Our FLIR has a germanium lens.
That's a thin-film bolometer, similar in general character to the ones used in modern uncooled microbolometer cameras. Compared with visible/NIR photodiodes, they're slow and very insensitive, but they sure do have a wide wavelength range.
20 years ago I built an interesting system called "Footprints" that I've discussed here a few times over the years. It used an array of 96 carbon-ink pixels screen-printed on a 9-um thick free-standing film of PVDF (which is basically fluorinated Saran Wrap). The pixels were 3 x 5 mm on a 6-mm pitch, to leave room for the wiring. The readout multiplexer was a single red display LED per pixel.Diodes ideally conduct in only one direction. The particular LEDs I used leaked less than 50 fA between -5V and +0.5V bias. Interfacing them to an AC-only sensor such as a pyoelectric requires a bit of bias current, which in my gizmo was supplied by four green display LEDs, which allowed the processor to adjust the average bias current between 0 and about 5 pA per LED.
The optical system was a moulded Fresnel lens made out of HDPE (bleach-bottle plastic).
When it was done, it had very competitive sensitivity: noise equivalent delta-T of just over 0.1 K, not bad for something so minimalistic.
We tried licensing it, but couldn't because the parts cost was too low.
Cheers
Phil Hobbs
Germanium makes really good IR lenses. Besides the built-in filtering action, it has a refractive index of 4. The optical power of a given surface is (n-1)/r, where n is the index and r is the radius.
Glass is generally around n = 1.5-1.8, so a Ge lens of a given power has
4-6 times the radius of curvature. Aberrations are much reduced due to the weaker curvature, so a simpler lens can have better performance. Also of course the diffraction spot size goes like lambda, so in terms of the diffraction limit a Ge lens at 10 um is like 100 times easier to design than a glass lens at 500 nm.Cheers
Phil Hobbs
Oh, and the dispersion of Ge out in the thermal IR is much less than glass in the visible, so you don't even need to achromatize it. The tempcos of index and of optical path length are quite large, so you do need to athermalize in general, something that's rarely required in the visible.
Cheers
Phil Hobbs
That lens stays in focus up to the point it touches a part. It will clearly show the hot spot on an 0603 resistor.
Here's a tiny dual transistor,
obviously two separate chips inside.
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