Lighting Design Glossary

Luminous Flux Density
Illuminance
Luminous Exitance

Illumination is a deprecated term for Illuminance.
Luminous emittance is a deprecated term for Luminous Exitance.

(Terms of photometry)

Luminous flux density is photometrically weighted radiant flux density, which means luminous flux per unit area at a point on a surface where the surface can be real or imaginary. An imaginary surface can for example be used to measure or calculate illuminance anywhere in space, maybe to determine the daylight factor on the workplane.

There are two cases:

• Illuminance (usually 'E' in formulas) is the total amount of visible light illuminating (incident upon) a point on a surface from all directions above the surface. This "surface" can be a physical surface or an imaginary plane. Therefore illuminance is equivalent to irradiance weighted with the response curve of the human eye.

  Standard unit for illuminance is Lux (lx) which is lumens per square meter (lm/m²).

  There are several older units of illuminance:

  footcandle	1 fc = 10.764 lx.
  dalx (in canadian safety regulations)	1 dalx = 10.764 lx.
  phot	1 ph = 10'000 lx

  Typical illuminance values are:

  1 lx	full moon
  10 lx	street lighting
  100-1'000 lx	workspace lighting
  10'000 lx	surgery lighting
  100'000 lx	plain sunshine

  A surface will receive 1 lx of illuminance from a point light source that emits 1 cd of luminous intensity in its direction from a distance of 1 m.
  When using the nonstandard US units, this translates into 1 fc received from a 1 cd source 1 ft away.

• Luminous exitance or is the total amount of visible light leaving a point on a surface into all directions above the surface. Therefore luminous exitance is equivalent to radiant exitance weighted with the response curve of the human eye.

  Unit for luminous exitance is lumens per square meter (lm/m²).