Luminance/Reflectance/Apostilbs

• Illuminance (lux) x reflectance = luminance (apostilbs).
• Apostilbs is not an SI unit. To convert this to SI (candelas m-2) divide by pi (or multiply by 0.318).
• If the Munsell "Value" reference of a color is known the reflectance can be approximately calculated using Reflectance = V(V - 1) where V = value.
• If Munsell "value" is 6, reflectance = 6 x 5 = 30% = .3.
• Since in most rooms the different surfaces will have different colors, they will reflect differing amounts of light and this will affect the distribution of light in the room.

Reflectance

• For a perfectly diffusely reflecting surface, the ratio of the reflected luminous flux to the incident luminous flux is the reflectance.
• Luminance = illuminance x reflectance / pi
• Reflectance = illuminance / luminance
• When the surface is not perfectly diffusely reflecting, reflectance is replaced by a

  luminance factor

  Luminance factor is the ratio of the luminance of a surface viewed from a particular position and lit in a specified way to the luminance of a diffusely reflecting white surface viewed from the same direction and lit in the same way. Here Luminance = illuminance x luminance factor / pi
  In lighting practice

  illuminance

  and

  luminance

  are most frequently used to characterise the effect of lighting.