Hi, I am trying to collimate light from a mercury arc lamp. I know this question has been asked before here https://www.physicsforums.com/showthread.php?t=413437&highlight=collimation but it was first posted last year and since my situation is a bit different it didn't quite answer all of my questions. I am using a 100W Hg lamp housed in a OBB PowerArc lamp housing. The lamp has an ellipsoidal reflector with an f number of 2.5. The specs on the lamp (http://www.obb1.com/LightSources/Compact-High-Intensity/PowerArc.html) lead you to believe you get 10W in a 1.2mm spot about 15 cm from the front of the housing. What we didn't expect was all of the diffuse/surrounding light. We put a iris to select only the most intense portion of light but that dropped our power significantly and not to mention burning through irises faster than we can buy them. We have tried a variety of different lens configurations to collimate the light but we are losing up to three orders of magnitude to get a nice collimated beam. Some things we have tried: a bestform lens, two plano-convex lens, an aspheric lens with a variety of plano-convex, biconvex lens. At this point we have taken the iris out to try to retain the power but now collimating has become impossible. We would like to have a beam diameter of 10 mm or less with at least 6 watts of broadband power. Any advice or suggestions? Thank you for taking time to read this.