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PI Physik Instrumente - Space Qualified Steering LW 1-15 MR
Photonics Dictionary

t number

The equivalent f number of a fictitious lens that has a circular opening and 100 percent transmittance, which would give the same central illumination as the lens being considered:



where EFL is the equivalent focal length, A is the area of the entrance pupil, and t is the transmittance of the lens system. For an unobscured system:

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