1. The problem statement, all variables and given/known data A projection aligner has an object to lens distance of 500mm. Calculate lens diameter for the aligner if NA = 0.28. Assume the usable area of lens is 90% of its diameter. 2. Relevant equations NA = r / D Note: NA is the numerical aperture. 3. The attempt at a solution I know that NA = r / D which means 0.28 = r / 500 and then r = 140mm so the diameter of the lens would be 280mm but I'm unsure how to use the idea of 90% of the lens being usable.