I was just curious to know whether the Apparent Magnitude of stars could be used to calculate the Lux they apply upon any given body? If so, is there an equation?
I'm wondering because I'd like to know how much light a star in a binary system would cast upon a body orbiting the other.
I posted this on another forum and had no replies at all so any info would be greatly appreciated.
Thanks in advance,
Danial
Bightness Calculations
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granthutchison
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Re: Bightness Calculations
The zero of the apparent visual magnitude scale corresponds to 2.54 x 10^-6 lux, so you can calculate the correspond value for other magnitudes from there.
It's strictly accurate for only one spectral subclass (I can't remember which), because the magnitude scale and the SI units use slightly different stimulus curves. But the approximation should be pretty good except for very hot and very cool stars.
For instance, the Sun's apparent visual magnitude from Earth is -26.8, giving an illuminance of 2.512^26.8 x 2.54x10^-6 = ~130000 lx, which is the accepted value outside the atmosphere.
Grant
It's strictly accurate for only one spectral subclass (I can't remember which), because the magnitude scale and the SI units use slightly different stimulus curves. But the approximation should be pretty good except for very hot and very cool stars.
For instance, the Sun's apparent visual magnitude from Earth is -26.8, giving an illuminance of 2.512^26.8 x 2.54x10^-6 = ~130000 lx, which is the accepted value outside the atmosphere.
Grant
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Topic authorDanial
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Re: Bightness Calculations
Thank you so much.
Very appreciative,
Danial
Very appreciative,
Danial
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Topic authorDanial
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Re: Bightness Calculations
One more quick question about Lux:
Is it additive? For instance, if the Moon and Sun are in the sky together, do you just add the 2 values (however insignificant it may be)?
Is it additive? For instance, if the Moon and Sun are in the sky together, do you just add the 2 values (however insignificant it may be)?
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granthutchison
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Re: Bightness Calculations
Remember that the illuminance is calculated for a surface at right angles to the incoming light rays. Since a surface can't be at right angles to two separate sources simultaneously, you can't realistically pull off the addition.Danial wrote:One more quick question about Lux:
Is it additive? For instance, if the Moon and Sun are in the sky together, do you just add the 2 values (however insignificant it may be)?
For oblique illumination, take the calculated lux and multiply by the sin of the angle between the illuminated surface and the incoming rays. (Notice that gives you, intuitively enough, a correction factor of one when the surface is at right angles to the rays, and of zero when the surface is parallel to the rays.)
Once you've allowed for the obliquity of illumination from each of several sources, you can sum the results to get the total illuminance.
Grant