# Definition of candela

1. May 15, 2007

### Milind_shyani

Hi
I cannot Understand properly the correct definition of candela.Can you please explain me.I know the definition by heart but i donot understand it properly

2. May 15, 2007

### rbj

Candela is, in my opinion, an unnecessary base physical unit. i don't think that it (or the mole) should exist in SI.

essentially it is a unit of luminosity which is a perceptual quantity and only indirectly related to a physical quantity of radiant intensity (1/683 watt per steradian that can be translated to a quantity of power because there is $4 \pi$ steradians comprizing all directions in 3 dimensional space) but defined at a particular frequency of visible light 540 x 1012 Hz (which is, i think yellowish green). but to talk about equivalent luminousity for other frequencies or colors, you might need to refer to data that looks like this:

http://en.wikipedia.org/wiki/Candela
http://www.fourmilab.ch/documents/specrend/

all's i can say is that if you have an omni-directional radiating element with total radiant power of 0.018398785 watts that radiates at frequency 540 x 1012 Hz (a wavelength of 555 nm), you have something that has, by definition, a perceived brightness of 1 candela. if it's another frequency but the same power, the luminosity will be less than 1 candela (i think you use that solid black curve in the Wikipedia article).

it's not a physical unit, it has all sorts of heuristic perceptual dependency in it, and it doesn't belong as a base unit in a system of units like SI. it's kinda a dumb inherited thing.

3. May 15, 2007

### cesiumfrog

Isn't it just "the number of burning candles you'd need, to replace whatever you're measuring, so that the brightness looks the same"?

4. May 15, 2007

### Claude Bile

The Candela is the unit of Luminous Intensity. Luminous Intensity is simply radiometric intensity (i.e. Watts per steradian), with a weighting function that is dependent on wavelength which is based on the human response of the eye.

The Candela is part of a larger set of units called photometric units. Photometric units are units based on how we perceive the radiation with our eyes, this is why the response of the eye has to be taken into account for these units.

Astronomers tend to use these units more than others in my experience, perpetuated most likely due to the abundance of amateur astronomers out there. Professional astronomers have little reason to use photometric units these days because all of their measurements are done radiometrically anyway.

Claude.

5. May 16, 2007

### rbj

but is that weighting function standardized in the SI as part of the definition of Candela? i cannot see that it is when i look at the NIST site about this.

6. May 16, 2007

### Claude Bile

I believe there are two standard functions, one based on normal vision and another on dark-adapted vision.

EDIT: http://physics.nist.gov/Document/sp330.pdf - see the chapter under photometric units.

Claude.