The pupil diameter of an objective lens represents the maximum diameter of a parallel light beam, which can enter from the rear side of the lens, along the optical axis.
This diameter is calculated from the following formula.
If beam light with homogeneous intensity distribution enters from the rear side of an objective lens, the light converges to a spot of a specific size due to its diffraction phenomenon even if the objective lens is an absolute aplanatic lens. As a guide, the beam spot diameter is calculated using from the following formula.
Note: If a light source such as a laser with intensity distribution is used, the spot diameter is not governed by the above formula.
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