GB1579214A - Screens for optical imaging systems - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO SCREENS FOR OPTICAL IMAGING SYSTEMS (71) We, CENTRAL ELECTRICITY GENERATING BOARD, a British Body Corporate, of Sudbury House, 15 Newgate Street, London, EC1A 7AU, and JOHN MAURICE WEBSTER, a British Subject, of Avery Lodge, Marchwood, Southampton, Hampshire, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to optical imaging systems of the kind having a screen on which an image is focused.

Focusing screens are used in many instruments, for example, scientific optical instruments, photographic microscopes, technical cameras and reflex cameras. Such a screen applies diffusion to the light falling upon it. Thus, when located in the focus plane of the objective lens or a secondary lens, it provides a surface on which the image can be visually observed whilst the focus is optimised. It is therefore desirable that the image should be bright and free of granular appearance.

It is, at present, the usual practice to make a focusing screen by grinding or, in some other way, forming a diffusing surface on a sheet of glass or on a prism surface.

The screen is made in such a way that sufficient diffusion is obtained so as to provide even illumination when viewed and to give as fine grain structure as is possible. The fine grain is necessary for sharp focusing of the image. Present techniques for producing a ground glass screen result in a visible granularlity which is objectionable and inhibits focusing particularly if the image is inspected with the aid of an auxiliary magnifier, as is a common photographic and microscopical practice.

Because of the objectionable granularity of such glass screens under magnification, many other techniques have been suggested for focusing screens, e.g. oiled screens, the use of split images, or the oscillation of the screen in its own plane. Such techniques are expensive and do not overcome all the problems. In particular they do not avoid the dull image usually obtained with ground glass screens.

It is an object of the present invention to provide an improved form of focusing screen for use in an optical imaging system.

According to this invention, in an optical imaging system having a screen for viewing the image in the focal plane of an optical system comprises a hologram of a diffusing screen. Such a hologram constitutes a scatter plate which scatters incident light, by a process of diffraction, in a complex manner.

The resultant diffusion or scattering of the light however is within a controlled and predetermined angle which is defined in producing the hologram. In many forms of imaging system, the image is to be viewed only from directions within a predetermined solid angle; for example the construction of the apparatus may determine the possible positions of the eye of the observer with respect to the viewing screen. It is possible therefore, in producing the hologram, to ensure that the light is scattered mainly over this required viewing angle. This results in a significant increase in brightness compared with a conventonal ground glass screen in which the incident light is scattered in all directions.

The hologram of a diffusing screen does not carry the grain structure of the original diffusing screen. As is well known, a hologram enables a light wave front to be reconstructed but it does not in itself constitute an image. The hologram for use in the present invention may be formed using known holographic techniques for forming, on a plate, a hologram of a diffusing screen, e.g.

a ground glass screen. In accordance with known techniques, the holographic plate may be bleached to increase the light transmission and hence further to improve the brightness of the image.

In holography, a holographic plate would be used with quasi coherent light, e.g. light from a laser source. In the imaging system of the present invention, however, the hologram is used as a diffusing screen. It does not have to form any optical image and it will diffuse equally well with monochromatic or polychromatic non-coherent light. It retains however the characteristics of efficiently controlled diffusion with forward scatter over a limited solid angle determined in the manufacture of the plate.

The aboveaescribed imaging system may, for example, be used as a focusing system for a microscope or photographic camera.

Using a bleached holographic scatter plate as described above results in a bright image because there is little or no loss through unwanted scatter. The image formed upon the screen, when viewed, is virtually grainless, compared with conventional ground glass diffusing screens, since the grain size of the hologram is now that of the photographic emulsion or the holographically formed fringes.

A further advantage of using a holographic plate as the viewing screen is that it is readily possbile to print photographically a graticule or other required markings onto the plate either before or after the holographic exposure.

The inlcusion of a Fresnel lens or a lenticular plate in the original diffusing screen will even further improve the undirectional or preferred directional properties of the light transmitted in the direction of the view point by the scatter plate hologram of the diffusing screen.

WHAT WE CLAIM IS:- 1. An optical imaging system having a screen for viewing the image in the focal plane of an optical system wherein the screen comprises a hologram of a diffusing screen.

2. An optical imaging system as claimed in claim 1 and in which the image is to be viewed from directions within a predetermined limited solid angle, wherein the hologram is arranged to scatter light only over the required viewing angle.

3. An optical imaging system as claimed in either claim 1 or claim 2 wherein the hologram is on a plate with a graticule also printed thereon.

4. An optical imaging system substantially as hereinbefore described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. be bleached to increase the light transmission and hence further to improve the brightness of the image. In holography, a holographic plate would be used with quasi coherent light, e.g. light from a laser source. In the imaging system of the present invention, however, the hologram is used as a diffusing screen. It does not have to form any optical image and it will diffuse equally well with monochromatic or polychromatic non-coherent light. It retains however the characteristics of efficiently controlled diffusion with forward scatter over a limited solid angle determined in the manufacture of the plate. The aboveaescribed imaging system may, for example, be used as a focusing system for a microscope or photographic camera. Using a bleached holographic scatter plate as described above results in a bright image because there is little or no loss through unwanted scatter. The image formed upon the screen, when viewed, is virtually grainless, compared with conventional ground glass diffusing screens, since the grain size of the hologram is now that of the photographic emulsion or the holographically formed fringes. A further advantage of using a holographic plate as the viewing screen is that it is readily possbile to print photographically a graticule or other required markings onto the plate either before or after the holographic exposure. The inlcusion of a Fresnel lens or a lenticular plate in the original diffusing screen will even further improve the undirectional or preferred directional properties of the light transmitted in the direction of the view point by the scatter plate hologram of the diffusing screen. WHAT WE CLAIM IS:-

1. An optical imaging system having a screen for viewing the image in the focal plane of an optical system wherein the screen comprises a hologram of a diffusing screen.

2. An optical imaging system as claimed in claim 1 and in which the image is to be viewed from directions within a predetermined limited solid angle, wherein the hologram is arranged to scatter light only over the required viewing angle.

3. An optical imaging system as claimed in either claim 1 or claim 2 wherein the hologram is on a plate with a graticule also printed thereon.

4. An optical imaging system substantially as hereinbefore described.