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Fresnel number

In a lens, the square of the radius of its aperture divided by the product of the focal length and the wavelength. It measures the importance of diffraction in the image formed by the lens. A small Fresnel number indicates greater effects of diffraction. The Fresnel number has been used in lasers with the distance to the following aperture in a resonator replacing the focal length, and it has proved efficient in determining the diffraction loss.
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