The invention relates to a projection screen which is intended to effectively enhance image or picture contrast, partly in the case of projectors in which the screen is illuminated with broadband white light and partly in the case of projectors in which the screen is illuminated with light in a plurality of narrow bands.

There are at present available commercially many types of projection screens which are intended for use with broadband illumination and also for use with illumination which is generally central in relation to the screen.

When the projector is pronouncedly offset laterally in relation to the projection screen, particular reguirement are placed on how the screen reflects light into the eyes of the viewers.

In those cases when the projection screen is illuminated with light in a plurality of narrow bands, there is at present no access to narrow band light sources which are sufficiently strong to obtain sufficient contrast in pictures, images, on available projection screens at normal room lighting. This problem is particularly relevant to a novel type if display, namely DMD which is an acronym for Digital Micromirror Device, this display being described in an article "The Digital Micromirror Device (DMD) and its Transition to HDTV" by the authors J.M. Younse and D.W. Monk in "The 13th International Display Research Conference", Strasbourg, August 31-September 3, 1993, pages 613-616. In this regard, it is difficult for practical reasons to achieve colour projection by means of space-multiplexing, i.e. by generating image or picture information in space-separated parts of the display unit or units. In practice, one is referred to the use of time- multiplexing, which means that the same parts of the display unit are used to reproduce the different colours. The problem

has been solved at present by using a rotary colour filter which is transilluminated with a lamp as a light source. This is a clearly disadvantageous solution, among other things with regard to light efficiency. A simpler and more effective arrangement could be obtained with the aid of narrow-band, fast switchable light sources, such as laser diodes, for instance.

The object of different aspects of the invention is to essentially reduce the aforesaid problems.

It is, of course, highly important that the light which reaches the screen is then reflected into the eyes of the viewers as effectively as possible. This can be achieved by providing the screen with micromirrors or reflective DOEs which send the light back within an angular range in which the eyes of the viewers can be expected to be found. The screen will not, of course, obtain uniform properties, since the angle of incidence of the light will vary across the screen and because the eyes of the viewers will lie in varying angular positions in relation to different parts of the screen. There is found within each pixel on the screen a plurality of mirrors which have a random tilt angle, both horizontally and vertically. The tilt angle distribution is calculated with regard to incident light and desired angular ranges of reflected light, and the number of mirror elements is chosen with regard to diffraction, so as to obtain a uniform light distribution. One possible manufacture could be obtained, for instance, with the aid of embossing forms manufactured with the aid of electron beam lithography or a "laser-scanner", for instance.

In order to reduce the reflection of ambient light on the screen, and therewith reduce the contrast in the image, the screen can be coated with a filter (for instance, a thin film filter) which has high reflectance for the wavelengths when the light which passes the image transmission elements has

high intensity, and high absorption for the wavelengths when said light has a low intensity. Naturally, this can be achieved most simply when the light sources have very narrow bands. In the case of a three-colour projector, such a filter can then be constructed as a filter having two stop bands with high absorption between the blue and the green colours and between the green and the red colours respectively. According to one preferred embodiment, the screen is constructed as a diffusor having a flat transparent coating on which the thin film coating is applied. The diffusor may advantageously be provided with micromirrors according to the aforedescribed principle.

The aforedescribed method using a filter coating can be applied with front projection and with rear projection on a transparent screen.

Fig. 1 illustrates schematically a projection arrangement that is displaced in the height direction and laterally in relation to the centre of a vertical display screen.

Fig. 2 illustrates schematically the construction of a reflecting screen provided with micromirrors.

Fig. 3 is a diagram which illustrates how the diffuse reflectance is formed with the aid of a thin film technique when using narrow band light sources.

Fig. 4 is a cross-sectional view of a piece of the projection screen.

Fig. 1 illustrates schematically a projection arrangement 10 which is displaced in the height direction and in a lateral direction relative to the centre of a vertical display screen 11.

Fig. 2 illustrates schematically the principle of constructing the projection screen with micromirrors. In principle, the mirrors 45 cover a pixel on the screen and reflect light so that all viewers can see this pixel. Light reflected by mirror 45a has a different direction to the light reflected by mirror 45b. The number of mirrors and the angular distribution can be calculated with regard to diffraction, so as to obtain a uniform light distribution over the eyes of all presumptive viewers.

Fig. 3 illustrates how the diffuse-reflection factor for the screen 11 should be formed optimally with the aid of a thin film technique. The axis r is graduated in diffuse-reflection factors, whereas the axis 1 is graduated in wavelengths, with the wavelengths in the illuminating wavelengths R, G, B being marked. The curve 8 shows the diffuse-reflection factor that is obtained, for instance, by coating a diffuse-reflective surface with a thin-film filter which absorbs the light between the colours B and G and between G and R respectively. The curve 8 can therefore also be perceived to show the transmission factor of the thin-film filter, the axis r being graduated in transmission factors. Because the major part of the ambient light that falls on the screen and lies between the colours B and G and the colours G and R respectively will be absorbed, there is obtained a considerable increase in image contrast. However, this effect can also be utilized to reduce the luminance of the light sources in the projector, thereby making a significant energy saving. The transmission curve 8 can be configured generally so that essentially only the projected light wavelengths will be reflected while light having wavelengths outside these wavelengths will be absorbed essentially by the screen. In order to obtain a noticeable effect, the ratio between the transmission factor at the illumination wavelengths R, G, B and the transmission factor of a wavelength between the illumination wavelengths R, G, B should be greater than 1.4, wherein the observed intensity difference corresponds to a factor 2. When the spectral

bandwidth of the light sources is relatively small, it is possible in this way to obtain a noticeable improvement in contrast in normal room lighting, since a major part of the ambient light is absorbed in the projection screen 11.

Fig. 4 is a cross-sectional view of a piece of the projection screen 11, in which the filter layer is referenced 1, the diffusor layer, which may be comprised of micromirrors, is referenced 3, the transparent layer between diffusor layer and the filter is referenced 2, and the supporting base layer is referenced 4. Naturally, the filter 1 may alternatively be applied directly to the diffusor layer 3, without including the intermediate transparent layer 2.