Luminous efficiency is a measure of the proportion of the energy supplied to a lamp that is converted into light energy. It is calculated by dividing the lamp's luminous flux, measured in lumens, by the power consumption, measured in watts. Luminous flux is a measure of the energy emitted by a light source in all directions. ... For alternate uses of the term lumen, see lumen In physics, specifically photometry (optics), the lumen (symbol: lm) is the SI derived unit of luminous flux. ... The watt (symbol: W) is the SI derived unit for power. ...
See the article light bulb for the specific efficiency of various types of electric light sources. The incandescent light bulb uses a glowing wire filament heated to white-hot by electrical resistance, to generate light (a process known as thermal radiation). ...
Historically, the engineers' unit of power, the watt, has been separated from the unit of luminous intensity, which is also a form of power, because the eye has a varying sensitivity over the visual spectrum, being relatively insensitive to blue and to red light.
Luminous emission is not the same as the perceived brightness of the source when you look at it.
The luminous intensity of a "white" light source is defined by multiplying the watts emitted at each wavelength by the efficiency of that wavelength in exciting the eye, relative to the efficiency at 555 nm.
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