Information Technology Reference
In-Depth Information
Figure 5.11:
(a) The response of the eye to various wavelengths. There is a pronounced peak in the green region.
(b) The different types of cone in the eye have the approximate responses shown here which allow colour vision.
Figure 5.11 shows an approximate response for each of the three types of cone. If light of a single wavelength is
observed, the relative responses of the three sensors allows us to discern what we call the colour of the light. Note
that at both ends of the visible spectrum there are areas in which only one receptor responds; all colours in those
areas look the same. There is a great deal of variation in receptor response from one individual to the next and the
curves used in television are the average of a great many tests. In a surprising number of people the single
receptor zones are extended and discrimination between, for example, red and orange is difficult.
The triple receptor characteristic of the eye is extremely fortunate as it means that we can generate a range of
colours by adding together light sources having just three different wavelengths in various proportions. This
process is known as
additive colour matching
which should be clearly distinguished from the subtractive colour
matching that occurs with paints and inks. Subtractive matching begins with white light and selectively removes
parts of the spectrum by filtering. Additive matching uses coloured light sources which are combined.
5.5 Colour difference signals
An effective colour television system can be made in which only three pure or single wavelength colours or
primaries
can be generated. The primaries need to be similar in wavelength to the peaks of the three receptor
responses, but need not be identical. Figure 5.12 shows a rudimentary colour television system. Note that the
colour camera is in fact three cameras in one, where each is fitted with a different coloured filter. Three signals,
R
,
G
and
B
must be transmitted to the display which produces three images that must be superimposed to obtain a
colour picture.
