Figure 10.2. Spectral sensitivity
of the eye.
The colour sensitivity of the eye
changes at very low lighting levels, however for normal artificial lighting photopic vision
applies.
The brain can modify the image
received by the eye and this is called colour consistency. When the colour of light changes the
image seen by the eye changes. However for familiar objects the brain knows they have not actually
changed colour and thus adjusts the visual signal.
This effect minimises the changes
in daylight and the local effects of reflected light. If you stand in a green field your face will
be greener but this will not be noticed unless your photograph is taken. The camera and film
records the actual situation and this will be apparent when looking at the photograph.
Colour Rendering
The eye and mind like to see colours appear natural and normal and that requires a light
source with good colour rendering.
Such a lamp requires the presence
of all colours in a continuous spectrum. See Figure 10.3 (page 6).
The spectral distribution curves
give an indication of how a light source will make objects appear. In general:
-
Incandescent and tungsten halogen
lamps produce continuous spectra.
-
Fluorescent lamps produce low level
continuous spectra but are dominated by one or more peaks.
-
HID lamps produce light in discrete
bands or lines.
Lamps with continuous spectra
generally produce less distortion of viewed colours and create better colour rendering.
Lamps with selective spectra
generally intensify certain colours and reduce others so produce a distorted colour image, which is
poor colour rendering.
The two lamps shown in Figure 10.4
(page 6) are made up of different colour constituents but their "white" colour appearance would be
very similar.
Specifying Lamp Colour
Two measures are needed to describe lamp colour completely:
Chromaticity (colour appearance) and colour rendering.
There are two ways used to describe
chromaticity:
-
CIE chromaticity co-ordinates. See
Figure 10.5 (page 6).
-
Correlated colour temperature or
Kelvin scale. This only applies to incandescent lamps or lamps whose colour coordinates fall on or
very near the "black body" locus. This line is indicated on
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