Candela = ??

[waion]

(sorry if this is a repeated question)

Could anyone pls tell me the meaning of candela? Is it equals to (different people told me):

"a 1 candela light source emits 1 lumen per steradian in all directions, isotropically" (hence 1 candela = 12.6 lumens) OR

"a measure of visible power per solid angle" (hence 1 lumen/steradian = 1 candela)

Thank you for helping me solve this really confusing question.

[waion]

Sorry for missing the following link given by one of those who told me the above meanings:

http://www.intl-light.com/handbook/ch07.html

[SilverFox]

Hello Wajon,

I think this can be simplified for our use in measuring the beams from flashlights to:

1 candela (beam) = 1 foot candle (at 1 foot) = 1 lux (at 1 meter).

If you are measuring room lighting then mean spherical candela would be more meaningful, and you would take into consideration the area being illuminated.

Tom

[waion]

Thank you, Tom.

[FlashlightOCD]

[ QUOTE ]
waion said:
... 1 candela = 12.6 lumens ...

[/ QUOTE ]

Candela ratings are very confusing and misleading. The above 12.6 conversion factor is only true if the light source is emitting 1 candela in all directions at the same time, but most flashlights [and especially LEDs] are very directional.

Example: A laser pointer would have a very high candela rating, but it's total lumens output would be a tiny fraction of it's candela rating.

I hope that helps.

[The_LED_Museum]

One of my green laser pointers gives an "OL" on my Meterman LM631 light meter, even when the meter is set to its highest range X10. But if the beam were widened, it wouldn't be much better than a few green LEDs aimed at the ceiling. :-\

[waion]

Thank you, FlashlightOCD and The LED Museum. I can see and totally agree with what you mean. Candela is really not a good indicator of the light output.
However, I really want to know what actually does Candela mean-as the defination or as the intensity unit defined in my first post? I think it could only be one of them as they are not the same.

[FlashlightOCD]

The following is over simplified, but it may help you get the general idea.

Lumens is a measure of total photons emitted from the source [in all directions].

Candela/FC/Lux are all measures of the photons hitting a surface area.

The Lux reading in the center of a flashlight beam is usually higher than in the corona, and the corona is usually higher than than where the beam does not go.

Candela can not be converted to Lumens unless you know a lot of information about the beam geometry and distribution.

[waion]

I see. Thanks, FlashlightOCD. That means Candela is more a light intensity measure unit than the other definition, rite?

[idleprocess]

I once found a webpage with a relatively straightforward method of converting mcd/cd into lumens. I worked up a spreadsheet that would handle the conversion, since it's hard to get a good idea of the overall light output based on mcd alone (example: a LED that puts out 280 mcd over 120 degrees puts out a crapload more light than one that does 6000 mcd over 20 degrees).

The mcd-lumens conversion isn't 100% correct, but it's been close enough for rouch comparison on the few occasions that I've had a spec sheet that lists both mcd*angle and lumens.

As soon as my webhost comes online, I'll upload it.

I can email it to anyone that wants it beforehand.

[Doug S]

[ QUOTE ]
waion said:
(sorry if this is a repeated question)

Could anyone pls tell me the meaning of candela? Is it equals to (different people told me):

"a 1 candela light source emits 1 lumen per steradian in all directions, isotropically" (hence 1 candela = 12.6 lumens) OR

"a measure of visible power per solid angle" (hence 1 lumen/steradian = 1 candela)

Thank you for helping me solve this really confusing question.

[/ QUOTE ]

The correct one is :
"a measure of visible power per solid angle" (hence 1 lumen/steradian = 1 candela)

I would prefer substituting "light flux" for "visible power".