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Homework Help: Calculate lens diameter using NA

  1. Jul 12, 2012 #1
    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.
  2. jcsd
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