Lenses are an integral component in most optical systems, where they are used to focus, collimate, expand, collect and image light. Many optical tasks r equire several lenses in order to achieve an acceptabl e level of performance. This selection guide will review the singlet spherical lens shapes offered by CVI Laser Optics and offer some practical guidance on determining the best material type and lens quality for your application. For a complete discussion of lens theory, use, and aberrations please refer to the Fundamental Optics and Gaussian Beam Optics sections of the Technical Guide. Lens Shape CVI Laser Optics offers four lens types for convergingor focusinglight. A bi-convex lens is the classic symmetric lens, possessing two convex surfaces of equal radii. Bi-convex lenses have positive focal lengths and form both real and virtual imag es. It is the best singlet l ens for imaging at unit magnification; spherical aberration is minimized, and coma, distortion, and transverse chromati c aberration exactly cancel each other out for a perfectly made lens (longitudinal chromatic aberration is not corrected). This is true regard less of the material used or wavelength, although use of a remote stop can reduce the degr ee of cancellation. Aberrations incr ease as conjugate ratios (object distance/image distance) depart from unity . Bi-convex lenses can also be used for focusing applicati ons, in particular when a lower f-number (ƒ/CA) is required , even if they do not have the best shape for this conjugatio n. They are recommended for virtual imaging of real objects and for positive conjugate image ratios from approximately 0.2 to 5 (note that these values are wavelength sensitive). Away from unity, the singlet lens shape that best minimizes spherical aberration at a given conjugate ratio is called a bestform lens, in which the two convex sides are of different radii. The marginal rays are equally r efracted at each of the lens/air interfaces for this shape, and surface-refl ection loss is minimized. Another benefit is that absolute coma is nearly minimized for bestform shape, at both infi nite and unit conjuga te ratios. It does not, however, perform well in wide-field applications, except in very specific configurations (with a meniscus for instance). At infinite conjugate ratio, the best form lens is not the optimum singlet shape, since it results in high field curvatur e. Positive bestform lens es are of exceptional performance and provide the smallest spot size available in a singlet lens. When working at infinite or near -infinite conjugate ratio, plano-convexlenses with the convex side toward the infinite conjugate perform nearly as well as the best-form lens. Convex on one side and flat on the other , these positive focal length lenses cost much less to manufacture than a bestform lens. This lens shape also exhibits nea r- minimum transverse spherical aberration and near-zero coma when used off-axis. Longitudin al aberration is low, but is only minimized when using a best form lens. If bulk light collection is required at minimal cost, an aspheric glass condenser lens may be the solution. Aspheric lenses provide better performance by reducing aberrations when used in low f-number, high-throughput applicati ons. One surface is asp heric; the second surface is flat, or spherical-conv ex. A flat second surface minimizes aberration, while a spherical-convex second surface How to Select a Spherical Lens Fig 1: Bestform lens
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