The PAC095 Visible Achromatic Doublet Lens is a 400 to 700 nm antireflection coated, 3 inch (76.2 mm) diameter, N-BK7 and N-F2 precision achromatic doublet lens with an effective focal length of 250 mm. This achromat is computer designed to effectively minimize spherical aberration and coma when operating at infinite conjugate ratio, yielding smaller focused spot sizes. Unlike singlet lenses, this results in a constant focal length independent of aperture and far better off-axis performance. Achromats are superior to singlet lenses for monochromatic applications at any visible wavelength.
Achromatic Lens Design
An Achromat, or Achromatic lens, is usually made up of multiple optical elements, which can be air-spaced or cemented together. These typically alternate a positive lens of low index material, such as crown glass, and a negative lens of high index material, such as flint glass. The elements are selected and optimized so that dispersion in the positive element is corrected by the negative element, resulting in minimized chromatic aberration at two well-separated wavelengths. Achromats offer a nearly constant focal length across the visible spectrum, providing superior off-axis performance. Doublet lenses offer users many more design options than singlet lenses, which allows for superior performance in visual imaging and laser beam manipulation applications. Our lenses are computer designed, minimizing aberrations and providing a smaller focused spot size. They are offered with multi-layer or single-layer AR coatings. Standard diameters and focal lengths may be interchanged with our singlet lenses. Further, achromats are available in a wide variety of configurations, including positive, negative, triplet, and aspherized. These lenses are produced as doublets or triplets (three elements). The collimated beam should enter or depart from the surface (R1 in drawing) with higher curvature (smaller radius) for optimized performance.
MgF2 Visible Antireflection Coating
Our single layer Magnesium Fluoride visible AR coating is the most common choice that offers extremely broad wavelength range at a reasonable price. It is standard on achromats and optional on our N-BK7 plano-convex spherical lenses and cylindrical lenses. Comparing to the uncoated surface, the MgF2 provides a significant improvement by reducing the reflectance to less than 1.5%. It works extremely well over a wide range of wavelengths (400 nm to 700 nm) at angles of incidence less than 15 degrees.
Superior Performance in Monochromatic Applications
Achromats are superior to singlet lenses for monochromatic applications at any visible wavelength. Negative achromatic lenses are used primarily to extend focal lengths, which is often combined with positive achromatic mirror systems and imaging lenses. Generally a diffraction limited spot may be achieved when using a monochromatic source such as laser.
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