GB2187911A

GB2187911A - Wide-angle collimated display apparatus

Info

Publication number: GB2187911A
Application number: GB08606340A
Authority: GB
Inventors: Richard Allen
Original assignee: Link Miles Ltd
Current assignee: Link Miles Ltd
Priority date: 1986-03-14
Filing date: 1986-03-14
Publication date: 1987-09-16
Also published as: GB8606340D0; GB2187911B

Abstract

A monitor CRT21 presents on its screen a colour image formed by dots or stripes scanned in raster fashion. A beam splitter 22 reflects the screen image to the focal surface of a Mangin mirror 23 (i.e. a concave mirror combined with a meniscus lens) so that a viewer 36 sees the image at infinity. The mirror 23 has a spatial filter at its reflecting back surface 25. The inherent structure of the image imposed by the screen and its raster scanning is substantially eliminated by the spatial filter, and a smaller mirror 23 and CRT21 can be used than hitherto since the Mangin mirror 23 allows a larger volume of viewer movement without spherical aberration degrading the viewed image. <IMAGE>

Description

SPECIFICATION Wide-angle collimated display apparatus The invention relates to wide-angle collimated display apparatus.

Vehicle simulator systems frequently require a visual display to provide important cues. The direct viewing of a monitor cathode raytube is not generally acceptable where a distant scene is to be generated.

Furthermore multiple displays on cathode raytubes cannot be abutted to increase the field of view without creating a disturbing gap between adjacent displays.

To overcome these shortcomings use is generally made ofaWideAngleCollimator.The principle purpose of a wide angle collimator isto proiect an image towards infinity. Current practice in-vehicle simulator systems isto use an arrangement such as thatillutrated in Fig 1 oftheaccompanying drawings.

The centre point of the image on a monitor CRT 11 is located, by means of a beamsplifter 12, at the focus of a front surface concave mirror 14 which forms an image at infinity. The screen of the CRT 11 is part of a substantially spherical surface so that it lies substan tiallyinthefocalsurface of the mirror 14. The total available field of view is typically 40 horizontal by 36 vertical. As a result primarily of spherical observation, the window provided by the mirror 14, when viewed from the normal position which is at the centre of curvature ofthe mirror 14, is limited to a viewed image within approximately 46" horizontal by 28 vertical.

Furthermore, the volume through which the viewer may move is limited by the aberrations of the mirror, primarilyspherical aberration. To provide an acceptable viewing volume, a mirror with a radius of typicaily about 50 inches must be used, thereby making necessary the use of a 26 inch (diagonal) monitor CRT. Such a system is described in US Patent Specification No.2482115.

The object ofthe presentinvention is to provide a wide-angle collimated display apparatus which, in comparison with the known apparatus, has; 1. reduced size; 2. reduced weight; 3. improved-visual performance due to reduced visibility of screen dot structure and raster; 4. similarviewingvolume; and 5. reduced costs.

This invention provides a wide angle collimated display which achieves the performance criteria of the conventionat wide angle collimated display but with a linear size reduction of two to one, in all three orthogonal axis. A Mangin mirror is used with an optical filter incorporated to reduce the visibility of CRT screen dot/stripe and raster structures.

A preferred embodiment of the invention may employ a plastics material as a substrateforthe mirror.

The invention will now be described in more detail, solely by way of example, with reference to Figs. 2 and 3 ofthe accompanying drawings, in which Fig. 2 is a schematic axial ray diag ram of a wide-angle collimated display apparatus embodying the invention; and Fig. 3 is a ray diagram on a largerscale of part ofthe apparatus of Fig. 2 In the apparatus of Fig. 2, an image to be viewed is produced on the substantially spherical screen of a monitor CRT21, by an array of dots orstripes in three colours-- red, green and blue, and may show a raster structu re.Th is source image is reflected by a beamsplitter22to a position in thefocal surface of Mangin mirror 23.The Mangin mirror23 has an anti-reflection coating apptied to its front surface 24to minimise unwanted reflections. The back surface 25 of thins mirror has a spatialfilter fi It r layerwith an aluminium overcoat. The spatial filter layer is an array of optical apertures each having a diameter in the range 0.5 millimetres to 1.0 millitres. When placed between the eye and infinity, the spatial filtercauses diffraction resolution limiting similarto the caused in the same circumstances buy a single such aperture. The use of a phase-modulated filter is proposed in orderto preserve the overall transmission of the system.

A point 26 on the screen of the CRT21 emits rays which are intercepted at points (three shown at 27,28 and29) in a plane bythe beamsplitter22 and in part are reflected towards the mirror 23. In passing through the front surface 24 of the mirror 23 at points 30,31,32 the three rays shown are refracted by asmall angle, as shown in greater detail foroneexample in Fig. 3, in which forclaritytheangles of incidence and refraction are exaggerated. At the back surface 25 ofthe mirror 23spatialfiltering is effected and the rays reflected.

The three rays leave the mirror 23 through the front surface 24, again undergoing refraction and pass through the beamsplitter at (33,34,35) towards the viewer 36. The emergent rays are collimated at infinity.

The effect of the refraction at the front surface 24 of the Mangin mirror 23 is well known as reducing the spherical aberration as described for example at page 94of Fundamentals of Optics, by F. A. Jenkins and H.

E. White, Third Edition published in 1957 by McGraw Hill Book Company Inc. However, in accordance with the present invention a spatial filter is applied to the back surface of the Mangin mirror. The purpose of this filter is to providea spatial low pass function or a band rejectfunction to minimise the visibility of the CRT raster and the colour dots/stripes of which its screen composed. This fiter may be made in numerous ways; one method is to apply a black matrix with a regular array of clear holes between the mirror substrate and aluminised reflecting coat. A second method may use a clear matrix with phase modulation to produce the required effect. Holographic techniques offer alterna tivetechniqueswhich may realise abetterfilter. In each casethe mirror/filterwill be backed with a reflective coating.

The size of the collimator apparatus of Fig. 2 is half that of conventional wide-angle coltimator apparatus, which does not use a Mangin mirror. The use of the Mangin mirror in accordance with the present invention provides the required viewing volume, thereby allowing optimal use to be made of the 13V (13 inch diagonal) high definition monitors CRTs now becom ing available.

Claims

1. Wide-angle collimated display apparatus com prising means for presenting a source image, a concave mirror, and meansforestablishing an image ofthe source image in the focal surface ofthe concave mirror, characterised in that the concave mirror is a Mangin mirror and has spatial filtering means arranged to filter out features of the source image due to image forming patterns ofstructure inherent in the meansfor presenting the source image.

2. Wide-angle collimated display apparatus according to claim 1, wherein the spatial filtering means comprises a black matrix with a regular array of holes between a substrate ofthe mirror and a reflecting coating.

3. Wide-angle collimated display apparatus according to claim 1, wherein the spatial filtering means comprises a clear matrix providing phase modulation.

4. Wide-angle collimated display apparatus sub stantial ly as described hereinbefore with reference to Figs 2 and 3 of the accompanying drawings.