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Computer-Optimized Achromatic Doublets - 12567 Computer-OptimizedAchromatic Doublets

Multielement Lenses Melles Griot achromats closely satisfy the Abbé sine conditionat the infinite conjugate ratio: they are nearly free of spherical aber- ration and coma. Achromats are therefore vastly superior to sim-ple singlet lenses for monochromatic applications at any visiblewavelength. For example, they are well suited for low-power laser beam manipulation.Freedom from spherical aberration means that the focal lengthis independent of aperture. Simple lenses require considerable refo- cusing as the aperture is varied, but computer-optimized achro-mats do not. Freedom from coma means that off-axis achromatperformance is far superior to that of simple lenses. Optimized spherical achromats can maintain almost uniform performance throughout a field of up to 2.5 degrees angular radius; whereas thespot size from a simple lens, under optimum conditions, increasesby as much as 4.5 WAVELENGTH INSENSITIVITY OF PRECISION OPTIMIZED ACHROMAT SERIES The following graph shows the variation of focal length of aprecision-optimized achromat (01 LAO 123) as a function of wave- length between 0.3 and 1.1 m m. This achromat has a nominal focallength of 100 mm at 546.1 nm. It can be seen that focal lengths forthe shortest and longest of the three design wavelengths are approx- imately equal, but both focal lengths differ from the center designwavelength focal length by about one part in 2000. The optimiza-tion process has slightly relaxed the achromatization constraint in favor of monochromatic aberration suppression, resulting in a supe-rior achromatic lens. Precise focal-length equality at the extremedesign wavelengths is excessive and impairs overall performance. Because of the superior imaging properties of Melles Griotachromats, there is often interest in using them outside the visiblespectrum. We are frequently asked how much focal length changeswhen moving from the mid-visible to 1.064
Cylindrical Optics # at 2.5 degrees. STANDARD AND PRECISION m m. The answer is some-where between a 0.584-and 0.939-percent increase over the publishedgreen-line nominal value. This variation results from the severaldifferent glass combinations utilized in the series. For this same change of wavelength, variations in back focal length (
Mirrors f COMPUTER-OPTIMIZED ACHROMATS Melles Griot currently offers two achromat ranges. Both aremanufactured to test-plate specifications, which means that their sur-faces are checked for power and irregularity by interference meth- ods with precision master test plates. Tight centration tolerances aremaintained in all dimensions to ensure optimum performance andconformance with their specifications. Focal and principal-point positions are accurately known and are essentially independent off-number, since spherical aberration is very well corrected. Instru-ments can be designed around these lenses with confidence that little alignment will be required when the lenses are installed.The LAO series of achromatic doublets consists of differentfocal lengths and diameters for general-purpose imaging tasks.Focal lengths of many of these achromats can be significantly reduced, without performance degradation, by using a carefullymatched aplanatic meniscus lens (go to pages 7.22-7.24).The LAL computer-optimized achromats are specificallydesigned for laser-beam manipulation and focusing. Optimizationis slightly different from the LAO series. LAL achromats are man-ufactured to the highest tolerances with excellent cosmetic surface quality (20-10 scratch and dig). In certain nonlaser applications,they are superior to corresponding LAO lenses. If you have anyquestions regarding selection of achromats, contact a Melles Griot applications engineer.
Prisms andRetroreflectors b ) and sec-ondary principal-point position also seem to be between 0.5 and1.0percent of their respective green-line nominal values.
APPLICATION NOTE Beamsplitters,Windows, Optical Flats

Lens Selection for Specific Applications

An achromat will often perform better than asinglet. This performance can be in the form ofbetter wavefront quality, smaller spot sizes, orimproved modulation transfer function (MTF). If your application requires an achromat with superiorperformance, we recommend our precisionlaser-grade achromats, (go to pages 7.19-7.21) which are manufactured to the most stringent standards. As a result, they have better surface accuracy and tighter focal-length tolerances. The surface quality is 20-10 scratch and dig, which makes them ideal for reducing scatter and stray light reflections.
Polarization Components Filters 7.2

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