Compared to many other forms of optical infrared lenses, meniscus lenses are rarely offered off-the-shelf. While meniscus infrared lenses are primarily used for focusing to small spot sizes or collimation applications, plano convex infrared lenses often offer a superior price-to performance ratio. There are scenarios, though, where meniscus infrared lenses provide significantly superior performance at only a moderate increase in price.
Due to the spherical nature of a lens, spherical aberrations cause parallel rays at different distances from the optic axis to not converge at the same point. While spherical aberrations can be corrected by using multiple elements that overcorrect other elements, for many infrared lenses systems where materials are significantly more costly than visible materials, it is ideal to minimize the number of elements. In lieu of using multiple elements, it is possible to minimize spherical aberration for a single lens by shaping the lens into its best form.
For a fixed index of refraction and lens thickness, an infinite number of radii combinations exist that can be used to create a particular focal length lens. These combinations of radii create different lens shapes which directly result in spherical aberrations and coma due to the degree in which a ray bends as it travels through the lens.
Benefits of Meniscus Design
When working in the visible, glass indices typically range from 1.5 to 1.7 and the shape for minimum spherical aberration is nearly plano-convex. In the infrared lenses, however, higher index materials like germanium are typically used. Germanium, with its index of 4.0, greatly benefits from a meniscus lens design by significantly reducing spherical aberrations.
The minimum spherical aberration occurs when the ray is bent equally at both interfaces. While the first surface of a germanium meniscus lens causes light to bend slightly more than a comparable PCX lens, the second surface of a PCX lens causes light to bend significantly more, resulting in an overall increase in spherical aberration.
Which compares the performance of a 25 x 25mm germanium PCX lens to a 25 x 25mm germanium meniscus lens, it is easy to see how the PCX lens causes light to bend significantly more with respect to the lens surface than the comparable meniscus lens. This increase in bend causes an increase in spherical aberration. The germanium meniscus lens demonstrates a drastic decrease in resulting spot size, making it more ideal for use in demanding infrared lenses applications.
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