The brightness of a light source, or its luminous intensity, is measured in 'Candelas'. This SI unit replaced the 'international candle' in measuring luminous intensity, also known as candle power.
The candela is defined as the luminous intensity of part of the surface of a black body at a specific temperature and pressure, perpendicular to the surface.
A Black body is a hypothetical object which absorbs all electromagnetic radiation hitting it. It is therefore completely black.
It also has the property of emitting all frequencies of the electromagnetic spectrum. The intensity of each frequency increases as the temperature increases. Also, as the temperature increases, the frequency at which the radiation is most intense decreases. In fact, the temperature is inversely proportional to the frequency.
All this means that at 1772°c, the melting point of platinum, a black body produces radiation at the frequency of visible light.
The candela must be measured under a pressure of 101325 Pascals, which is average at atmospheric pressure, this value being known as 1 atmosphere.
The definition must come from 1/600000 of a square metre of the surface of the black body, which, by my calculation, gives an area of 1 2/3 square millimetres.
The only remaining problem is, as you may have noticed, a black body is a hypothetical object, and whilst the concept works, any tangible object does not absorb all the radiation cast upon it - some is reflected. Because of this, an object must be made which will reflect little light, as small an amount as possible.
A light meter is an instrument used to measure the strength of light. Different light meters have been invented for use by astronomers, and illumination experts. Illumination experts use a light meter called an illuminometer, foot candle meter, or lux meter to measure the lighting in buildings. Photographers use exposure meters to tell them how to correctly set their cameras.
Most light meters used today are photoelectric light meters. They use photo cells made of selenium, a substance which produces a weak electric current when light shines on it.
As more light falls on to the photo cell, the greater the electric current. The strength of the light falling on the cell, or its luminous intensity, can be found by measuring the electric current with a sensitive electric meter.
Other photo electric light meters use a cadmium sulfide cell to control an electric current supplied by a battery. The amount of current the cell passes depends on how much light shines on it. This current then runs an electric meter.
Light meter scales give readings in foot-candles, lumens, candelas or a number that can be translated into other units.