So i've been doing a lot of photonic / optics reading lately and I'm beginning to get a better understanding. So I'm beginning to look at some interferometry and I understand the most basic setup (Michelson - Morley experiment). As the mirror moves the light gets shifted by twice the distance and thus you get an interference pattern. But in modern apparti my understanding is that to measure the 'flatness' of an object they don't use a reference lightsource since no surface made can be 100% flat. http://upload.wikimedia.org/wikipedia/en/thumb/b/ba/Fizeau_optical_testing_with_computer_generated_hologram.png/550px-Fizeau_optical_testing_with_computer_generated_hologram.png [Broken] So they use computer generated holograms to compare the laser somehow with the test piece. How exactly is this working? I don't understand holography yet, is there some sort of book people would recommend?