GB2131569A

GB2131569A - Astigmatism compensated optical scanning refractive polygon

Info

Publication number: GB2131569A
Application number: GB08322014A
Authority: GB
Inventors: J F Menke
Original assignee: Elektro Optik GmbH and Co KG
Current assignee: Elektro Optik GmbH and Co KG
Priority date: 1980-05-29
Filing date: 1983-08-16
Publication date: 1984-06-20
Also published as: GB2076992B; SE454027B; GB2076992A; FR2483629B1; DE3020342C2; SE8103046L; GB8322014D0; DE3020342A1; FR2483629A1; GB2131569B

Abstract

An optical scanning device for the production of cartesian images with the aid of heat rays, comprising an optional entry lens 1, an optional deflecting mirror (7), a rotatable refractive polygon (2) with plane parallel surfaces (2a, 2b), an optional optical transformation system (5) and optionally a detector (4) is compensated for astigmatism in image production by the polygon using an aspherical mirror (8) in the ray path from the polygon (2). The mirror (8) has curvature so matched to the optical characteristics of the polygon as to compensate for the astigmatism. The mirror (8) may be formed by mechanically having a planar mirror (Fig. 2). <IMAGE>

Description

1 GB 2 131 569A 1

SPECIFICATION

Astigmatism Compensated Optical Scanning Refractive Polygon The present invention relates to an optical scanning device for use in the production of cartesian 5 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 10 Ay' can be calculated for a certain angular setting of the polygon by the equation:

d cosw a cose AY, = -{tgnO - -) - tg-0 - -)} n cosw 2 cose, in which d = width across the flats of the polygon, n = refractive index of the polygon, co = area angle 360'/F, a = image angle c = angle between the aperture and the perpendicular to the surface, wherein, sin w n. sin w', sin c n. sin c', a tgn = tg# + tg2 and a 30 e=CD + e. = (D + n 2 (D 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 39 119.

For the correction of image field curvature and astigmatism, it is known from DE-OS 27 39

119 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 40 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 lenses is expensive and 45 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 the present invention there is provided 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.

The aspherical mirror can comprise a planar mirror which is mechanically bowed by means of 55 a device in such a manner as to provide an ashpherical course of the reflecting surface.

The exact data for the dimensioning of the aspherical deflecting mirror must be calculated individually for the 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.

An embodiment 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 first scanning device, with an aspherical deflecting mirror arranged in the ray path, embodying the invention; and Figure 2 is a schematic view of a second scanning device, in which an aspherical deflecting 65 2 GB 2 131 569A 2 mirror is produced by mechanical bowing of a planar mirror, embodying the invention.

Referring now to the drawings, in Fig. 1 there is shown an optical scanning device comprising an entry lens 1, a deflecting lens 7 arranged after the lens 1, and, as a scanning optical element, a rotatable refractive polygon 2 with plane parallel polygon surface pairs. The polygon is mounted straight in the ray path, so that its central axis 3 and its rotational axis 6 coincide. 5 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 and a further deflecting mirror 8. 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. 10 In Fig. 2 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

CLAIMS 1. 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 20 optical transmission characteristics of the element as to compensate for astigmatism in image projection by the element. 2. A device as claimed in claim 1, wherein the mirror comprises a planar reflective element mechanically bowed by force exerting means. 3. An optical scanning device substantially as hereinbefore described with reference to Fig.

1 of the accompanying drawings.

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

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

2 of the accompanying drawings.

Printed for Her Majesty's Stationery Office by Burgess Et Son (Abingdon) Ltd-1 984. Published at The Patent Office, 25 Southampton Buildings, London, WC2A l AY, from which copies may be obtained.

v 1 l, i 7 A-

2 of the accompanying drawings.

CLAIMS (9 Feb 1984) 1. 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 transmisssion characteristics of the element as to compensate for astigmatism in image projection by the element, the mirror comprising a planar reflective element mechanically bowed by force-exerting means.