This invention relates to image projection systems, by which is meant optical systems for projecting an image of a small-scale original towards a display screen on which a magnified image of the original may be viewed.

Projection systems are known which use a slide or overhead projector to project the image on the front of the screen, to enable it to be viewed by reflection. This has the disadvantage that the projector and its operator have to be in front of the screen, which prevents some members of the audience from having an uninterrupted view of the screen.

While this disadvantage could be overcome by using a rear projection screen in front of the projector, on which screen the image is viewed by transmitted light, this requires there to be a large space behind the screen, which makes inefficient use of the floor space.

The present invention aims at providing an optical projection system in which the projector and operator are to one side of the screen. Accordingly this invention provides an image projection system which is as claimed in the appended claims.

The present invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a front view of one form of projection system of this invention, using an overhead projector, in a working environment;

Figure 2 is a view, similar to Fig. 1 , showing the use of an electronic presentation system;

Figure 3 is a perspective view from a point to one side of the system shown in Fig.

;

Figure 4 is a diagrammatic view of another form of the invention, showing a mobile stand for the projector, screen and mirror, and

Figure 5 is a diagrammatic plan view of the optical ray path of the system shown in Fig. 4.

In all the drawings, the reference numerals applied to different components remain unchanged.

As shown in Fig. 1 , in one form of the invention a rear projection screen 2 is positioned in front of one shallow angle of a room 4 having support rails 6 fitted on its walls 8. Positioned behind the screen is a large mirror 10. Hanging from a rail 6 is a table 12 on which an overhead projector 14 is mounted. The projector has a Fresnel lens 16 on which may be positioned a transparency of the original image to be displayed. Above the lens 16 is an angled mirror 18 arranged to direct its output beam 20 towards the mirror 10. As shown more clearly in Fig. 4, one side of the beam 20 just clears one side of the screen 2 before falling on the remote end of mirror 10. In this way, all the light in the beam 20 falls on the rear face of screen 2 after reflection from mirror 10. Thus the operator of the projector, even when standing in front of the projector in order to exchange transparencies, is out of the line of sight of most people sitting in chairs 22 and facing the screen.

In that form of the invention shown in Fig. 2, the projector is in the form of a data projector 24 coupled to a laptop computer 26. The beam 20 from the projector passes in front of a whiteboard 28 before falling on a mirror (not shown) behind the rear projection screen 2. As is known, the projector may be fitted with an anti- keystone lens to compensate for distortion in the system, so that all the vertical lines in the original are vertical when they fall on the screen to produce the displayed magnified image 30.

As shown more clearly in Fig. 3, the beam from the projector falls on mirror 10. In order to prevent distortion of the final image, the shape of a rectangular-sectioned beam where it falls on the mirror is of an irregular trapezoid. While this means that some of the area of the mirror is unused, there would be no effective economies in making the mirror 10 trapezoidal, because the projector may be positioned to either side of the screen, and one does not want to have to invert the mirror when moving the projector from one side to the other.

In that form of the invention shown in Fig. 4, the mirror and screen are mounted on a stand 32 running on four castors 33. The two limbs of the stand are hinged together at one end (with the hinge axis being hidden behind the screen, as viewed), and at their other end they are joined by a tie 36 of adjustable length, so that the angle β which they subtend (see Fig. 5) may be adjusted over a small range. Pivotally mounted on one upright of the framework is a cantilever arm 38 supporting the table 12 for the projector 24 at its free end. This arm 38 may be constrained to move through a relatively-small range of angles, and it may even be of adjustable length, to accommodate the use in the projector of lenses of different focal lengths.

The screen 2 may be of known construction, having a rear face in the form of a large surface area Fresnel lens, and a front face made from a monolithic lenticular lens. Between them these have the effect of the rear face collimating the incident beam before its passage to the front face, and of the front face directing the output light uniformly over a relatively-wide range of angles, as indicated diagrammatically in Fig. 5, so that most viewers see the screen as being evenly illuminated, without their having to sit on a perpendicular to the screen. Such screens are known as optical rear projection screens, and are sold by DNP.

Thus it will be seen that the present invention provides a system by which a screen may be viewed from a wide range of angles without the view being interrupted by a projector or its operator.