IRST wavelength?

[RadicalDisconnect]

When it comes to IR sensors, I notice that the infrared band has quite a few subcategories, including short wave, mid wave, and long wave IR. And different IR sensors use different wavelengths. For example, the F-22's AN/AAR-56 missile launch detector uses short wave IR, while the F-35's DAS uses the mid wave IR cameras. I'm not sure what wavelengths the F-14's AN/AAS-42 or the PAK FA's 101KS-V operates in. On Wikipedia, it is said that long wave and mid wave IR is considered thermal infrared while short wave and near infrared is considered reflected infrared. It's also said that longer wavelengths are less absorbed by the atmosphere. So in general, what are the practical tradeoffs when it comes to the different wavelengths for IRST purpose? Among the considerations is that short wave behave more like visible light, and might be more scattered by atmospheric conditions compared to longer wavelengths.

 

[Void]

- SWIR emissions are zero at room temperature and only become significant hundreds of degrees above it
- MWIR emissions are low at room temperature and reach their relative peak two or three hundred degrees above it
- LWIR emissions are at their relative peak at room temperature

SWIR is only suitable for detecting very hot objects, like exhaust plumes from rockets or jet engines or hot turbine parts. Contrast is quite high, nothing else contributes significant SWIR radiation except sunshine. As an IRST it is basically unsuitable because it is quite possible for an aircraft or missile to have little or no SWIR emissions. Or only in the rear aspect. Relative to the other infrared bands, SWIR wavelengths are very short and more susceptible to scattering by particles suspended in the air.

MWIR is exceptionally good at detecting all objects heated above room temperature. Background emissions are significant but weak. All forms of engine exhaust and friction heating are readily detectable. Sunshine is quite strong in this band though and solar reflections create significant amounts of clutter. Aircraft signature is still highest from the rear aspect, but it is significant in all aspects. Constrast for aircraft and missiles in general is very good.

Contrast for both MWIR and SWIR between hot objects and the background is significantly higher at night.

LWIR is good at detecting everything. At room temperature a black bodies emissions are right in the middle of the LWIR spectrum and everything on earth produces substantial emissions. Sunshine does not dominate over earthshine in this band so solar reflections do not contribute much directly to background clutter (though objects heated by the sun do). Contrast is lower than MWIR for hot targets though and background clutter is ubiquitous. LWIR radiation emitted from or reflected by (metal tends to act as an infrared mirror) airframes contribute substantially to LWIR signature even for aircraft that are not being heated by friction, so signature is relatively independent of aspect. Particles large enough to interfere with LWIR transmission are cleared rapidly from the air and LWIR propagation through dust clouds is generally excellent. LWIR radiation is absorbed strongly by water vapor however, so it is not especially suitable for humid/maritime environments.

Arguably LWIR is the best choice for land-based IRSTs because the good transmissions characteristics of LWIR radiation minimizes it's downtime in low visibility conditions. MWIR provides the best contrast for the targets of interest in air combat though, and the effects of water vapor mean it is almost mandatory for ships and maritime aircraft operating at low altitudes.

 

[RadicalDisconnect]

Thanks for the info. It would seem like if your clutter filtering software is highly sophisticated, LWIR would be the way to go?