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Imagingoptics

A wide variety of thin Fresnel lenses are molded in plastic, the width of the rings being only a few thousandths of an inch, for use as field lenses with ground-glass screens in cameras and small projectors to increase the brightness of the outer parts of the screen.

A one-piece molded-glass Fresnel lens is convenient for spotlights, floodlights, railroad and traffic signals, and decorative lights in buildings. Cylindrical Fresnel lenses are used in shipboard lanterns to increase visibility.

Fresnel lens, succession of concentric rings, each consisting of an element of a simple lens, assembled in proper relationship on a flat surface to provide a short focal length. The Fresnel lens is used particularly in lighthouses and searchlights to concentrate the light into a relatively narrow beam. It would be almost impossible to make a large lighthouse lens of the usual solid glass-disk type because the thickness and weight would be prohibitive; the lighter Fresnel lens is constructed of elements that are separately ground and polished from suitable glass blanks and assembled to make up the complete lens.

Opticssurplus

Attenuated total reflection Fourier transform infrared (ATR/FT-IR) spectroscopy has been used to determine optical constants in the infrared region of the spectrum for a variety of materials. Usually, the Kramers-Kronig transform is used to obtain optical constants from spectra, given that the baseline refractive index (anchor point) is known or can be determined. This determination often involves performing a separate experiment in which the refractive index of the sample is measured in a nonabsorbing region of the visible part of the spectrum, and the result extrapolated to the infrared. However, this is not feasible for opaque samples or for ones that contain domains large enough to scatter visible light. In this paper, we present a method for concurrently determining the anchor point and the optical constants using only variable-angle ATR/FT-IR spectroscopy and the subtractive form of the Kramers-Kronig transform. The method for determining the anchor point involves ratioing pATR (the negative log of the intensity of the ATR spectrum) values from weak absorption bands from spectra recorded at different angles of incidence. The anchor point can then be determined by computer. The only requirements are that the ATR system use unpolarized radiation and that the sample be thicker than the sampling depth of the IR radiation.

Brent M. DeVetter, Nicole K. Scharko, Bret D. Cannon, Tanya L. Myers, and Timothy J. Johnson Appl. Opt. 57(22) 6587-6597 (2018)

Georges-Louis Leclerc de Buffon (1748) originated the idea of dividing a lens surface into concentric rings in order to reduce the weight significantly. In 1821 this idea was adopted by Augustin-Jean Fresnel in the construction of lighthouse lenses.

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