The candela is the foundation unit for the measurement of visible light. It is one of the seven foundation SI units. It's formal definition is:
The candela is the luminous intensity, in a given direction of a source that emits light that has a wavelength of aprox. 555 nm (yellowish-green) and that has a radiant intensity in that direction of 1/683 watt per steradian. One candela is 1000 milicandelas or 1 cd = 1000 mcd.
The candela value is independent of distance. One can think of it as the emission from the lamp without the interest in what happens to the photons it has ejected. The candela is mostly used when dealing with focused light - for LEDs, flashlights or spots.
The candela can be used for measuring the light intensity of nonfocusing light sources but this can lead confusion . For example: a LED made by CREE can have up to 100 Cd while a 100 Watt incandescent light bulb has around 105 Cd. Does that mean that the LED is a bright as a 100 W light bulb? The answer comes from view angle: Cree 80 deg, light bulb: 360 deg. vs
To clarify this problem we will give some examples of light intensity in day to day life (in candelas per square meter):
Except for focusing light sources light is measured in lumens which we explain below.
|Activity||Illumination on task surface(lux)|
|Public areas with dark surroundings||20 - 50|
|Working areas where visual tasks are only occasionally performed (storage rooms)||100 - 150|
|Easy Office Work, Classes||250|
|Normal Office Work, PC Work, Study Library, Groceries, Show Rooms, Laboratories||500|
|Supermarkets, Mechanical Workshops, Office Landscapes||750|
|Normal Drawing Work, Quality control||1,000|
|Detailed Drawing Work, Very Detailed Mechanical Works (watchmaking)||1500 - 2000|
|Performance of visual tasks of low contrast and very small size for prolonged periods of time||2000 - 5000|
|Performance of very prolonged and exacting visual tasks||5000 - 10000|
|Performance of very special visual tasks of extremely low contrast and small size||10000 - 20000|
The viewangle shows the angle of radiation for a light source. For example a view angle on 30 deg means the light has a shape of a cone and its borders form a 15 deg angle with the center axis. A change in the view angle affects the luminous intensity of an LED (candela) but not the luminous flux (lumen).
Viewing angle is actually the most important element in the performance of a light source. A very wide viewing angle means that a large percentage of light ends up going in non-useful directions ( for example: up). Incasing the light source and using a reflector has limited efficiency.
By taking into the account both viewing angles and lumen output the comparison between LEDs and other light sources looks like this:
|Light source||Lm||Lm/W||Viewing angle||Usefull viewing angle*||Useful lm|
|CREE X-RE R2 White LED||242||92||90||100%||242|
|100 Watt incandescent||1200||12||360||33%||396|
|25 Watt Halogen||260||9||360||33%||85.8|
|15 W T8 neon||1350||90||360||33%||445.5|
* without case or reflector
Next time when you go out at night look up in the sky. Do you see stars, or the Milky Way? In you live in a big city the answer is: only a white glow. For people that live outside a big city in North America, Europe, India, Japan or China the answer will be: a white glow and a few stars. The Milky Way can only be seen from remote regions or non-developed countries.
The reason? Bad lighting design and use of sources with ultra wide view angles creates light pollution, light that is wasted in the sky. The effects of such pollution are wide, they affect animals and insects and also our sleep.