Photometry is the study of the measure of light, developed taking into account our perceptual and neurological interpretation of its nature. Photometric lights are used when a physically-based lighting simulation is desired to achieve real-world illumination effects. Four photometric quantities come into play when using photometric lights. They are:

• Luminous flux
  • This term refers to the amount of light energy arriving, leaving or going through a surface per a unit time.
  • The unit of luminous flux for both the International and American Systems of Units is the lumen (lm).
• Illuminance
  • The measure of luminous flux incident on a surface of unit area. It is used to describe illumination levels on a surface without being size-dependent.
  • The International Unit is the lux (lx), quantified as 1 lm/m².
  • The American Unit is the footcandle (fc), quantified as 1 lm/ft².
• Luminance
  • This refers to the light reflected off a surface in a given direction. This quantity is converted to colors to create a realistic rendering.
  • The standard units of luminance are the candela per square meter and the candela per square inch.
• Luminous Intensity
  • Luminous Intensity measures light energy per unit time from a point source in a given direction. It shows how the luminous intensity of a light source varies as a function of its outgoing direction.
  • Candela is the standard unit of measure for luminous intensity.

Alright, now that we're done with the red tape and photometric nomenclature, we can start talking about using these lights. There are a couple things that one must keep in mind when using photometric lights. The first is that all photometric lights attenuate using the inverse-square falloff. The next important point is that because photometric lights use real-world algorithms to compute lighting, your scene must use real-world units for its scale in order for the lighting effect to look realistic. With each photometric light, you have a choice of distribution methods. There are eight different varieties of photometric lights, each of which are described below:

Target Point Light	Free Point Light	Target Linear Light	Free Linear Light
Target Area Light	Free Area Light	IES Sun Light	IES Sky Light

Download the tutorial scene here.

Isotropic Distribution

Spotlight Distribution

Web Distribution