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.

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

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 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.