Check out some similar questions!

chandrahasan · Jul 8, 2009, 11:00 PM

Sun light is scattered when it enters into atmosphere. But how the sun light intensity is distributed with angle of incidence on any given area on earth when sun is in zenith position.

ebaines · Jul 9, 2009, 05:36 AM

When the sun is at the zenith, that means it is directly over head. Which means its angle of incidence is 90 degrees. So are you asking how the intensity of sunlight varies as the sun moves from the zenith? Very roughly, the intensity is proportional to the sine of the angle of incidence. So at 90 degrees (sun at zenith) it's at its maximum; at 30 degrees it's intensity is half, and at 0 degrees (sunrise or sunset) it's zero.

FlyYakker · Jul 10, 2009, 04:17 AM

Keeping in mind that some parts of the world never experience the sun being directly overhead. I believe the tropics of cancer and capricorn define the limits on either side of the equator where a true "overhead" sun can occur.

chandrahasan · Jul 14, 2009, 01:48 AM

Originally Posted by ebaines

When the sun is at the zenith, that means it is directly over head. Which means its angle of incidence is 90 degrees. So are you asking how the intensity of sunlight varies as the sun moves from the zenith? Very roughly, the intensity is proportional to the sine of the angle of incidence. So at 90 degrees (sun at zenith) it's at its maximum; at 30 degrees it's intensity is half, and at 0 degrees (sunrise or sunset) it's zero.

For a given position of sun,say zenith, the intensity varies if we direct a telephotometer at different angles.I like to know if any such luminace/radiance data is available.It will be of great help to me for stray light analysis of a field operated projection systems.

ebaines · Jul 14, 2009, 05:43 AM

Got it. You're looking for the brightness of the sky given the angle from the sun, right?

I would think this would be dependent on the wavelength of light you're interested in, and would be greatly influenced by variables such as cloudiness, humidity, levels of dust and smog, and elevation of the observer. So the answer will vary depending on where you are.

I don't have an answer, but here's a discussion on this topic - I can't vouch for its accuracy or relevancy to what you're trying to do:

Google Answers: total eclipse sky brightness vs zenith sky brightness