Hi, I need help or any sudgestions with the next scenario - I'm designing a sensor for a wind wave tunnel - can be found in the google search using the key words 'wind wave flume' The sensor should give me the speed and frequency of the cappilary gravity waves created in the tunnel by a wind in it. The sensor is of two main units; the bottom unit under the tunnel and the head unit above the tunnel. On the bottom unit placed a laser propagating a beam toward inclined spinning mirror (1800 rpm) so that the laser beam creates a holow cone shape. Then the beam passes a plano-convex lens in order to enter perpendicularly to the bottom of the tunnel. This way no diffraction appears in the air-water interface. When the water are still (no wind) I'll get a circle shape on the surface of the water, BUT when wind exists in the tunnel, diffraction will occure in the water-air interface. If I'll know the diffraction ammount, I'll be able to calculate the speed and frequency of the waves. In order to measure the diffraction I decided to use a PSD (Position Sensetive Detector) placed on the head unit which entered into the tunnel above the water surface, with as less as possible interference to the wind in. Since I want to keep a reasonable distance from DC wave height (~40mm) the delta's are quite big, somewhere aroung 12mm in every direction (X and Y), so it makes me to use a large area PSD something like 45x45mm, but this PSD is very expensive! I'd like to use a small PSD something like 12x12mm and in order to use such a PSD I need some optic system of a lens or two, to decrese the difractions and collimate them with optical axis before they reach the PSD surface. The problem is that wave's slopes are changing rapidly (different diffraction gradient) every single moment, so I don't know what optical system will work for me. Can someone help me with this one? Thank you, Gary Diamant.