Flatness question

When BillA measures waterblock bases with the optical flat and sees all the lines, how do you tell where the high and low spots are? I think I understand the theory behind it, (interference of reflected light waves), but I can't figure out how to quantify which is a hill and which is a valley. Can anyone explain this to me, or point me in the direction of some good reading material?

Hills are spread out contours, and valleys are where the contours are closer together.

Very good question Calvin. I followed some links when the he first began posting about the optical flat. I remember reading that the table the piece sits on, needs to be moved up/down in relation to the glass. This will cause the lines to appear to "roll" uphill or downhill. Depending on which direction you are moving the table and which way the lines are "rolling" you can determine if you are looking at a hill or a valley. I'll see if I can find the link for you, as I'm paraphrasing the info.

peace.
unloaded

Mark, I'm not sure if I understand what you're saying. Could you please explain more?

So I'm guessing this is like a topographical map, but without marked elevations, right? From a still picture, you can tell that there are changes in surface height and where the changes are more severe, but without more information you cannot really tell which is high and which is low?

I have a number of refernces, but no clear concise link (but I am sure it exists somewhere)

convex



concave



if you study the images a bit you can generally sort them out

be cool