being restricted to ~visible spectrum is a design goal for a camera, since we expect it to produce images that look like what we see. Your usual camera sensor will see a bit further into infrared than humans do (hence why cameras usually contain a filter filtering that out), but not so far that it is in any way useful for things that aren't burning hot. E.g. with cheap digital cameras with bad filters you might see the coals of campfire being slightly tinted wrong, because those get hot enough to be visible, but everything else is not affected, and you'd think a camera was bad if it were
The visible detectors are kind of a lucky accident, that the visible spectrum is within the range that can be detected by a silicon sensor. Therefore, visible sensors are cheap and easy to make thanks to silicon IC technology. Sensors that detect IR beyond about 1 micron wavelength (the bandgap of silicon, where it becomes transparent) have to be made out of more "exotic" materials that are also more expensive. The problem might be surmountable, but fewer people want to set up and optimize "fabs" for those materials.
The problem with detecting heat with silicon is that the intensity is extremely low, and the amount of background radiation high. Getting a temperature measurement that isn't corrupted by other effects is going to be difficult.
silicon's bandgap of 1.1 electron volts corresponds to about a 1100-nm wavelength; is that the minimum photon energy a cmos sensor can detect?
if i did this calculation right, the spectral radiance at 1100nm should be a couple million times dimmer than the 700nm radiance at the draper point
You have: h c / boltzmann 700 nm 300 K
You want:
Definition: 68.513185
You have: h c / boltzmann 2000 nm 300 K
You want:
Definition: 23.979615
You have: h c / boltzmann 700 nm tempC(525)
You want:
Definition: 25.751996
You have: h c / boltzmann 1100 nm 300 K
You want:
Definition: 43.599299
You have: exp(43.6-25.8)
You want:
Definition: 53757836
so it's not literally impossible to detect but it seems like you would probably need special conditions like a light-tight darkroom or a thermal emitter switching on and off at a particular known frequency for millions of cycles
A physicist once emailed me to say that it's possible to pick up NIR emissions from a soldering iron, but only in a dark room.
Beyond the sensor limitations, MWIR and LWIR require special optics, because regular glass is opaque to them. The last time I looked into it, the glass was usually germanium-based.
One can do some neat stuff with actual thermal imaging. I was lucky enough to get a FLIR E4 when they could have the firmware replaced to turn them into an E8. 320x240 resolution is still pretty low, but having seen the difference between that and the stock 160x120, I'd not want to use a sensor with a resolution any lower than 320x240 or so.
[1] Scroll way down to the "Stovetop Bokeh (Stove on Medium-High)" section of https://www.beneaththewaves.net/Photography/Thermal_versus_N...
or you can use mirrors
[0] quite what is "forward looking" about them I don't know
Old sensors worked like side-scanning sonar, where the image is built up one row at a time as your vehicle moves forward. Forward looking means you can just point it somewhere and take a 2D picture.
The name’s just stuck around since.
> it would seem a bit weird to me if they just happened to be restricted to exactly the same part of the spectrum as our eyes as a hardware limitation.
That's a feature, for the leica m8 sensor "sees" UV and IR, as a result you need to use a UV/IR filter on your lens otherwise you get weird colors and blurrier images (since UV/visible light/IR all focus at different plans)
https://www.35mmc.com/14/02/2016/leica-m8-infrared-cut-filte...