Light Output

Increasing Light Emission

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.

A table of ambient illuminances. For our glowing bacteria to be seen from a given distance, they must be at least 1/20th of this value.

Illuminance	Example
10⁻⁵ lux	Light from Sirius, the brightest star in the night sky
10⁻⁴ lux	Total starlight, overcast sky
0.002 lux	Moonless clear night sky with airglow
0.01 lux	Quarter moon
0.27 lux	Full moon on a clear night
1 lux	Full moon overhead at tropical latitude
3.4 lux	Dark limit of civil twilight under a clear sky
50 lux	Family living room
80 lux	Hallway/toilet
100 lux	Very dark overcast day
320–500 lux	Office lighting
400 lux	Sunrise or sunset on a clear day.
1,000 lux	Overcast day
10,000–25,000 lux	Full daylight (not direct sun)
32,000–130,000 lux	Direct sunlight

To summarise, the luminosity function allows one to convert between radiant energy and luminous (i.e., visible) energy.