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Light Output

Increasing Light Emission

Relevant Physics

  • The Candela measures how much light perceivable to the human eye is being emitted by a source of radiation, the Luminous Intensity (or luminous flux. It is related to Radiant intensity (or radiant flux) (which is measured in watts per steradian) but has been weighted by a Luminosity function. This alters the power output so that it is proportional to the sensitivity of the human eye: Definition of the candela with respect to Radiant intensity. Therefore, an object which is black may be emitting a lot of radiation (measured in watts per steradian) but since it is not perceivable to the human eye it is emitting 0cd of radiation.
  • The lumen is simply cd*sr. It is thus the analogue to the Watt
  • The Lux measures how much light is incident on a suface, the Illuminance. It is related to irradiance (measured in Watts per m^2) but has been weighted again by the luminosity function. 1lux=1cd/m^2
  • The luminance of an object is how much light is emitted from it. This is also measured in Lux, but it is different contextually.

10−5 lux Light from Sirius, the brightest star in the night sky
10−4 luxTotal starlight, overcast sky
0.002 luxMoonless clear night sky with airglow
0.01 luxQuarter moon
0.27 luxFull moon on a clear night
1 luxFull moon overhead at tropical latitude
3.4 lux Dark limit of civil twilight under a clear sky
50 luxFamily living room
80 luxHallway/toilet
100 lux Very dark overcast day
320–500 lux Office lighting
400 lux Sunrise or sunset on a clear day.
1,000 luxOvercast day
10,000–25,000 lux Full daylight (not direct sun)
32,000–130,000 luxDirect sunlight
  • To summarise, the luminosity function allows one to convert between radiant energy and luminous (i.e., visible) energy.

Human Vision