The invention relates to a colour filter cell to produce light of a desired colour from a light beam comprising all wave lengths of visible light, which cell includes at least one filter element with filter portions of three different basic colours.

As to spotlights used especially for stage lighting, the colour of the light sent by projectors is still mainly changed by means of a so-called sema- phore device comprising a few light filter foils each of one colour tumable in front of a light beam co ing from the projector. In some devices, separate filter foils are connected to a long band, which can be coiled to and fro in front of the projector. Both fil- ter types are characterized in that by means of them it is possible to produce only few preselected colours in practice.

It is generally known that by combining at least three suitably selected basic colours in varying proportions it is, in principle, possible to produce any colour visible for the human eye. This phenomenon is made use of e.g. when generating an image in a colour television; the colours to be combined are red, blue and green. Many kinds of colour-light projectors based on the combination of three basic colours have been de¬ veloped. According to the most of them (patent publi¬ cations EP 00 08 639, DT 25 50 891 and GB 21 23 941), the colours are combined by means of semi-translucent mirrors or prism surfaces. A drawback of such methods is except for the complicated structure also a consid¬ erable loss of luminosity occurring at reflections and transmissions.

One type of projector (patent publication

DE 27 24 304) produces colours by means of three par¬ allel filter strips of different colours, which strips can irrespective of each other be moved to cover a greater or smaller part of a light beam. A drawback of this method is an uneven colour distribution of the light beam obtained - after the filter, a special dim filter or a rough mirror surface is needed to mix the colours with each other. It is evident that such an equalization of a light beam creates lots of diffused light and consequently, the loss of light is consider¬ able especially when a narrow beam with sharp outlines is desired.

A usable method to separate a desired spectral band from a ray of light is also offered by a so- called dichroid interference filter. This filter is characterized in that a wave length band transmitted through the filter changes when the light is passing in angles of different sizes through the filter foil. In practice, it is not possible to get very pure colour shades by means of one filter foil, and there¬ fore, 3-4 such foils to be turned separately are needed successively to provide a satisfactory range of colours (patent publication EP 01 40 994 and the rec¬ tified version US 4 602 321). Drawbacks of this method are also the loss of light occurring at the transmis¬ sion through several junctions and the rather compli¬ cated structure as well as the quite expensive inter¬ ference filters required by the method. In practice, this type of projector also needs some kind of a colour equalizer before the objective, because it is not possible for the light to pass fully parallel through planar filter foils and thus, rays of light passing in different directions get shades deviating from each other. The object of this invention is to provide a

colour filter cell avoiding the drawbacks mentioned above, by means of which cell it is possible to create almost any shade by turning the cell. This object is reached by means of a colour filter cell of the inven- tion, characterized in that said filter portions of three different basic colours are arranged substan¬ tially perpendicular to each other to form an element of three colours.

By means of the colour filter cell of the in- vention, considerable advantages are reached compared to the methods described above. The loss of light remains small, because the light beam to be filtered must pass through one filter strip only. Generally, it is not necessary to lead the light obtained from the filter through any kind of colour equalizer, because the rays of different colours generated in the cell already along a short distance get mixed with each other producing a homogeneous shade at the object to be lighted. Also a defect in colour due to transmis- sion angles of different sizes connected with a wide light beam can be almost entirely eliminated by forming the filter cell into a suitable form. Addi¬ tionally, it is easy to manufacture the cell by con¬ ventional machining methods and therefore, the manu- facturing costs remain low.

In the following, the invention is described in detail with reference to the drawing enclosed. Figure 1 shows a perspective view of a three-colour element of a colour filter cell, Figure 2 shows the colour filter cell assembled of three-colour elements vertically from the front. Figure 3 shows a cross-section of a colour-rectified convex cell, Figure 4 shows a cross-section of a colour-rectified

cell, the elements of which are deformed, and Figure 5 shows the colour :cell with its control de¬ vices connected to-the projector. Figure 1 shows a basic form of a three-colour element 9, in which the element is formed of square colour filter strips 1, 2 and 3 (generally red, blue; and green) placed vertically to each other. For the sake of visualization, broken lines 4 show the ele- ment complemented to a cube. It depends on the angle in which the element is looked at, in which area pro¬ portion the different filter strips are seen. By vary¬ ing two angles of deflection independent of each other (e.g. swing angles around axles 5 and 6), it is possible to obtain an arbitrary combination of three basic colours. When such a cell is placed in a light beam, it is subjected to the effect of three different colour filter strips in a desired proportion.

A colour filter cell 8 of Figure 2 is created by combining elements shown in Figure 1 with each other. The filtering effect of the cell can be ad¬ justed by declining the cell in the same way as a separate element. In practice, the cell can be de¬ clined e.g. by a suitable lever mechanism either manually or even by means of two electric motors.

Figure 3 shows how in the case of a conically opening light beam 7 a deviation between colour shades arising in different points of a planar colour filter cell can be rectified by making the cell 8 convex in a suitable way. A deviation from colour can be entirely rectified by means of certain angles of deflection, e.g. by making the cell spherically curved, whereby no angle error at all is created in the light beam coming straight from the front. Figure 4 shows an alternative way to carry out

a rectification of angle: instead of making the whole cell 8 curved, separate elements 9 in different points of the cell are deformed so that their projections in the direction of the light rays coming into the cell are indentical in every point of the cell in the same way as in the convex cell shown in Figure 3.

One embodiment of the three-colour filter cell is shown in principle in Figure 5. To a projector 10 is connected a filter unit consisting of a housing 11 for device, of a possible heat filtering glass 12 placed at the output aperture of the light beam, of a possible colour equalizer 13 placed at the output aperture of the light beam, of a three-colour filter cell 8 turned by means of electrically controlled motors 14, and of a possible fan 15. In practice, it is possible to create by means of such a remote-con¬ trolled filter - by using a filter cell manufactured of a lightweight and heat resistant plastic material and e.g. repeater motors to turn the cell - a light beam of any colour in a few hundredths of a second. The purpose of the heat glass 12 is to absorb a great part of the heat radiation from the incoming light and thus to protect the cell 8 from overheating. The pur¬ pose of the fan 15 is also mainly to cool the cell 8. As colour equalizer 13 can be used e.g. a frosted glass plate; it may be necessary to use an equalizer, if the opening angle of the light beam coming from the filter is very large.

The embodiments described above are by no means intended to limit the invention, but the invention can be varied within the scope of the claims in many dif¬ ferent ways.