KevinWuFaji
Newly Enlightened
- Joined
- Jun 9, 2011
- Messages
- 1
What's the difference between the unit "lm" & "mcd"?
and for instance, how much of "lm" is the same as 20000mcd?
and for instance, how much of "lm" is the same as 20000mcd?
That's not entirely correct.
<SNIP>
That's not entirely correct.
candela is a base unit in the SI system. It measures how much light a light source puts out in one direction at the specific wavelength of 555 nm. You can use it to compare different light sources, if you define the direction (it's not in the candela definition). Even so, this is only practical for similar light sources. It works quite well for comparing incandescents, but cannot easily be used to compare incans to CFLs or LEDs, since the light output at just one wavelength does not compare to the total light output in a similar way for different technologies. The candela definition specifies how to construct a specific light source, which can then be used to calibrate other light sources or detectors, but it is not very useful for comparing light sources that are not constructed to this specification.
Lumens are different. As stated, it measures the light output of the light source in ALL directions. It also measures ALL wavelengths (in the visible range of 400 .. 700 nm). Also computed in is the sensitivity of the human eye. If you want to compare different light sources, and how well they will illuminate your surroundings, you have to go with lumens.
You cannot calculate lumens from candela for different light technologies, as the spectra of the light sources are vastly different.
this is true. On the forums the term "lux" is often used interchangeably with candela. This is because in the special case where your surface is exactly 1m from a 1-candela point-source, the surface will be lit to an illuminance of 1 lux (or 1 lumen/square-meter).Both units have little to do with lux (which measures how much light ARRIVES at a certain spot and distance from the light source). To calculate that, you have to specify the direction from the light source (which is not specified in the candela definition), and the lumens number of the light source does not tell how the light is distributed around the light source (if the light source throws all light in one direction, you get a high lux number in THAT direction, but if another light source with the same lumens throws light in all direction, that same spot will have a low lux number).