Calculation help needed:

Ugh, physics gurus: Whats the correct conversion from lumen to candela? I've always thought it goes as follows: lumen is defined as candela/sr. Ok. So, for 18lm flux, the calculation goes as follows: 110 degrees (of 360) is 122.22222 in steradians. Ok. Then multiply the degree value w/flux totalling 2200 Candelas for 1W white Luxeon star in whole beam.

Huh?

BR,
R.S

Your problem is in your degree to steradian conversion.

A steradian is the solid angle which would cut out a chunk of area 1 unit square on the surface of a 1 unit sphere. Since the surface area of a sphere is 4*pi*r^2, the total steradians in a sphere is only 4*pi, or about 12.6.

The number of steradians in a 110 degree cone is clearly less than 6 steradians, but I don't currently know what the actual conversion is. If you figure it out, please post

Originally posted by R.S:
Ugh, physics gurus: Whats the correct conversion from lumen to candela? I've always thought it goes as follows: lumen is defined as candela/sr. Ok. So, for 18lm flux, the calculation goes as follows: 110 degrees (of 360) is 122.22222 in steradians. Ok. Then multiply the degree value w/flux totalling 2200 Candelas for 1W white Luxeon star in whole beam.

Huh?

BR,
R.S

Click to expand...

<font size="2" face="Verdana, Arial">Strictly speaking, there *is* no conversion since the lumen and candela are measures of different things [flux and intensity, respectively]. It is true, that a flux of 1 lumen distributed uniformly within a solid angle of 1 steradian will yield an intensity of 1 candela.
Candela = lumen/steradian

For small angles, lumens = candela * 0.00024 * angle^2. I don't know how quickly the error builds, haven't really looked into it.

Since most light sources do not have even light distribution, you cannot convert from lumen to candelas. For the isotropic source, an object that radiates even light to all directions, 1 candela = 4*pi lumens.

Luxeon Star will have different caldela values depending on what angle you measure.

Originally posted by CCW:
[QB]For the isotropic source, an object that radiates even light to all directions, 1 candela = 4*pi lumens.

QB]

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<font size="2" face="Verdana, Arial">This statement in not correct. CCW is confusing candela with "candle" [also "standard candle"] It is true that 1 candle =4*pi lumens. An isotropic source of one candle will have an intensity of 1 candela.

Doug S:

I think you are not quite up-to-date. The term "candle" is not used any more (at least as an official engineering unit). Candela is the correct unit.

This, straight from Lighting Handbook 8th Edition by Illuminating Engineering Society of North America, pages 28;
"For a sphere of unit redius, it can be shown by simple geometry that the area of the sphere's surface is equal to 4pi square unit; thus the isotropic source having a intensity of 1 cd produces a total luminous flux of 4pi lm."

The same page defines cd as a candelas.

There used be a term "MSCP(Mean Spherical Candle Power)" used by most US incan mfrs. MSCP would been what you think as a "candle" or "standard candle".

Originally posted by CCW:
Doug S:

I think you are not quite up-to-date. The term "candle" is not used any more (at least as an official engineering unit). Candela is the correct unit.

This, straight from Lighting Handbook 8th Edition by Illuminating Engineering Society of North America, pages 28;
"For a sphere of unit redius, it can be shown by simple geometry that the area of the sphere's surface is equal to 4pi square unit; thus the isotropic source having a intensity of 1 cd produces a total luminous flux of 4pi lm."

The same page defines cd as a candelas.

There used be a term "MSCP(Mean Spherical Candle Power)" used by most US incan mfrs. MSCP would been what you think as a "candle" or "standard candle".

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<font size="2" face="Verdana, Arial">CCW, you are quite correct in everything you say in this post [OK, with with the exception of your assessment of my "up-to-dateness" I am well aware that Candle is an archaic unit] This may seem like splitting hairs to some, but please note that you originally said:

For the isotropic source, an object that radiates even light to all directions, 1 candela = 4*pi lumens.

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<font size="2" face="Verdana, Arial">This clearly implies that candela and lumens are different units for the same thing. This is very different from your [correct] statement above that "For a sphere of unit radius, it can be shown by simple geometry that the area of the sphere's surface is equal to 4pi square unit; thus the isotropic source having a intensity of 1 cd produces a total luminous flux of 4pi lm."

You will never find a table of conversions that says 1 candela = 4*pi lumens.
Candela is a unit of *intensity*
Lumens, MSCP, Candles are all units of *luminous flux*.

Doug:

Semantics, I guess. I can see why you are reading that sentence that way. What I was trying to say was that isotropic source with 1 cd will emit 4pi lumens. Poor English...

Originally posted by CCW:
Doug:

Semantics, I guess. I can see why you are reading that sentence that way. What I was trying to say was that isotropic source with 1 cd will emit 4pi lumens. Poor English...

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<font size="2" face="Verdana, Arial">I hope you don't think that I'm picking "nits"

No, not at all. Thanks.