Case study: F/2 MWIR imager from stock optics

Published 5.9.2014

Designing a fast MWIR imaging optics using only stock components.

For prototyping purposes, our company engages in creating different applications with stock optics. This time we focused on a fast MWIR (3-5μm) optics. Available stock optics are severely limited, but we managed to pull off a 50° FFOV diffraction-limited objective. Field would benefit from custom optics, but most prototyping and proof-of-concepts are quite possible with just stock optics.

I started with the field problem, creating a field lens and a collimator. Various makers have a good selection of CaF2 glass lenses, but not enough for an impressive field. Second field lens fixed this. Large field necessitates lenses quite larger than the aperture, but we found only Φ25.4 mm CaF2 lenses and didn't want the aperture to be too small, so I settled for the 50° FFOV. The collimator lens was also CaF2.

Index of refraction falls with wavelength at reducing rates, which is the reason why UV and VIS are sometimes troublesome to achromatize. At MWIR band this is a lesser problem, and CaF2 has a very high Abbe number in the first place, so chromatic aberration was controlled quite easily while bending the field rays into collimated, more or less unidirectional rays. After that it was no chore to choose a standalone imager lens. A germanium hybrid lens with numerical aperture of one looked nice, although its design NA was no way achievable, as the aperture of the hybrid was larger than the diameter of the collimated beam. With the field lens, NA = 0.24. System stop was located between the collimator lens and the imager.

Being happy with the optical design, I begun to gather the optomechanical components. I could have opted for individual lens mounts and connect them with bars, but the rebel in me opted for closed tube assembly, because heck with it, life is short, y'know. With a system length of about 65 mm, to fit all in one tube would have taken a 3" tube. But preserving the field from vignetting, the image plane ended up inside the tube. Back to optical layout. Luckily an equivalent solution with optical length (minus working distance) of less than 2" presented itself in no time. The final optical length between the first and last surfaces fit (just barely with retaining rings) inside a 2" tube, making the setup quite compact and light-weight.

In short, two plano-concave CaF2 lenses provide field, a double convex CaF2 lens collimates, system aperture limits and a germanium hybrid lens focuses a 50° full field MWIR (3-5 μm) band beam of 6 mm system aperture. System focal length is 12 mm and the working distance (in air) is 9 mm.