Conventional projection systems typically use 3 primary colours (red, green and blue) for the reproduction of colour images. The colour gamut that can be produced by an additive combination of the 3 primary colours nevertheless is limited. The colour gamut depends on the dominant wavelength (hue) and on the excitation purity (saturation) of each of the primary colours. The visible colours, the primary colours and the gamut of produced colours are usually represented in a chromaticity diagram e.g. CIE 1931 chromaticity diagram or CIE 1976 U.C.S. (Uniform Chromaticity Scale).
One solution to produce a wider colour gamut is by increasing the excitation purity of the primary colours, or in other words to narrow the spectral pass band of each of the three primary colours. An alternative solution to produce a wider colour gamut may be the use of more primary colours, such as e.g. using 4, 5, 6 or more colours instead of 3 colours. In printing devices, where subtractive mixing of colours is performed (by different ink cartridges), the commonly used colours are cyan, magenta, yellow and black (CMYK). In special printers, dedicated for certain types of printing, additional colours such as e.g. indigo also are used. In display technology, several techniques are known to apply more than three primary colours. One option is to generate a wider colour gamut by generating more than three primary colours by filtering them from a white illumination source and modulating them sequentially according to image data using a single chip modulation system. In such a system, typically the amount of illumination from the illumination source that is not used for displaying the image is relatively high resulting in a less efficient system. In another option, the wider colour gamut is generated by guiding more than three primary colours each to a primary colour dedicated modulator, where the primary coloured sub-beam is modulated. The latter typically results in complex and expensive systems. An example of such a system is a display system using four primary colours, each primary colour sub-beam modulated by its own primary colour dedicated modulator, as described for example in “Four primary colour projection display” by Roth and Caldwell in SID 05 Digest (2005) 1818.
Adding more primary colours also introduces an electronic puzzle as to how to describe the colour space, as each colour to be displayed needs to be produced by combining the different primary colours, i.e. by subtracting in the case of printing, or by adding in the case of a display system. A method of handling the data conversion for generating images to be displayed using more than three primary colours has been described e.g. in European Patent EP 0 897 641 B1 by BARCO N. V.