Jonathan,

Not to mention the dirt on the windows :mrgreen:

I'm still gonna take advantage of the light - windows, dirt and all :smile:

Are you absolutely certain of that Tony :question: I kinow glass filters out UV, but blue filtered out by atmosphere :question: I don't know

Have you ever seen a sunset? What colour is it? :razz:

Got me there, but is that not angle of incidence :question:

Partly, but the amount of atmosphere a photon must traverse is directly proportional to the angle of incidence. If that photon is aiming for the equator it enters the atmosphere at a vector normal (i.e., perpendicular) to the surface of the earth. So it has the least amount of distance to travel. Also all his buddies are all hitting of surface area of the earth that is the smallest in comparison to the same number of photons hitting a surface at any degree other than zero. So more photons per unit area and least amount of atmosphere.

At our latitude thus there are fewer photons per unit surface area, and more atmosphere.

At sunset there are fewer photons yet (thus least amount of intensity) and the furthest distance to travel through atmosphere.

....

And now my brain hurts. THANKS a lot, I haven't had to sing and dance like that since I was in school!! :razz: