Illumination

The correct scientific term is 'illuminance'.

There are basically three measures, and an SI unit associated with each, that a quizzer needs to get their head around. Even as someone who had a scientific education, I find it very confusing. What follows is my best attempt to summarise; please do not take it as gospel, and if anyone can help to enlighten me (not to say illuminate me!), please get in touch!

Illuminance can be thought of as the amount of light (technically, luminous flux) that falls upon a particular surface. The SI unit of illuminance is the lux; one lux is equal to one lumen per square metre.

The lumen is the SI unit of luminous flux. Luminous flux is a measure of the total amount of light emitted by a source in all directions. One lumen is equal to one candela steradian. This is the measure that I have most trouble in understanding.

The candela is the SI unit of luminous intensity. Luminous intensity is defined as 'the (wavelength–weighted) power emitted by a light source in a particular direction per unit solid angle'. It's wavelength–weighted to reflect the variation in light intensity perceived by the human eye at different wavelengths within the visible spectrum.

The candela is one of the base units in the SI system, along with the metre, kilogram, second, ampere, kelvin and mole. All other units in the SI system (including the lux and the lumen) are derived from these seven, plus the two 'dimensionless' units – the radian and the steradian, which are the units of an angle and a solid angle, respectively.