A thin achromat can be corrected for third-order spherical aberration, coma, and axial chromatic aberration. The combination consists of a positive and a negative element. To eliminate axial chromatic aberration, a simple relation between powers and Abbe numbers of the two elements must be met. This relation is
And, since φ = φ
A+ φ
B,
The process of deriving the complete prescription for such a doublet is straightforward but too cumbersome to be included here.1
The radii as a factor of an achromat’s focal length are listed in the table below for two objectives: one for the MWIR and one for the LWIR region. For the MWIR lens, silicon and germanium have been the materials chosen, and the combination Amtir-1/zinc sulfide was selected for the LWIR objective. Amtir is an acronym for amorphous material transmitting infrared radiation. The composition of Amtir-1 is 33% Ge, 12% As, and 55% Se.
Radii of lens elements for two selected achromats.
| Spectral region | MWIR
(3 – 5 μm) | LWIR
(8 – 12 μm) |
| Front element | Silicon | Amtir-1 |
| R1 | 0.97f | 1f |
| R2 | 3.25f | 6f |
| Rear element | Germanium | Zinc sulfide |
| R3 | 4f | −6f |
| R4 | 2f | −24f |
These choices are a sound starting point that lead quickly to good solutions in terms of optimization with the computer after adding thicknesses and spacings.
Reference
- M. J. Riedl, “The Thin Achromat,” Electro-Optical Systems Design, Cahners Publishing Co. (September 1981), pages 49–52.
