Lightness and Brightness and Other Confusions

Examples of colorimetric concepts and units
Tristimulus values
Chromaticity coordinates
CIELAB diagram
CIELUV diagram
Hue angle
MacAdam ellipses
One important use of colorimetry is to measure and specify the chromatic
qualities of light sources. For light sources consisting of a glowing material
the colour temperature is expressed in Kelvin (K). The temperature here
refers to a theoretical black body which, when glowing, emits differently
coloured light depending on its temperature – from slightly red when starting
to glow through white to bluish at very high temperatures. The correlated
colour temperature of a light source is calculated through colorimetric comparisons
between its emitted light and that of the theoretical black body.
Another important quality of a light source is its colour rendering capacity.
One aspect of colour rendering deals with the colour gamut, i.e. how many
different colours you can perceive under this light source. Another aspect
deals with the character or colour differences. The colour rendering capacity
of a light source is usually expressed as its Colour Rendering Index, CRI (Ra),
which is colorimetrically established. In principle, colour samples with standardized
reflectance curves are illuminated with the light to be controlled, and
the reflected light is compared to that which appears when the samples are lit
by a standardised reference light source. In practice, once you have the spectral
distributions of the samples and the light sources in question, all this is
done mathematically.
Physiological processes behind visual perception
The V( ) curve and other basic assumptions behind photometric and colorimetric
technology were established through psychometric matching experiments,
belonging to classical experimental psychology. Since then, the understanding
of mechanisms in the human visual system has made large progress
through the additional input from brain research. Such new knowledge is,
however, not always incorporated in the theoretical foundations of photometry
and colorimetry. For example, today’s understanding of the sensitivity
and interaction of retinal receptors could possibly lead to the abandonment of
the V ) curve in favour of other theories (Liljefors 2010). Should this be done,
it would change all the photometric concepts, units and measuring tools –
that is, the very basis for lighting technology.
62