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.