Photometry is the measurement of a quantity of light energy, weighted by the sensitivity of the eye. If we consider the quantity of light which arrives on a surface we measure an illumination (Lux) while if we measure a quantity of light emitted by a source we speak of luminous intensity (Cd) or luminance if we consider the source as spatial unit (display screens in Cd / m²)
Colorimetry analyzes light energy according to 3 components (red, green and blue) and always according to the sensitivity of the eye. These three components X, Y and Z make possible to calculate color coordinates in several color spaces (x; y for CIE 1931 or u, v for CIE 1976)
Coupled with a goniometer, the photometric measurement makes possible to obtain the complete distribution of light in the space. This distribution is essential to define the quality of lighting and to simulate the behavior of a luminaire or an automobile light.