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Glossary
Introduction-
Historically there has been a lot of confusion about how to measure light. Originally all measurements were made relative to a candle. In order to try and standardise this people began to make the measurements relative to a British Standard Candle of specified wick, wax and diameter.
However, this was not a very accurate benchmark for measuring light so they began trying to measure it relative to a black body (an object which absorbs and emits all radiation perfectly). The best black body possible was made; an oven was heated to the freezing point of platinum and a square of 1cm2 was cut out of it. All measurements were made relative to the light emitted from that square.
This lasted untilinstruments to measure the spectral distribution of light accurately were invented. It was realised that humans see very little radiation, only what’s in the visible spectrum, and the eye doesn’t detect all of that light equally. Scientists came up with a system of units for measuring photometric (visible) quantities by measuring only the light given out in the visible range, and weighting the units towards the wavelength at which it is easiest to see (555nm). This method is still not perfect since every eye is different, and changes with age, but it gives the most accurate measure of visible light that we have.
Luminous Flux-
The visible power a source emits in all directions is measured in lumens. Thus, two bulbs of the same wattage will produce different amounts of luminous flux. The formal definition of a lumen is the luminous flux of a source which emits monochromatic (only one wavelength) radiation of frequency 540 x 1012 Hz, has a radiant intensity of 683-1 watts and emits the light evenly over a sphere of 4π steraidians.
http://www.lamptech.co.uk/Documents/SO1% 1
Solid Angle-
To understand the other quantities involved with light it is necessary to know what a solid angle is. Imagine a point source evenly emitting light in all directions. A sphere of light would be produced. The solid angle of a light cone is the projected area of the cone divided by the total surface area of the sphere.
A solid angle is measured in steradians, where 1 steradian corresponds to the solid angle subtended by a cone of unit slant height and unit projected area. A sphere has a solid angle of 4π steradians.
http://www.schorsch.com/kbase/glossary/s 1
Luminous Intensity-
Luminous intensity is the flow of luminous energy, or lumens, per steradian. The unit is called the candela, which originates from the term ‘candle power’.
We can see that the candela and the lumen are linked such that a point source that has a luminous intensity of 1 candela will have a luminous flux of 4π lumens, assuming the source radiates uniformly in all directions.
Illuminance-
This is the rate of flow (lumens) per unit area. It is used to describe what happens as light falls onto a surface, and the unit is the lux.
Luminance-
Luminance is the rate of flow per unit area per steradian. In other words it’s the flow of light from one unit area, making the unit the candela per square metre. This unit is useful because a close dim object and far off bright object could have the same illuminance, but their luminances would be different.
Efficacy-
Efficiency of an object is the ratio of the power output over the power input. However this is an inappropriate measure for lighting since power output can also include infra-red light (heat) or ultra-violet light. Therefore a more useful ratio is efficacy, the lumens produced divided by the power input in lumens per watt.