138 TECHNOLOGY / LIGHT SOURCES

Figure 4: Equation to transform from the CIE XYZ to the CIE RGB space.

Figure 5: SPDs of a 2800K incandescent and 5000K fluorescent tube.

for calculating the luminous flux. The SPD of an incandescent lamp and a fluorescent is shown in figure 5. The relative SPD is often calculated because the luminance of lighting fixtures and other light sources are handled separately and so a spectral power distribution is often normalized to unity at 555 or 560 nanome- ters, coinciding with the peak of the eye’s luminosity function.

It is also possible to transform from the CIE XYZ to the CIE RGB space using the equa- tion seen in figure 4.

SPECTRAL POWER DISTRIBUTION (SPD) In colour science and radiometry, a spectral power distribution (SPD) describes the power per unit area per unit wavelength of an illumination, or more generally, the per- wavelength contribution to any radiometric quantity and as we have seen it is crucial

OVERVIEW The human eye perceives colour according to a wide range of factors that have been modelled for nearly a century and have been shown to approximate three distinct colour functions approximated by the CIE as standard observer functions. These functions can be used to model the human eye response to artificial light and can be transformed into a variety of math- ematical spaces using simple calculations. The human eye varies its spectral sensitiv-

ity response according to the ambient light conditions so the eye has three defined vision states including photopic, scotopic and Mesopic. Finally, the perceived colour from an object to the human eye can be defined by several processes as follows: 1) The characteristic spectral power density of the light source being emitted 2) The surface and material characteristics of the object being illuminated 3) The spectral and intensity response of the human eye observing the light being emitted from the surface of the illuminated object. In the next issue I will cover the definition of CCT and CRI as well as how to mathemat- ically calculate CCT using the chromaticity coordinates of LED lighting sources. g.archenhold@mondiale.co.uk