Optical: systems and elements – Glare or unwanted light reduction – With absorption means
Reexamination Certificate
2000-02-22
2001-04-24
Sikder, Mohammad (Department: 2872)
Optical: systems and elements
Glare or unwanted light reduction
With absorption means
C359S615000, C359S884000, C359S859000
Reexamination Certificate
active
06220715
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is in the field of electronic reproduction technology and is directed to a method and to an apparatus for the elimination of light by absorption.
The method and the apparatus are employed, for example, in an electronic recording device for point-by-point and line-by-line exposure of a recording medium with a laser beam.
In a recording device, also called an exposer or recorder, a laser beam modulated by a video signal 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. In the case of an inside-drum recording device, the recording material is fixed on a stationary holder or exposure trough shaped like a cylindrical segment, and the laser beam is conducted across the recording material point-by-point and line-by-line with a rotating light beam deflection unit. The recording device, however, can also be designed as a drum device or flat bed device.
For direct exposure of printing plates, a laser beam having a high light power is required, this being generated in a laser light source for example in the form of a YAG laser. The modulation of the laser beam by the video signal occurs in a video modulator that, for example, is designed as an acousto-optical modulator (AOM). The acousto-optical modulator uses the input laser beam to generate essentially a used output laser beam of the first order, referred to below as an illumination beam, and an unused output laser beam of the 0 order, referred to below as a secondary beam.
While the illumination beam is being conducted across the recording material point-by-point and line-by-line with the rotating light beam deflection unit, the undesired secondary beam must be absorbed in a device, called a light sump or a light trap (beam dump).
There is a similar problem when a controlled light beam deflector that deflects the light beam employed for the illumination into the light trap during the illumination pauses is employed instead of the video modulator.
In order to achieve a good recording quality given the recording device, back reflections of light into the laser light source must be avoided, since they excite disturbing oscillations in the laser light source. Such back reflections can be caused directly by the unused secondary beam or by reflections of the secondary beam at optical components and housing parts, being caused as stray light when the light power of the undesired secondary beam is not completely and designationally eliminated in the light trap. Given the traditional light trap in the form of an absorption block, back reflections in the laser light source cannot, however, be completely avoided, so that the recording quality can be deteriorated.
EP 0 533 346 A and U.S. Pat No. 5,457,567 A already disclose laser recording devices wherein an illumination beam used for registration and an unused secondary beam are generated from an input laser beam in an acousto-optical modulator (AOM), and wherein the unwanted secondary beam is absorbed in a light trap. No specific exemplary embodiments for such light traps are recited in these publications.
Patent Abstracts of Japan, vol. 013, no. 407 (E-818), Sep. 8, 1989 and JP 01 146386 A (Toshiba Corp.), Jun. 8, 1989, already disclose a light trap that is composed of a cylindrical hollow body, whose one face side forms the light entry aperture, and of an axially symmetrically arranged, conically shaped reflector.
However, these known light traps do not allow back reflections into the laser light source of a laser recording device to be adequately avoided, so that the recording quality can be deteriorated.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to improve a method and an apparatus for the elimination of light by absorption, preferably in an electronic recording device for point-by-point and line-by-line illumination of the recording material, such that disturbing back reflections into the light source are avoided in order to achieve a high recording quality.
According to the method and apparatus of the invention for illumination of light by absorption, light is conducted through a light entry aperture into a hollow member having a light-absorbent inside surface. Light incident into the hollow member is conducted substantially onto an inside surface of the hollow member with a reflector situated in the hollow member. The hollow member with the light-absorbent inside surface is designed as a hollow sphere. The light entry aperture of the hollow sphere is selected so large that unwanted light to be absorbed proceeds into the hollow sphere without light scatter. A reflection face of the reflector is designed at least as a spherical section facing toward the light entry aperture and arranged at an inside surface of the hollow sphere facing away from the light entry aperture. The reflection face of the reflector is positioned within the hollow sphere such that the light incident through the light entry aperture and reflected from the reflection face is not reflected onto itself, such that an optimally slight part of the reflected light does not proceed toward the outside through the light entry aperture as stray light, and such that an optimally great part of the reflected light is absorbed by the inside surface of the hollow sphere.
The invention is explained in greater detail below with reference to FIGS.
1
through
3
.
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Patent Abstracts of Japan—01146386—08-06-89.
Fischer Jorg Achim
Jacobsen Thomas
Heidelberger Druckmaschinen Aktiengesellschaft
Schiff & Hardin & Waite
Sikder Mohammad
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