Luminous intensity
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Candela
Brightness is stated in candela (cd), which tells how bright the light is in a specific direction. This is used to state the brightness from reflector lamps and shows the intensity of the light in the beam of light. Using this value one can calculate brightness levels. Some low voltage halogen lamps have different focus angles, and the candela will be different with a wide beam of light (60 degrees) then with a narrow beam (10 degrees) with the same amount of watts. Luminance is stated in cd/m2 and tells how the eye perceives the light in a illuminated surface. This depends on the brightness level and the reflection of the surface, this is influenced by e.g. contrast.
Lumen
Illumination output is stated in lumen and tells how much light a light source emits in radius. Therefore this symbol can not be used for light sources with reflectors, for that you need candela. Luminous efficiency is stated pr. watt (lm/W) snd tells how much light (lumen) you get with regards to the power consumption (watt). The more efficient a light source is the higher the number. A normal bulb can normally have 10 lm/W, while a compact fluorescent lamp can have from 45-50 lm/W depending on the quality.
