Illuminance vs. Luminance: Know Differences!


In the lighting world, many of the lighting terms often sound very  similar but have vast different meanings which can lead you to the confusion and a misunderstanding of lighting terminology. Illuminance and luminance both the terms are the most confusing term and are more or less used incorrectly by people, sometimes even by the people in the same industry!

illumination & Luminance


  • Simply the illuminance determines the amount of light that covers a surface. Illuminance is invisible, generally it is light passing through space and not seen unless you look at the source (e.g. a light bulb) or a surface it reflects off or we can only see luminance.


  • The illuminance or light level is the amount of light energy reaching a given point on a defined surface area, which is the luminous flux (i.e. lumens) per square meter.


  • In other words, At a point on a surface, the ratio of the luminous flux incident on an infinitesimal element of the surface containing the point under consideration to the area of the element.
  • (NOTE — The unit of illuminance (the measurement of illumination) is Iux which is 1 lumen per square meter.)
  • Luminance is an amount of visible light that comes to the eye from surface or it can also be said as a state or quality of radiating or reflecting light.
  • Luminance is often used to describe emission or reflection from flat or diffuse surfaces. The luminance indicates how much luminous power will be detected by an eye looking at the surface from a particular angle of view.
  • Therefore luminance is thus an indicator of how bright the surface will appear.
  • Note: Luminance = Illuminance + Reflection


  • In other words, it is the quotient of the luminous intensity in the given direction of an infinitesimal element of the surface containing the point under consideration by the orthogonally projected area of the element on a plane perpendicular to the given direction. The unit is candela per square metre (cd/m2).
Also Read: Components of Daylight Factor


illumination-light flow-cover area

Luminance-Luminous intensity-illuminance

  • Standard unit for illuminance is Lux (lx) which is lumens per square meter (lm/m2).
  • Typical illuminance values/ranges for our understanding are as follows:
1 lxFull moon
10 lxStreet lighting
100-1’000 lxWorkspace lighting
10’000 lxSurgery lighting
100’000 lxPlain sunshine
  • The SI unit for luminance is candela per square metre (cd/m2).

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