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Posted by Kris Krieger on July 12, 2008, 6:57 pm
Please log in for more thread options http://en.wikipedia.org/wiki/Candela This page notes that a *unifromly-emitting* 1 candela light source yields 12.5 lumens - the frst reference I'd found which mentioned this did not specify the "uniform" part. ANyway, the age has a succinct chart that allows conversion of mcd into "real" lumens based upon the full-angle cone of the source's emission. THere is also a link to a nifty calculator. THis helped me a lot, because before this, I'd not realized that bit about the *uniform* source. Now I know :) Hopefully, this will be useful to others who have similar questions! - Kris | ||||||||||||||||||||||||||||||||||||||||
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Posted by Ian Stirling on July 13, 2008, 3:47 pm
Please log in for more thread options Assuming that the full-angle emission is in fact completely uniform. This is often very, very far from the truth. | ||||||||||||||||||||||||||||||||||||||||
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Posted by JosephKK on July 19, 2008, 9:55 am
Please log in for more thread options wrote:
>Just in case this might help others:
> >http://en.wikipedia.org/wiki/Candela > >This page notes that a *unifromly-emitting* 1 candela light source yields >12.5 lumens - the frst reference I'd found which mentioned this did not >specify the "uniform" part. > >ANyway, the age has a succinct chart that allows conversion of mcd into >"real" lumens based upon the full-angle cone of the source's emission. >THere is also a link to a nifty calculator. > >THis helped me a lot, because before this, I'd not realized that bit about >the *uniform* source. Now I know :) > >Hopefully, this will be useful to others who have similar questions! > >- Kris I suspect that is candela to lux rather than lumens. That 12.5 is actually 4*pi. | ||||||||||||||||||||||||||||||||||||||||
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Posted by Kris Krieger on July 19, 2008, 9:37 pm
Please log in for more thread options
> wrote:
> >>Just in case this might help others:
>> >>http://en.wikipedia.org/wiki/Candela >> >>This page notes that a *unifromly-emitting* 1 candela light source >>yields 12.5 lumens - the frst reference I'd found which mentioned this >>did not specify the "uniform" part. >> >>ANyway, the age has a succinct chart that allows conversion of mcd >>into "real" lumens based upon the full-angle cone of the source's >>emission. THere is also a link to a nifty calculator. >> >>THis helped me a lot, because before this, I'd not realized that bit >>about the *uniform* source. Now I know :) >> >>Hopefully, this will be useful to others who have similar questions! >> >>- Kris >
> I suspect that is candela to lux rather than lumens. That 12.5 is > actually 4*pi. > > Oh no <groan>...now I'm confused again :(
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Posted by Phil Hobbs on July 20, 2008, 2:25 pm
Please log in for more thread options Kris Krieger wrote:
>
>> wrote:
>> >>> Just in case this might help others:
>>> >>> http://en.wikipedia.org/wiki/Candela >>> >>> This page notes that a *unifromly-emitting* 1 candela light source >>> yields 12.5 lumens - the frst reference I'd found which mentioned this >>> did not specify the "uniform" part. >>> >>> ANyway, the age has a succinct chart that allows conversion of mcd >>> into "real" lumens based upon the full-angle cone of the source's >>> emission. THere is also a link to a nifty calculator. >>> >>> THis helped me a lot, because before this, I'd not realized that bit >>> about the *uniform* source. Now I know :) >>> >>> Hopefully, this will be useful to others who have similar questions! >>> >>> - Kris >> I suspect that is candela to lux rather than lumens. That 12.5 is
>> actually 4*pi. >> >> >
1 lumen is the unit of photometric 'power'--at the peak (540 nm), 1 W of
> Oh no <groan>...now I'm confused again :( > optical power is 683 lumens (probably the largest prime number ever used for unit conversion). 1 lux is 1 lumen/m**2 1 candela is 1 lumen per steradian. An isotropic source at the centre of a sphere illuminates the entire sphere surface uniformly--which is 4*pi steradians. Such an isotropic source might be a localized air plasma created by an induction source, for instance, or the Sun. If the light shines only in one direction, it illuminates a hemisphere (2*pi steradians). If instead of an isotropic illuminator, you pick a flat surface, then in addition there's obliquity to worry about. If you have a uniformly illuminated diffuse white card, it looks smaller if you see it obliquely, but the surface appears equally bright. Another way of putting this is that if you looked at it through a drinking straw, you couldn't tell by the surface brightness if it was tilted or not. Real surfaces aren't quite this diffuse, but things like white paper and packed MgO powder are close. A source that's perfectly uniform with angle (i.e. it passes the drinking-straw test) is said to be Lambertian. Light from a Lambertian source has a cosine obliquity term, because a flat plate tilted by theta appears to be cos(theta) times its real length. When you do the integral over the hemisphere, you get Effective solid angle = integral (0 to 2*pi)dPhi integral(0 to pi/2) sin(theta) cos(theta)dTheta = 2*pi*[cos(0)-cos(pi/2)] = pi steradians. Cheers, Phil Hobbs | ||||||||||||||||||||||||||||||||||||||||
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>
> http://en.wikipedia.org/wiki/Candela
>
> This page notes that a *unifromly-emitting* 1 candela light source yields
> 12.5 lumens - the frst reference I'd found which mentioned this did not
> specify the "uniform" part.
>
> ANyway, the age has a succinct chart that allows conversion of mcd into
> "real" lumens based upon the full-angle cone of the source's emission.
> THere is also a link to a nifty calculator.