Lightness and Brightness and Other Confusions
Lightness and Brightness and Other Confusions
Lightness and Brightness and Other Confusions
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
Examples of photometric concepts <strong>and</strong> units<br />
Luminous flux (lumen)<br />
Luminous intensity (c<strong>and</strong>ela)<br />
Luminance (c<strong>and</strong>ela per square metre)<br />
Illuminance (lux)<br />
Luminous efficacy (lumen per watt)<br />
The above concepts are part of the SI unit system<br />
Psychophysical aspects of ’colour’<br />
The psychophysical methodology for colour is called colorimetry. Its primary<br />
aim is to identify <strong>and</strong> quantify visual differences between colour stimuli in<br />
order to ensure production stability <strong>and</strong> to formulate levels of tolerated deviation.<br />
Colorimetry was developed from the same basic assumptions as photometry.<br />
Observers were asked to adjust mixtures of three monochromatic light<br />
stimuli to match the colour of a single monochromatic stimulus. From this<br />
was calculated a number of models, in which every colour stimulus is characterised<br />
by three physical variables (tristimulus values), typically dominant<br />
wavelength, luminance <strong>and</strong> spectral purity (Tonnquist 1995, p 54). CIE has<br />
published different mathematical functions – algorithms – to be used under<br />
specified circumstances, including a choice of st<strong>and</strong>ard illuminants. These<br />
functions are illustrated as diagrams – ‘colour spaces’ – which can be used for<br />
specifying colour coordinates, such as CIELAB, <strong>and</strong> colour difference scales,<br />
such as E (delta E). Such models can roughly indicate the appearance of the<br />
perceived colour evoked by the stimulus, but this is not their purpose <strong>and</strong> the<br />
indication is far from precise.<br />
60