Optical Lenses Information

Optical Lenses Information

Optical lenses are transparent components made from optical-quality materials and curved to converge or diverge transmitted rays from an object. These rays then form a real or virtual image of the object. There are many types of optical lenses. Achromats or achromatic lenses consist of two or more elements, usually of crown and flint glass, which have been corrected for chromatic aberration with respect to two selected wavelengths. Aspheric lenses compensate for spherical aberrations and are used primarily for their light-gathering ability. Ball lenses provide short focal lengths for use with collimated light and fiber coupling applications. Cylindrical lenses have at least one cylindrical surface and are used to correct or produce astigmatisms. Fresnel lenses resemble a planoconvex or planoconcave lens cut into narrow rings and flattened. If the steps are narrow, the surface of each step is generally conical and not spherical. Gradient index (GRIN) lenses focus light through a precisely controlled radial variation of the lens material's index of refraction from the optical axis to the edge of the lens. Meniscus lenses can increase the numerical aperture of a positive lens assembly without an undue increase in the number of aberrations. Spherical lenses or singlets are transparent optical components that consist of one or more pieces of curved optical glass.

Materials

Optical lenses are made from optical materials that are selected for their Abbe v-value and index of refraction. The Abbe v-value is the constant of an optical medium that describes the ratio of its refractivity to its dispersion. The index of refraction determines the degree to which light entering a material is bent. There are two major types of optical glass: crown and flint. Crown glass is harder than flint glass and has a lower index of refraction. Crown glass has an Abbe v-value greater than 50 and an index of refraction from 1.495 to 1.54. By contrast, flint glass has an Abbe v-value from 35 to 40 and an index of refraction from 1.595 to 1.64. BK7, a borosilicate crown glass, has an Abbe v-value of 64 and an index of refraction of 1.51. SF5 and SF11 are common flint glasses made from alkaline silicate materials. SF5 has an Abbe v-value of greater than 35 and an index of refraction less than 1.63. SF11 is a higher-quality glass than SF5, but has a similar Abbe v-value and index of refraction. In addition to crown and flint glass, optical lenses often include acrylic, polycarbonate, polystyrene, calcium fluoride, germanium, sapphire, and zinc selenide materials. UV-grade fused silica offers improved transmission in the ultraviolet (UV) range compared with crown glass.

Selection

Selecting optical lenses requires an analysis of surface quality and surface flatness specifications. Scratches are defects whose lengths are many times greater than their widths. Digs are defects that are nearly equal in length and width. Optical lenses with a 10-5 scratch/dig surface quality have scratches with an average length of .10 mm and digs with an average diameter of .05 mm. Optical lenses with a 20-10 surface quality have an average scratch length of .20 mm and an average dig diameter of .10 mm. Products with 40-20, 60-40, and 80-50 scratch/dig quality measurements are also available. Surface flatness, another important specification, is expressed in terms of  where  = 633 nm. Typically, the largest variation in surface height is no larger than /2, /4, /5, /8, /10, or /20. Other important specifications for optical lenses include effective focal length, diameter or length, width, and lens application. Ultraviolet (4 nm to 380 nm), visible (380 nm to 750 nm) and infrared (750 to 2500 nm) products are commonly available.

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