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 (Figure 1). While spherical aberrations can be corrected by using multiple elements that overcorrect other elements, for many IR 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.
As shown in Figure 4, 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 IR applications.
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