s1m0n wrote:I don't get the reflectance distinctions. Can you elaborate in a way that a lit major might grasp?
If you shine a light onto a highly polished surface of a white metal like platinum or aluminum, almost all of that light will be reflected back. That's a high-reflectance surface. If you shine it onto a pane of glass, 4% will be reflected back. That's low reflectance. Silicon is in between, about 35%.
Another way to look at it is, if you look at a mirror, you see a good image of yourself. If you look at a pane of glass, you might see yourself, but it would depend on what's behind the glass since it's passing most of the light -- if you're inside at nighttime, you can see yourself well, if it's daytime outside, you see what's outside.
The most precise way to measure the shape of an optical surface is by comparing it to another surface. This is done by splitting a beam of light into two arms, reflecting those off of the reference optic and the optic under test, and recombining them. This results in a series of fringes -- stripes, bullseye pattern, or something else. The best contrast in the fringes happens when the two beams have the same intensity, which is when the two surfaces reflect the same.
So if you're measuring the shape of a metal mirror, a pool of mercury would provide a good "perfect" flat surface. If you're measuring a glass surface, a pool of water would be the appropriate perfect reference.
I hope that's a little clearer. Feel free to PM or email me or ask further on the board; I love talking about this stuff.
Cheers, Charlie