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LWIR, MWIR & SWIR side by side

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I had a little time to call my own today and some new-ish toys to play with so I thought I'd try comparing imaging at three different wavelengths: LWIR (7-14µm), MWIR (3-5µm) and SWIR (0.9-1.9µm).

I used a Therm-App Pro (640 x 480) with 19mm lens for LWIR, an Agema THV 450 (120 x 120) with 20 degree lens for MWIR and an ElectroPhysics  MicronViewer 7920A (see later for resolution).

There is a tremendous size difference between the three cameras. The oldest, the mid-1980s Agema, is the size of an early ENG camera-recorder. It represented a huge technological advance, for its time, being one of the very first completely portable MWIR thermal cameras that could operate from batteries. Unlike many cameras of its time it didn't require liquid nitrogen or a limited-life Stirling cooler; instead, it used a single-pixel 'SPRITE' detector (cooled to about -70°C by a stack of Peltier devices), with the raster image being got via a system of moving mirrors. There is plenty of documentation on the web (particularly on EEVblog, thanks to Fraser, who did a very well-illustrated and detailed teardown of the very-similar THV 470). It produces an image of 120 lines x about 120 pixels at 20Hz, 'dropped in' to a standard PAL TV frame (along with image data) by an integral 7-bit scan converter circuit. I'm still learning to drive this camera and it may be possible to get better-quality results than in these first experiments.

In the middle, size-wise, is the ElectroPhysics MicronViewer 7920A. These are still relatively current, having only gone out of production in the last few years as the supply of a critical part dried up. Despite that, the technology really has its roots back in the 1960s, possibly earlier, as the camera is based around a vidicon tube. Unlike the pyroelectric vidicons in first-generation fire cameras (waves at Argus 1), the short-wave (red to about 1.9µm) tube doesn't require a chopper or a fancy frame store; it's basically the same technology as you'd find in a Pye Lynx. Vidicons are normally operated at ambient temperature - if you feel comfy, the tube will too. This camera uses the NTSC (525/60Hz) video standard which means it's a bit difficult to express the resolution in good ol' computer-friendly pixels. It is, roughly speaking, 640 x 480 but until I delve into the tube data sheet I won't be able to say exactly what the true optical resolution is. But 640 x 480 is a good-enough place to start.

The LWIR camera is the newest, and smallest of the gang: a Therm-App Pro, 640 x 480 25Hz Android dongle-style camera.

I used a cheap USB video capture dongle running on a phone to digitise the MWIR and SWIR video outputs. I adjusted the black level, gain and (particularly) gamma of the resulting images to give what I felt was the best possible result.

Let's start with a look at the three cameras side by side.

So, what about the images? Here is my standard test target, my little shed. You can see what I mean about the MWIR resolution being well behind the others - but the camera (and its technology) is nearly 35 years old.

Now a pile of trash in front of a near-derelict conservatory that's full of more stuff that I need to get rid of. Interestingly, the glass is transparent to the SWIR camera. I think it was opaque to the MWIR camera but the resolution wasn't really good enough to be sure.

And last in this series, a selfie. Notice my spectacle lenses are opaque at LWIR and MWIR but transparent to SWIR.

Finally, here are three unretouched images exactly as they came from the respective cameras.

The three cameras have different fields of view so it isn't really possible to do a 'perfect' side by side comparison. I compromised by having the cameras the same distance from the 'still life' subjects but adjusted my selfie position till it looked 'about right'. I'm not convinced I got the focus spot-on for any of the selfies except the LWIR one.

And that, my friends, was an afternoon's work. I had hoped to increase the spectral range to include near-IR (0.75-1.1µm), visible, plus UV (380-320nm) but time was against me. Another day, perhaps.


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