Lightness and Brightness and Other Confusions
Lightness and Brightness and Other Confusions
Lightness and Brightness and Other Confusions
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Examples of colorimetric concepts <strong>and</strong> units<br />
Tristimulus values<br />
Chromaticity coordinates<br />
CIELAB diagram<br />
CIELUV diagram<br />
Hue angle<br />
MacAdam ellipses<br />
One important use of colorimetry is to measure <strong>and</strong> specify the chromatic<br />
qualities of light sources. For light sources consisting of a glowing material<br />
the colour temperature is expressed in Kelvin (K). The temperature here<br />
refers to a theoretical black body which, when glowing, emits differently<br />
coloured light depending on its temperature – from slightly red when starting<br />
to glow through white to bluish at very high temperatures. The correlated<br />
colour temperature of a light source is calculated through colorimetric comparisons<br />
between its emitted light <strong>and</strong> that of the theoretical black body.<br />
Another important quality of a light source is its colour rendering capacity.<br />
One aspect of colour rendering deals with the colour gamut, i.e. how many<br />
different colours you can perceive under this light source. Another aspect<br />
deals with the character or colour differences. The colour rendering capacity<br />
of a light source is usually expressed as its Colour Rendering Index, CRI (Ra),<br />
which is colorimetrically established. In principle, colour samples with st<strong>and</strong>ardized<br />
reflectance curves are illuminated with the light to be controlled, <strong>and</strong><br />
the reflected light is compared to that which appears when the samples are lit<br />
by a st<strong>and</strong>ardised reference light source. In practice, once you have the spectral<br />
distributions of the samples <strong>and</strong> the light sources in question, all this is<br />
done mathematically.<br />
Physiological processes behind visual perception<br />
The V( ) curve <strong>and</strong> other basic assumptions behind photometric <strong>and</strong> colorimetric<br />
technology were established through psychometric matching experiments,<br />
belonging to classical experimental psychology. Since then, the underst<strong>and</strong>ing<br />
of mechanisms in the human visual system has made large progress<br />
through the additional input from brain research. Such new knowledge is,<br />
however, not always incorporated in the theoretical foundations of photometry<br />
<strong>and</strong> colorimetry. For example, today’s underst<strong>and</strong>ing of the sensitivity<br />
<strong>and</strong> interaction of retinal receptors could possibly lead to the ab<strong>and</strong>onment of<br />
the V ) curve in favour of other theories (Liljefors 2010). Should this be done,<br />
it would change all the photometric concepts, units <strong>and</strong> measuring tools –<br />
that is, the very basis for lighting technology.<br />
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