conversion factor: apparent magnitude to footcandles?

[chilehed]

There's gotta be one. Anyone know what it is?

 

thks

[insane_alien]

nope there isn't any. for one, the scales are different, one is linear(foot candles) and the other is logarithmic(magnitudes).

[chilehed]

I don't understand. There's a conversion from watts/m^2 sound pressure to dB, why can't there be a conversion from footcandles to apparent magnitude?

 

Sems to me that an object is determined to hava a brightness of zero magnitude because of the amount of light falling on a meter. That gives a certain number of foot candles - so how many footcandles corresponds to zero magnutude?

[insane_alien]

ok, another reason why there is no conversion factor is that the measurements are for different things. foot-candles measure illuminance while magnitude measures luminosity. while similar, they are different.

 

also magnitude is unitless. while foot-candles obviously have units of foot- candles.

[swansont]

nope there isn't any. for one, the scales are different, one is linear(foot candles) and the other is logarithmic(magnitudes).

 

That shouldn't matter. But you've got to connect a number of different measures, since footcandle is a non-SI unit for illuminance, and you want to get to an SI unit for radiant intensity. I've done a similar conversion before, and it wasn't nearly as easy as you'd hope. This is because illuminance is dependent on wavelength, as it incorporates the sensitivity of the eye. So you have to account for the whole spectrum, and use the luminosity weighting function.

 

Here's a good place to start

http://en.wikipedia.org/wiki/Lux

[chilehed]

But you've got to connect a number of different measures, since footcandle is a non-SI unit for illuminance, and you want to get to an SI unit for radiant intensity....This is because illuminance is dependent on wavelength, as it incorporates the sensitivity of the eye. So you have to account for the whole spectrum, and use the luminosity weighting function....

The problem I'm having is that I can't find any information whatsoever about what radiant intensity coresponds to what magnitude.

 

I have a light meter that I use to measure the lighting in parking lots and such. The meter has a built-in filter that weights the results for frequency so that the meter returns the same value regardless of spectrum, as long as the human eye would perceive the same illuminance. It's essentially the same thing as a dBA measurement in sound intensity, and I'm wondering about the same kind of thing for the apparent magnitude of stars.

[swansont]

The problem I'm having is that I can't find any information whatsoever about what radiant intensity coresponds to what magnitude.

 

The sun's radiant intensity should be easy to find. It has an apparent magnitude of -26.73

[chilehed]

The sun's radiant intensity should be easy to find. It has an apparent magnitude of -26.73

Already done. I'm looking for confirmation that I got the right results.