed 
to the inside surface of a stationary, cylindrical segment-shaped holder 
or exposure trough. 
A YAG laser, for example, can be employed as light source 1. The optical 
isolator 4 is designed as an acousto-optical modulator of the 1.sup.st 
order that is switched on and off by gate pulses GI on a line 10. The 
video modulator 5 is an acousto-optical modulator of the 0.sup.th order, 
as a result whereof the optical modulation of the light beam 2 can be 
implemented with a high efficiency and with steep edges. The video 
modulator 5 is switched on and off by a two-level video signal (VS) on a 
line 11. The functioning and structure of acousto-optical modulators (AOM) 
are known. 
The gate pulses (GI) and the video signal (VS) are generated in a drive 
circuit 12 from an image signal (BS) that carries the information to be 
recorded and that is supplied to the drive circuit 12 via a line 13. 
For synchronization of the deflection motion of the light beam 2 with the 
signal generation in the drive circuit 12, a clock generator 14 generates 
a basic clock sequence T.sub.0 from which a first reference clock sequence 
T.sub.1 and a second reference clock sequence T.sub.2 are derived with 
frequency dividers (15, 16). The reference clock sequence T.sub.1 is 
supplied to the rotatory drive (8) for the light beam deflection means (7) 
via a line (17) and serves as reference value for a speed regulation. The 
second reference clock sequence proceeds via a line 18 to the drive 
circuit 12 for coordinating the signal generation. A pulse generator 19, 
which is coupled to the rotatory drive 8 in the illustrated exemplary 
embodiment, generates a line start pulse (ZI) once per revolution of the 
light beam deflection unit 7. The line start pulses (ZI), also referred to 
as a "Start-Off-Line" pulse or, abbreviated, as SOL pulses, determine the 
point in time in each line at which the gate pulse (GI) and the video 
signal (VS) are enabled for the recording. The line start pulses ZI are 
supplied to the drive circuit 12 via a line 20. An exemplary embodiment of 
the drive circuit 12 is indicated in FIG. 6. 
Due to the interposition of the optical isolator 4 between light source 1 
and video modulator 5, the disturbing back reflections from the video 
modulator 5 and the light beam deflection unit 7 onto the light source 1 
cited in the introduction to the specification are advantageously 
prevented since the optical isolator 4 detunes the wavelengths of the 
back-reflected light. It is thereby assured that the light source 1 
operates in stable design. Since the video modulator 5 fashioned as a 
modulator of the 0.sup.th order in fact exhibits a high efficiency but 
cannot completely suppress the light beam 2, there is the risk of 
disturbing misexposures of the recording material 9. These misexposures of 
the recording material 9 are likewise advantageously suppressed by the 
optical isolator 4 in that it completely shuts the light beam 2 off in the 
respective exposure pauses. 
Due to the activation and deactivation of the optical isolator 4 by the 
gate pulses (GI), thermal effects occur in the optical isolator 4 that 
have a negative influence on the operating properties. For example, the 
position of the light beam 2 can vary due to the heating of the optical 
isolator 4, this becoming disturbing on the exposed recording material (9) 
especially when the recorder works in what is referred to as start/stop 
mode. A recorder must always be operated in start/stop mode when it is not 
assured that the processor provided for editing the video signal, also 
referred to as raster image processor (RIP), cannot make the video signal 
for the modulation of the light beam continuously available with the 
required speed. 
As a result of the temperature compensation, the optical isolator 4 is 
thermally stabilized in an advantageous way in that it is held at a 
constant temperature in the exposure time spans and in the return time 
spans for the light beam 2 due to the type of drive with the gate pulses 
(GI). 
The temperature compensation of the invention is explained in greater 
detail below. 
First, FIG. 2 shows a recorder of the inside drum type in a sectional view. 
Such an inside drum recorder comprises a cylindrical segment-shaped half 
shell or exposure trough 22 with an aperture angle of, for example, 
180.degree. in which the recording material 9 is fixed. The light beam 
deflection unit 7 rotates around the cylinder axis 23 of the exposure 
trough 22 and conducts the light beam 2 over the recording material 9 
point-by-point and line-by-line in the direction of an arrow, whereby the 
light beam deflection means 7 moves in the direction of the cylinder axis 
23. 
The recording material (9) extends over an exposing region (24) inside the 
exposure trough (22). In every exposing period (line), the exposing region 
(24) identifies the exposure time span or, respectively, modulation time 
span in which the light beam (2) undertakes the exposure of a line on the 
recording material (9). The exposing region (24) begins following a start 
mark (25) at the aperture angle 0.degree. and ends preceding an end mark 
(26) at the aperture angle 180.degree.. A return region (27) that, given 
the indicated arrow direction, begins at the end mark (26) and ends at the 
start mark (25) identifies the return time span dead zone in which no 
exposing occurs and the light beam (2) is respectively conducted to the 
next line. 
FIG. 3 shows time diagrams for illustrating the time curve of the control 
signals for the optical isolator 4 and the video modulator 5 according to 
the prior art without the temperature compensation of the invention. A 
line start pulse (ZI) is generated at the start mark 25 for a time 
interval 28 within a line during the execution of the exposure. A gate 
pulse (GI) is forwarded to the optical isolator 4 in the time interval 29 
chronologically offset relative to the line start pulse (ZI), this gate 
pulse (GI) switching the optical isolator 4 on, whereby the duration of 
the gate pulse (GI) corresponds to the length of a line or, respectively, 
to the exposing region 24. At the same time, the two-level video signal 
(VS) in the exposure time span 30 for the corresponding line is supplied 
to the video modulator 5, this signal (VS) switching the video modulator 5 
on and off according to the respective signal level for exposing the line. 
All signals lie in that time interval that lies between the start mark 25 
and the end mark 26. Corresponding to the respective exposure sequence, 
the chronological lengths of the gate pulses (GI) and of the exposing time 
span 30 vary dependent on the respective length of the line or, 
respectively, of the exposing region 24. 
Without temperature compensation, the optical isolator 4 is not switched on 
by additional gate pulses (GI) within the return time span 31 that 
corresponds to the return region 27, so that the operating temperature of 
the optical isolator 4 drops during the return time span 31. 
FIG. 4 shows time diagrams for illustrating the signal curves given 
activated temperature compensation for the optical isolator 4 according to 
the invention. A compensation time span 32 is provided within the return 
time span 31 in addition to the gate pulse (GI) within the time interval 
29. The optical isolator 4 is additionally activated by a gate pulse (GI) 
of appropriate length within the compensation time span 32 so that it 
warms in the return time span 31 in order to keep the operating 
temperature of the optical isolator 4 nearly constant overall in the 
individual exposing periods, i.e. respectively in the exposing time spans 
30 and the return time spans 31. 
Given a time-variable duration of the exposing time span 30 or, 
respectively, given a time-variable duration of the gate pulse (GI) in 
every line, the compensation time span 32 is advantageously varied within 
the return time span 31 for the purpose of temperature compensation such 
that the sum of the time extents of the time interval 29 or, respectively, 
of the exposing time span 30 and of the compensation time span 32 is 
approximately constant in the lines. The particular aim is to achieve an 
exact constancy. However, an exact constancy is often not required in 
practice for adequately avoiding given parameter deviations due to 
temperature differences. 
FIG. 5 shows time diagrams with signal curves modified compared to FIG. 4 
wherein the duration of the time interval 29 for the gate pulse (GI) and 
of the exposing time span 30 is shortened but the duration of the 
compensation time span 32 is lengthened. 
This leads to the end of the exposing time span 30 migrating farther toward 
the left on the time axis "t" and the end of the compensation time span 32 
migrating farther toward the right. In order to make a comparison to FIG. 
4 possible, the end of the exposing time span 30 of FIG. 4 is additionally 
entered in FIG. 5 as a broken line 33, and the end of the compensation 
time span 32 of FIG. 4 is entered as a broken line 34. 
In FIG. 5 relative to FIG. 4, the compensation time span 32 has been 
lengthened by the time difference between the end of the compensation time 
span 32 and the broken line 34. This time span corresponds to the time 
difference between the broken line 33 and the end of the exposing time 
span 30. The sum of the time extents of exposing time span 30 and 
compensation time span 32 has thereby remained constant, as is preferred. 
A constant heating of the optical isolator 4 is managed by this constancy 
of the activation time spans of the optical isolator 4. As a result 
thereof, positional changes of the light beam 2 due to temperature 
fluctuations are advantageously avoided. By keeping the heating of the 
optical isolator 4 constant, moreover, disturbing density changes on the 
recording material 9 given start/stop mode of the exposure unit are 
avoided, so that a high overall recording quality is achieved. 
FIG. 6 shows an exemplary embodiment of the drive circuit 12 in the form of 
a block circuit diagram. The image signal BS on the line 13 is supplied to 
an editing stage 37 that is also input with the second reference clock 
sequence T.sub.2 on the line 18 and with the line start pulses (ZI) on the 
line 20. The video signal (VS) on the line 11 and the gate pulses (GI) for 
the time interval 29 or, respectively, the exposing time span 30 on a line 
38 are derived from the image signal (BS) in the editing stage 37. The 
video signal (VS) is supplied to the video modulator 5. 
A counter 39 can be input via a loading input 40 with a presetting that is 
stored in a memory 41 and that can be potentially modified for adaptation 
to respective application demands. Over and above this, the counter 39 
comprises a clock input 42, a start input 43 as well as a count input 44. 
The line start pulses (ZI) on the line 20 are applied to the start input 
43 in order to initiate a loading process of the counter 39 with the 
pre-setting value stored in the memory 41. The gate pulses GI on the line 
38 are adjacent at the count input 44. 
During the time duration of the gate pulses (GI), the counter 39 counts its 
counter reading down from its pre-setting value in the clock of the 
reference clock sequence T.sub.2 adjacent at the clock input 42. The 
current counter value is available at a counter output 45 and is forwarded 
to a comparator 46. The comparator 46 comprises a control input 47, a 
signal output 48 as well as a counter feedback 49. 
A signal that informs the comparator 46 about whether the light beam 
deflection means 7 is already aligned to the return region 27 is adjacent 
at the control input 47. When this is the case, the comparator 46 compares 
whether the value "zero" is adjacent at the counter output 45 of the 
counter 39. When this is not the case, then the comparator 46 activates 
the additional gate pulses (GI) for the compensation time spans 32. At the 
same time, the counter 39 is initiated via the counter feedback 49 to 
continue to lower its counter reading. As a result thereof, the 
compensation time span 32 extends until the counter 39 has counted down to 
"zero". The gate pulses (GI) for the exposing time spans 30 on the line 38 
generated in the editing stage 37 and the additional gate pulses (GI) for 
the compensation time spans 32 generated at the signal output 48 of the 
comparator 46 are operated with one another in an OR element 50 and are 
supplied to the optical isolator 4 via the line 10. 
Via a potentially occuring pre-setting of the comparator 46, it can be 
prescribed that the compensation time span 32 does not begin immediately 
follow the end mark 26 but that a predefinable distance is realized. 
The exact time positioning of the gate pulses (GI) is determined by the 
required length of exposure within the lines and by the starting position 
of the lines. As warranted, the time interval 29 can begin somewhat 
earlier and end somewhat later compared to the exposing time span 30 in 
order to compensate inertia effects of the optical isolator 4 with a lead 
and a lag and in order to assure that back reflections are avoided with 
the optical isolator 4 at least during the exposing time span (30). 
It lies within the scope of the invention to implement the temperature 
compensation not only at the optical isolator 4, as described, but in 
general at every modulator that is respectively activated in exposing time 
spans and deactivated in following return time spans for the light beam. 
In this case, the on intervals of the modulator are respectively 
determined in an exposing time span with reference to the video signal, 
and the modulator is switched on in the following return time span for a 
compensation time span that corresponds to the sum of the identified 
on-time intervals of the modulator. 
Although various minor changes and modifications might be proposed by those 
skilled in the art, it will be understood that our wish is to include 
within the claims of the patent warranted hereon all such changes and 
modifications as reasonably come within our contribution to the art.