Illumination unit and a method for point illumination of a...

Photocopying – Projection printing and copying cameras – Illumination systems or details

Reexamination Certificate

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C355S034000, C355S035000, C355S067000, C355S053000, C355S077000, C250S492200

Reexamination Certificate

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06529265

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to an illumination unit for point illumination of a medium where at least one light emitter is arranged to illuminate at least one illumination face via a microshutter arrangement, and to a method of point illumination of a medium by means of at least one light emitter arranged to illuminate at least one illumination face via a microshutter arrangement.
This takes place in a print-down frame under vacuum to ensure close contact between film and printing plate. The film, whose photo emulsion is sensitive in a given range, e.g. 350-450 nm, is illuminated through the film by a powerful lamp, such as e.g. a xenon or mercury lamp, whereby the image from the film is transferred to the printing plate.
The image on the film used has typically been produced in that a laser beam has drawn the image on the film in an image setter. The laser beam is turned on and off by means of a modulator which is controlled by digital information.
Thus two main process steps are involved: film manufacture and plate copying.
It is evident that both time and money may be saved by skipping the film manufacture and directly illuminating the image on the plate on the basis of the digital information. This solution would be particularly advantageous if it was possible to expose conventional commercially attractive printing plates.
However, it is problematic to obtain digitally controlled illumination sources or more particularly laser sources which are capable of illuminating the conventional printing plates at the correct wavelength and with sufficient optical power. This problem is particularly pronounced in the UV region and the blue region in the 350-450 nm range.
One way of obviating these circumstances may e.g. be to develop particularly highly sensitive plates which can be exposed by lasers at greater wavelengths. This technique is used today in so-called CtP machines (Computer-to-Plate Image Setters). However, these printing plates are expensive to obtain, just as these highly sensitive printing plates must be handled in consideration of the fact that they are exposed in daylight. Thus, it would be an advantage in several respects if it was possible to achieve sufficient illumination of more commercially attractive printing plates with lower sensitivity, which may be achieved by increasing the light intensity on the printing plate itself.
This solution, however, will give new problems, since the only commercially available light sources like those mentioned above do not provide sufficient optical power at the necessary wavelengths.
This problem may be obviated by using e.g. short arc lamps which can emit a greater optical power.
This technical solution, however, involves further complications, as a lamp cannot be modulated in the same manner as the known lasers, which may be solved, however, by a purely mechanical modulation of the light source.
The present applicant's European Patents EP 0 642 423 and EP 0 643 645 disclose applications of the above-mentioned type, wherein a film or a printing plate is illuminated directly through so-called microshutters by a so-called direct exposure. However, these applications with increased illumination resolution requirements will be relatively complicated and expensive in their structure, since each microshutter is illuminated by an associated light source in the form of an optical fibre and associated optics. In case of great illumination resolutions, these thus involve a very high complexity and the necessity of a very large number of optical fibres, which each have to be calibrated for the application concerned. In case of illumination systems of the above-mentioned type, the optical fibres will thus restrict the possible resolution in practice, just as the optical losses will be a limiting factor for the complete system, since utilization of this proximity exposure requires that the individual pinholes are illuminated evenly over the entire hole surface on the inlet side. If the light is merely conducted down toward the individual pinholes from e.g. a collimation lens, there will be a very great loss of light which falls outside the pinhole. If the light is focused down into the hole, e.g. by means of a microlens, the intensity profile of the light at the inlet to the hole will not be even, but resemble a Gauss curve, and only a small part of the beam can therefore be used to achieve an even illumination.
The object of the invention is thus at first to achieve a modulator type which may be used in. an illumination system for efficient and economic illumination of conventional printing plates requiring a relatively high illumination intensity.
SUMMARY OF THE INVENTION
When, as stated in claim
1
, at least one of the light emitters of the illumination unit is arranged to illuminate at least two microshutters via a first lens arrangement, said lens arrangement comprising at least one microlens arranged with respect to each microshutter so that the light emitted by the light emitter is focused in or in the vicinity of the optical axis of the light channel of the individual microshutters, an apparatus extremely suitable for illuminating photo-sensitive media with a lower optical sensitivity is achieved.
Light from the light emitter or one of the light emitters is thus focused in the individual microshutters, which can thus modulate the fed coherent light on an illumination place.
It has thus been found that when using commercially available plates sufficient “optical energy” or intensity may be provided from one light emitter to several microshutters at the same time. Thus, according to the invention it will be sufficient to subdivide the microshutters used into smaller groups.
This is achieved partly by the circumstance that it has been found to be mechanically possible to distribute a collimated light beam by subdividing this light beam into several light beams and focusing these in their respective microshutters by means of the first arrangement of microlenses in accordance with the invention. Further, it has been found that this distribution involves remarkably small optical losses, even though considerable optical losses might be feared by this division.
Within the scope of the invention, the microlens arrangements may be constructed in several different ways. An example of a variation within the scope of the invention might be a multilayer line arrangement.
It should be noted that power optimization in accordance with the invention may be achieved by focusing the entire or most of the light beam through the associated light channels without regard to a requirement for a flat intensity profile in the inlet of the entire light channel. A minimum or reduced power waste between the shutter apertures may thus be achieved by suitable focusing.
According to the invention, it is also possible to illuminate several shutters by a light emitter in the form of e.g. an optical fibre end.
As the illumination of a medium thus takes place in parallel using one or relatively few light emitters, this division makes it possible to achieve and maintain a high illumination intensity at the individual illumination points at a very high illumination rate.
Thus, reproducible illumination of a medium is possible on the basis of stored digital data by high rate exposure, as a high illumination intensity allows the opening times of the individual microshutters to be reduced, which, however, also requires a reduced rise/fall time. A reasonable structure of the micromechanical shutters enables a rise/fall time to be achieved which is so small that the shutter arrangement according to the invention may be used also when the overall system involves a relative movement between the illumination area and the illumination arrangement, which may e.g. be the case in connection with a scanning over an illumination area. A small rise/fall time will thus cause the illumination spot to approach its ideal shape.
According to the invention, a particularly advantageous embodiment of a microshutter will be achieved by a hole which forms a light chan

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