What is the meaning of lumen as far as brightness is concerned?

Answer 1

Lumen and brightness are unconnected.

Lumens are an imaginary measure of the amount of light (equivalent to a weight of apples for instance). A lamp will be said to emit X lumens.

Lumen figures are calculated from the amount of energy at any particular wavelength and then applying a correction to allow for the sensitivity of the human eye (which is greatest in the red-orange). Different correction figures are applicable for the two different types of human sight - that in high and low level illumination - photopic and scotopic respectively. Hence any light source will have both photopic and scotopic lumen figures. Photopic would be applicable to household or office lighting, scotopic to street lighting, for instance.

Different corrections are also known for other organisms, other than man, most importantly for plants, where plant lumens are a measure of the effect of any light-source on photosynthesis.

AnswerIn simple terms, the lumen is the photometry-equivalent of the watt. In other words, it defines the rate at which a source emits 'visible light' energy (as opposed to the watt, a radiometry unit, which defines the rate at which the same source emits allradiant energy (including visible light). Lumens are NOT used to measure 'brightness', or more accurately, luminous intensity (candelas) or luminance (candelas per square metre).

Note that the 'watt', described above, is used to measure the output from a light source, not its input.

Radiometry describes the measurement of electromagnetic energy across a very broad spectrum. Photometry describes the measurement of just the 'visible light' part of the electromagnetic energy spectrum, as perceived by the human eye.