Method and an apparatus for illuminating points on a medium

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Details

355 67, G02B 1324

Patent

active

055792405

DESCRIPTION:

BRIEF SUMMARY
This application is a 371 of PCT/DK93/00180 filed May 26, 1991 and published as WO93/24327 Dec. 9, 1993.


BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention concerns a method and an apparatus for illuminating points on a medium, said method and said apparatus being of the type defined in the characterizing portion of claim 1, respectively. In a preferred embodiment the apparatus of the invention is used for selective illumination of points on the surface of a medium, and said apparatus may comprise a central control unit having a store in which the information on a desired illumination pattern is stored.
2. Description of the Prior Art
It is known to expose photosensitive media with light to transfer information to it. This was originally done by illuminating the medium through a partly transparent original containing the desired information. To rationalize this process techniques have subsequently been developed, where a laser beam scans the medium for exposure, while the laser beam is modulated for the medium to obtain the correct illumination in all points. The information on the illumination pattern of the medium is now contained in a central control unit and is used for modulating the laser. For a satisfactory image quality to be obtained on the medium, the mechanical system retaining the original and the light source with respect to each other during exposure must satisfy strict stability requirements. For the required accuracy to be obtained, the medium for exposure is often arranged in a device of the type usually called "Internal Laser Image Drum Plotter". Devices of this type are described i.a. in U.S. Pat. Nos. 4,852,709, 4,595,957 and 3,958,250. Devices of this type provide a satisfactory image quality, but they have a very complex mechanical structure, because the light source and the medium are to be moved very accurately with respect to each other. Consequently, devices of this type have a relatively high cost price, which puts a limit to the number of users.


SUMMARY OF THE INVENTION

The object of the invention is therefore to provide a method of the type stated in the opening paragraph for illuminating points on a medium, where the apparatus is based on simpler and fewer mechanical components.
This object is obtained as stated in the characterizing portion of claim 1 by using proximity focusing in the exposure of the medium, as stated there, i.e. the medium exposed with the light is spaced very closely from an aperture or a pinhole causing diffraction of the light. It is a relatively undescribed physical effect which is employed. When collimated light enters an aperture, the light is diffracted, forming, after the aperture, a cone of light having an aperture angle determined by the wave-length of the light and the width of the aperture. This zone in which the light diverges is usually called the Fraunhofer field or region. However, the divergence of the cone of light only begins at a certain distance from the aperture, since the bundle of rays converges beforehand. The zone in which the bundle of rays converges is called the Fresnel field or region. When the bundle of rays leaves the aperture, it has a narrowing at a certain distance from the aperture. This distance depends on the diameter 2 R of the aperture, the wavelength .lambda. of the light and on the thickness of the wall in which the aperture is provided. If the wall thickness is negligable, e.g. a vaporized layer on a crystal, the distance to the narrowing may be expressed by R.sup.2 /.lambda..
Claim 2 defines an apparatus in which proximity exposure principles according to the method defined in claim 1 are used.
On the face of it, one would believe that by diffraction the light is spread too much for the bundle of rays to be used as a well-defined beam for exposure of points on a medium, but it has surprisingly been found that such a bundle of rays in the region around its narrowing can be used for this purpose. A satisfactory depth definition can be obtained by arranging the medium for exposure in the vicini

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