Photometry measures visible light in units that are weighted according to the sensitivity of the human eye. Our eye is a complex, nonlinear, detector of electromagnetic radiation with wavelengths between 380 and 770 nm. The sensitivity of the human eye varies with the wavelength. Figure 2.1 shows the CIE photometric curve (CIE stands for Commission Internationale d'Eclairage - International Commission on Illumination). This curve tells us that a light source of strength 1 will appear brighter if it emits light of wavelength 550 nm, than the same strength light source that emits light of 440 nm wavelength. Actually, all that photometry does is the weighting of radiometric units using the CIE photometric curve. The only difference between radiometry and photometry is in their units of measurement. All radiometric units have their photometric counterparts. We will mention only luminance as the counterpart of radiance. In fact it is just photometrically weighted radiance. Note that digital image synthesis simulates the light in an enviroment, and as a result radiances (or luminances) of particular wavelengths are computed. This radiances are stored in the raw-image, which is then mapped to the display device, using one of the mapping techiques.
Figure 2.1: CIE photometric curve for photopic, V(), and scotopic V'() [WySt82]
Up to now color nas not been taken into account. The next section, colorimetry, will give us a brief overview of color.