Internal drum scophony raster recording device

Abstract

An optical recording and data processing system for exposing an image on to a flexible, light sensitive medium, which includes a medium holder having an inner cylindrical wall portion against which is held said medium, and a light source having an approximately rectangular emitting aperture, with a short aperture axis and a long aperture axis, operative to emit a beam of light having a rectangular cross section with a long axis corresponding to the long aperture axis and a short axis corresponding to the short aperture axis. An optical modulator is aligned with the light source so as to intercept light from the beam of light and produce a spatial modulation pattern across the long axis of the beam of light. A pattern shifter for shifting the spatial modulation pattern across the length of the long axis at a constant rate is provided as is a pattern rotator for rotating the spatial modulation pattern at a rate equal to the rate of shifting of the spatial modulation pattern. A scanner for scanning the beam of light onto and across the circumference of the inner cylindrical wall portion and a driver for advancing the scanner mechanism, after scanning a row, to an adjacent row to repeat the scanning are both provided. The relative phase angle between rotation of the spatial modulation pattern and scanning of the pattern is maintained such that the direction of movement of the projected image of said shifting is parallel to the scan motion, but opposite in direction. An optical system is provided to project the modulated beam of light and focus it to produce an image of the shifting modulation pattern at the recording medium so that the rate of shifting motion cancels the scan motion.

Description

FIELDThe present invention relates to an optical recording system used to impart a raster composed image on to a flexible photo-sensitive medium. Such recording devices are well known in the industry of photographic film recorders, for exposing either finished display materials or photo-tool masters. The photo-tools are used as an intermediary, to transfer images on to a secondary medium.BACKGROUND OF THE INVENTIONThe printing industry has gone through an evolution designed to speed up the printing process and at the same time to lower costs. The film transfer to plate scheme simply exposes light sensitive photographic film with light from a low power source and then uses the processed film to transfer the image to a plate. More efficient plate imaging methods led to the direct imaging of the plates themselves. Although still using relatively low power light sources, such methods required complex plate composition and processing chemistry, and hence a large expense for the plates th...

AI Technical Summary

Benefits of technology

According to the invention there is provided an optical recording and data processing system for exposing a raster image on to a flexible, light sensitive medium, which includes a medium holder having an inner cylindrical wall portion against which is held the medium, and a light source having an approximately rectangular emitting aperture, with a short aperture axis and a long aperture axis. A lens system collects the emitted light and projects a beam having a secondary beam waist which has a rectangular cross section with a long axis corresponding to the long aperture axis and a short axis corresponding to the short aperture axis. An optical modulator is aligned with the light source so as to intercept light at the secondary beam waist and produce a spatial modulation pattern across the long axis which shifts across the length of the long axis at a constant rate. A rotatable multiple beam deflector is positioned in the path of the beam and rotates so as to rotate the spatial modulation pattern at a constant rate. The modulated light beam is directed and focused onto a scanning deflector interposed along the axis of the cylindrical wall portion, such that after undergoing scan deflection, the light is focused onto the inner cylindrical recording surface. The focused light at the recording surface forms an image of the spatial modulation pattern. The scanning deflector scans the beam of light around the circumference of the inner cylindrical wall portion at a constant angular rate, equal to the rate of rotation of the spatial modulation pattern. The relative phase angle between rotation of the spatial modulation pattern and scan means is maintained such that the direction of movement of the projected image of the shifting modulation pattern is parallel to the scan motion, but opposite in direction. The velocity of scanning at the inner cylindrical recording surface is constant and maintained equal to the shifting velocity times the optical magnification between the modulator and the recording surface. A scanning deflector advancement assembly advances the scanning deflector along the cylinder axis, one pixel spacing for each scan rotation, to expose successive lines of raster and provide complete coverage of the raster image.

Problems solved by technology

Although still using relatively low power light sources, such methods required complex plate composition and processing chemistry, and hence a large expense for the plates themselves.

In general, raster recording devices are limited in their exposing power by the brightness of the optical source.

This limitation is in contrast to a flood exposure system, used to photo-graphically transfer images from a master on to a secondary medium, in which the amount of light can be increased simply by increasing the size or number of sources.

Because the field of view of a raster recorder usually extends over only a very small fraction of the total image area, in many cases extending over only a single image pixel element, the maximum exposure limit can be restrictive in terms of exposing power.

The source power required to expose a conventional plate efficiently, with a raster film recorder, is prohibitive using present day technology.

Lasers are the favoured light source for many raster recording devices because of their inherent high brightness, but they are limited to known lasing materials which impose a number of design restrictions, such as the choice of available wavelengths.

In particular, ultra-violet laser sources are much more difficult to manufacture, and are considerably more costly than longer wavelength lasers.

However, they are only capable of emitting wavelengths in the near infra-red to red portion of the optical spectrum.

It is important to consider this effect when designing a raster optical recording system, because some architectures provide varying delays between adjacent lines of raster, which could result in exposure uniformity problems.

NIR lasers of equivalent power (approximately 5 watts), which produce a circular spot, are much more costly than such diode lasers.

The brightness of the laser diode is limited by material damage thresholds so that total power can only be increased by enlarging the emitting aperture.

The resulting rotational asymmetry imposes restrictions on the design configuration of an efficient raster optical system.

Complex corrective optics can be employed to partially compensate for the distortion, but they are expensive and ultimately restrict the number of resolvable pixel elements and the allowable source aperture aspect ratio.

A major problem with such a system resides in the requirement of having to rotate a large drum, of considerable mass and rotational inertia, at the high speeds necessary to achieve fast recording rates (state of the art systems plot at raster rates in excess of 200 lines per second).

By recording multiple raster lines in parallel, a high pixel throughput can be maintained, but at the expense of increased system complexity and cost.

If only circular symmetric, single spots are to be projected on to the recording plane, this rotation effect is unimportant.

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