Colour signals are the light spectra either from the source or from the interaction between the illuminations’ spectra and response properties of materials. CCD colour camera can be described as a filter which transforms continuous colour signals from the limited spectral area to three descriptors (“red”, “green”, and “blue”) values of a limited range. In this sense, colour cameras resembles the human eye; they cannot directly measure the spectra of colour signals because the spectral accuracy is sacrificed for the spatial resolution (Fortner & Meyer 1997). Since the spectral data for a point is described with three values, it is only an approximation of the true, incoming colour signal spectra. Also because of this spectral data compression, colour samples with different reflectances can become metameric, which means, for example, that they appear as two different colours under a certain illumination whereas under a second illumination they cannot be discriminated (Wyszecki & Stiles 2000). According to Fortner and Meyer (1997), there are four reasons why the human eye has only three different cones: 1) there are a limited number of available visual pigments, 2) the increasing number of different cones decreases the light sensitivity of the visual system because a photon can be detected only once, 3) cones need space; and if more different cones are required to form a point, the area needed for seeing a point increases and therefore reduces resolution, and 4) more different cones would mean increasing already the enormous information flow to brain. Cameras are usually monochromatic or colour. There do exist imaging spectrographs to capture more accurately spectral data, but for them, the image forming takes a much longer time due to decreased light sensitivity. This makes them unsuitable for real-time operations and susceptible to environmental changes. Only colour cameras are considered in this thesis. It is important to note that sensor sensitivities vary between colour cameras which makes the descriptors camera dependent. In addition, there are two types of CCD colour cameras: 1CCD and 3CCD colour cameras, depending on the number of CCD elements. The 3CCD cameras have separate CCD detectors for each colour channel, whereas in 1CCD cameras the colours for the output channels are approximated using filters covering the detector. The filters have either stripe or mosaic layout over the detector and they can produce directly the RGB signals or other colours like cyan, yellow, magenta or white (no colour filter) (Holst 1998). These signals are interpolated to produce the three output colour channels and in the case of filters other than RGB, the channels are converted to RGB colour space. An image taken by a 1CCD camera has poorer spatial resolution and colour reproduction quality than the one taken with a 3CCD camera because of the colour interpolation in 1CCD cameras (Klette et al. 1998). 1CCD cameras are susceptible to colour Moire effects which cause colour deviation. On the other hand, the 3CCD cameras are more expensive and need more intense light.
Although in the modelling of colour image formation the main factors are illumination spectral power distribution (SPD), spectral sensitivities of the camera, and surface reflectances, there are many other factors which can have an essential effect: scene and acquisition geometry, surroundings, camera settings, camera type and other nonidealities of the camera. The output of the colour camera is often digitized RGB (Red, Green and Blue). Because the RGB space is redundant, it is often preferred to do further processing in another colour space.