GB2076992A

GB2076992A - Astigmatism compensated optical scanning refractive polygon

Info

Publication number: GB2076992A
Application number: GB8114789A
Authority: GB
Original assignee: Elektro Optik GmbH and Co KG
Current assignee: Elektro Optik GmbH and Co KG
Priority date: 1980-05-29
Filing date: 1981-05-14
Publication date: 1981-12-09
Also published as: DE3020342C2; FR2483629B1; SE8103046L; GB2076992B; SE454027B; GB2131569A; FR2483629A1; DE3020342A1; GB2131569B; GB8322014D0

Abstract

An optical scanning device for the production of cartesian images with the aid of heat rays, comprising a rotatable refractive polygon (2) with plane parallel surfaces (2a, 2b), the central axis (3) of the polygon (2) being inclined by a polygon inclination angle ( beta ) relative to its rotational axis (6), and the inclination angle ( beta ) being selected in dependence on the inclined setting of the plane parallel surfaces (2a, 2b) in such a manner as to provide compensation of the astigmatism in image production by the polygon. Instead of an inclined setting of the polygon, an aspherical mirror may be provided in the ray path, the mirror having a curvature so matched to the optical characteristics of the polygon as to compensate for the astigmatism (see Fig. 2 not shown). <IMAGE>

Description

SPECIFICATION Optical scanning device The present invention relates to an optical scanning device for use in the production of cartesian images with the aid of heat rays.

It is known that images projected by refractive polygons include imaging errors, namely image field curvature errors and astigmatism, due to parallel displacement of the central ray.

In the case of dihedral polygons with plane parallel surface pairs, for example as disclosed in DE-OS 21 16 469, there results a displacement of the image plane y' by Ay'. The magnitude of Ay' can be calculated for a certain angular setting z of the polygon by the equation::

in which d = width across the flats of the polygon, n = refractive index of the polygon, = = area angle 360"/F, a = image angle = angle between the aperture and the perpendicular to the surface, wherein, sin co = n. sin cho', sin e = n. sin 1, a tgN = tgp + tg - 2 and cu 2 e=Pfg 4 > being equal to the instantaneous angular setting of the polygon.

The equation is not explained in greater detail, as it will be familiar to the expert and it is exactly described in, for example, DE-OS 27 39 11 9.

For the correction of image field curvature and astigmatism, it is known from DE-OS 27 39 11 9 to dispose in the ray path one or more correcting lenses which are so designed in accordance with the width across the flats of the polygon and with the refractive index and number of surfaces thereof that the astigmatism and image field curvature can be corrected thereby. In addition, it has been proposed to correct these image errors by means of air lenses which are mounted within the polygon. In all of these correction measures, technically satisfactory corrections are achieved. However, the measures for obtaining these corrections can be executed only with great difficulty. The manufacture of the correcting lensea is expensive and the mounting of an air lens requires division of the polygon into two.

There is accordingly a need for an optical scanning device in which correction of or compensation for the image errors occurring in single axis is possible in a simple manner.

According to one aspect of the present invention there is provided an optical scanning device for the production of cartesian images with the aid of heat rays, the device comprising a rotatable refractive polygon with plane parallel surfaces, the polygon being set inclined through an inclination angle at 90 to its direction of rotation, and the inclination angle being chosen in dependence on the inclined setting of the plane parallel surfaces of the polygon in such a manner that compensation for the astigmatism is brought about thereby.

In particular, this compensation may be achieved by virtue of the angle of the plane parallel polygon surfaces adding to or substracting from the polygon inclination angle.

If it is assumed that a certain error magnitude is permissible, then the smallest mean error in an optical arrangement can be determined as the mean error between a maximum and a minimum.

According to another aspect of the invention, however, a deflecting mirror can be arranged in the ray path. If the reflecting surface of the mirror is aspherical, then the mirror effects a correction of or compensation for the astigmatism in the ray path in just such a simple manner as the aforementioned inclined setting of the polygon.

The aspherical mirror can comprise a planar mirror which is mechanically bowed by means of a device in such a manner as to provide an aspherical course of the reflecting surface.

The exact data for the inclined setting of the polygon and for the dimensioning of the aspherical deflecting mirror must be calculated individually for each scanning device. The data substantially depends on the following magnitudes: width across the flats of the polygon, number of surfaces of the polygon, rotational angle, image ratio, refractive index, and focal length of the entry lens.

Embodiments of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a scanning device with a polygon, according to a first embodiment of the invention, Figure 2 is a schematic view of a scanning device with an aspherical deflecting mirror arranged in the ray path, according to a second embodiment of the invention, and Figure 3 is a view similar to Fig. 2, showing a modification in which the aspherical deflecting mirror is produced by mechanical bowing of a planar mirror.

Referring now to the drawings, in Fig. 1 there is shown an optical scanning device comprising an entry lens 1 and, as a scanning optical element, a refractive polygon 2 with plane parallel polygon surface pairs, for example 2a and 2b. Each polygon surface pair is inclined at a different angle to the central axis 3 of the polygon.

Arranged in the direction of light transmission from the polygon is a detector 4, which is optically redisposed onto the rearward polygon circumferential surface portion by means of an optical transformation system 5.

For correction of image errors, the polygon is set at an inclination in the ray path, i.e. its central axis 3 is inclined at an angle ss to the rotational axis 6. During rotation of the polygon through the angle 4X, the angle ss and the different angles y therefore add and subtract, respectively. As a result, the required correction is achieved in a simple manner.

The specific dimension of the angle ss may be different for each scanning device and depends on the above-mentioned optical magnitudes.

In Fig. 2 there is shown an optical scanning device comprising an entry lens 1 behind which is arranged a deflecting mirror 7. The polygon 2 in this embodiment is mounted straight in the ray path, i.e. its central axis and its rotational axis coincide.

The detector 4 and the optical transformation system 5 are present in like manner as in the embodiment of Fig. 1.

In the present embodiment of Fig. 2, however, a further deflecting mirror 8 is arranged between the polygon 2 and the optical transformation system 5. This deflecting mirror 8 is an aspherical mirror which effects the correction of the image errors. Its exact optical data may be different for each scanning device and dependent on the optical data of the rest of the device.

In Fig. 3 there is shown a simple method of providing the aspherical mirror 8, wherein a planar mirror is mechanically bent by forces acting in the directions of the arrows A and B. This method is possible in view of the fact that the required mirror curvature for image error correction need only be small.

Claims

1. An optical scanning device for use in thermographic production of cartesian images, the device comprising a refractive element having a plurality of pairs of parallel planar surfaces bounding a polygonal cross-section of the element, the element being mounted to be rotatable about an axis inclined at an angle to a central axis thereof at right angles to the plane of the cross-section and the pairs of surfaces being so inclined relative to said central axis as to compensate for astigmatism in image projection by the element during rotation thereof.

2. An optical scanning device for use in thermographic production of cartesian images, the device comprising a rotatable refractive element having a plurality of pairs of planar parallel surfaces bounding a polygonal cross-section of the element, and an aspherical mirror which is arranged in a ray path from the element and the curvature of which is so correlated with the optical transmission characteristics of the element as to compensate for astigmatism in image projection by the element.

3. A device as claimed in claim 2, wherein the mirror comprises a planar reflective element mechanically bowed by force exerting means.

4. An optical scanning device substantially as hereinbefore described with reference to Fig.

1 of the accompanying drawings.

5. An optical scanning device substantially as hereinbefore described with reference to Fig.

2 or Fig. 3 of the accompanying drawings.