It seems millimeter waves can be used for thermal imageing.

[Ben321]

According to http://www.trexenterprises.com/Pages/Products%20and%20Services/Sensors/security.html not only are um waves emitted in blackbody radiation, but mm waves are also emitted in sufficient quantities, allowing for mm wave cameras to be used passively for thermal imaging purposes, rather than an active imaging as most microwave systems require. This means there is no need for an emitter near the receiver like is required for RADAR. In the example used at that website, mm waves emitted from a warm human body will easily pass through clothing, but be blocked by a gun or other metal object the person is smuggling, allowing that person to be stopped before they can get on an airplane.

I had always assumed that THz and mm waves were so much longer than um waves typically used with thermal imaging, that the natural blackbody emission of these wavelengths would simply be too weak to detect, preventing passive imaging at these wavelengths. I always thought that these wavelengths would require an emitter pointed at the target, and that the receiver would pick up the reflected signal, just like with RADAR. But that above mentioned website seems to suggest that mm waves are emitted strongly enough, just from blackbody radiation, that they can be detected and used for passive imaging.

Anybody here have an idea how that actually works? There's not too many technical details about the system used on that website (probably proprietary trade secrets or something).

[Bill W]

It is not 'thermal imaging' but Terahertz imaging.  A lot of the same materials work though.

Wikipedia is now your friend

[Ben321]

It is not 'thermal imaging' but Terahertz imaging.  A lot of the same materials work though.

Wikipedia is now your friend

I said thermal, because the energy was being emitted by a thermal emission process, because in the use case described in that website, they depend on the human body's own blackbody emission, rather than using a separate THz signal generator. This means that the THz signal would be stronger for a hotter object than for a colder one. Thus I described it as thermal imaging.

[mzzj]

Anybody here have an idea how that actually works? There's not too many technical details about the system used on that website (probably proprietary trade secrets or something).

Superconductive bolometer sort of:
https://indico.cern.ch/event/659554/contributions/2709562/attachments/1527300/2388518/3EO2-06_Juha_Hassel_Room_34.pdf

[eKretz]

From what I gather, in this frequency range there has to be a certain amount of heat to allow this to work well enough to get a decent image. For instance on the "just sub-THz" system linked above, the sample image shows a slight amount of detail on the human body but the cooler area with the "concealed item" is basically a black blob. Gives an indication that something is there but no idea what. On a scene where temperature differential isn't very high it probably wouldn't be very effective or useful.

[Dark Volter]

This is well known, and this is what you are looking for, this is better than the grey airport images
http://www.vision4thefuture.org/s4_resources/2_passive.htm

Be advised, this does not stop at millimeter-wave, but goes to submillimeter and beyond. You can image extremely far down into the Spectrum, and I have seen images all the way down to 11 gigahertz of the environment.
I we'll try to find them and post them, but a lot of that sort of tech is extremely cutting-edge and a lot of it is still being researched, and after a certain point I don't think this is been tried, for longer and longer wavelengths. Radio telescopes do not count since they're looking at the sky not the environment.

[eKretz]

It's only better in that particular linked image because they are using reflected (basically active) imaging outdoors. Indoor images - i.e. low temperature span passive imaging - will still be crap, as I said before. Here is another image from your same link showing an example using indoors passive imaging. As you can see, it is nearly exactly the same poor quality as the one posted earlier.

[Ben321]

It's only better in that particular linked image because they are using reflected (basically active) imaging outdoors. Indoor images - i.e. low temperature span passive imaging - will still be crap, as I said before. Here is another image from your same link showing an example using indoors passive imaging. As you can see, it is nearly exactly the same poor quality as the one posted earlier.

The poor quality also has to do with the resolution of the sensor. That looks like it's about 32 pixels across, but upscaled for easier viewing.

At 300GHz, the wavelength is 1mm. An array of quarterwave receiving antennas on a chip, to make an imaging chip, would have a pixel width of 0.25mm (each antenna being 0.25mm long). So for a 160x120 sized imager you would have 40mm x 30mm physical chip size. In English units, that's 1.6in x 1.2in. That's pretty big, for the size of a chip, but it is possible.

[eKretz]

A slight bit, but it's mainly as I described. Here is an image from that link that clearly shows the difference between a scene with low differential and high differential. Notice that even the clear outdoor picture is very low resolution but features look very recognizable compared to the indoor image.

[Ben321]

A slight bit, but it's mainly as I described. Here is an image from that link that clearly shows the difference between a scene with low differential and high differential. Notice that even the clear outdoor picture is very low resolution but features look very recognizable compared to the indoor image.

Why does the outdoor one look better? Isn't the human body itself emit blackbody radiation in all wavelengths from THz through LWIR? So shouldn't the THz image look good, just from the body's own blackbody emission (regardless if inside or outside)? In fact, shouldn't the outdoor one be more obscured, because the background behind the person is heated from the sun's light, thus causing it to be nearly as (hot if not hotter than) the person's own body? So wouldn't the outdoor scene have low contrast between the background and human body? I notice that is the case for LWIR for sure. In fact, walking around with my Seek Compact Pro, people often are colder than buildings and other background objects (and at the worst times are at approximately the same temperature as the background). Only indoors, or at nighttime outside, do I find that LWIR makes people's skin stand out as brighter (hotter) than the background. I would assume the same would be true for passive THz imaging.

[eKretz]

At low heat levels the human body doesn't emit enough at that wavelength. If we were very very hot it would image better. The outdoors image looks far better because it is showing the waves from the sun that have bounced off the human skin and are reflected back to the camera, basically almost like active imaging.

[Ben321]

At low heat levels the human body doesn't emit enough at that wavelength. If we were very very hot it would image better. The outdoors image looks far better because it is showing the waves from the sun that have bounced off the human skin and are reflected back to the camera, basically almost like active imaging.

I thought maybe it was because the human skin was heated by the sun, and thus the skin started radiating more, not reflecting the sun's radiation. I doubt the skin is a very reflective substance.

[eKretz]

Do some more research, and you will find that it is indeed reflecting quite a bit at that frequency. It's not just the heat being reflected but also the lack of same, which produces the higher contrast outdoors. Your theory doesn't really make any sense... When you go outside, does your body temperature change much? In my experience, humans don't fluctuate much more than a few degrees. Maybe you are cold-blooded? 

[Ben321]

Do some more research, and you will find that it is indeed reflecting quite a bit at that frequency. It's not just the heat being reflected but also the lack of same, which produces the higher contrast outdoors. Your theory doesn't really make any sense... When you go outside, does your body temperature change much? In my experience, humans don't fluctuate much more than a few degrees. Maybe you are cold-blooded? 

You need to put a thermometer in your mouth to get your temperature. That's because your body temperature is fixed internally, but not the outside of your body. Your skin temperature varies with the weather. That's why you feel hot on a summer day (the surface temperature of your skin gets hot).

[eKretz]

Yes of course, but it's not going to vary enough to make a huge difference to a mm-wave camera, which is what we were discussing, no? If your skin reached 120°F for even 5 minutes you would be severely burned.

[Ben321]

Yes of course, but it's not going to vary enough to make a huge difference to a mm-wave camera, which is what we were discussing, no? If your skin reached 120°F for even 5 minutes you would be severely burned.

I thought the scalding temperature for the human body was 140degF. 120degF is merely painful but not damaging.

[eKretz]

Again, do some more research. The damage occurs over time - at 140° third degree burns can occur in seconds. At 120° they can occur after 5 minutes.

[Dark Volter]

Is that something that can be compensated for with more sensitive detectors, or is it simply tricky to use longer and longer wavelengths to image things that don't emit large amounts? (In regards to seeing people in THZ, MMW, Sub MMW and so on...)


I've heard of cases where for military research these longer wavelengths are starting to be explored because longer wavelengths penetrate even better through conditions like fog than LWIR does(at a certain point, so you have to go past THZ into MMW for this benefit)
And also, so human's don't have peak emission in MMW- but surely they "glow" in it a little? Or does  that sort of effect only happen further in the spectrum

(here is a 90 ghz image)
https://www.researchgate.net/figure/Photographs-and-corresponding-90-GHz-images-taken-with-the-ABOSCA-imaging-radiometer_fig2_225000895

I feel like there is a lot of extreme potential for imaging further and further into the spectrum(yes,  get tricky as the sensor gets bigger , but right up to tha point, passive imaging has a few applications..)

[Ben321]

Is that something that can be compensated for with more sensitive detectors, or is it simply tricky to use longer and longer wavelengths to image things that don't emit large amounts? (In regards to seeing people in THZ, MMW, Sub MMW and so on...)


I've heard of cases where for military research these longer wavelengths are starting to be explored because longer wavelengths penetrate even better through conditions like fog than LWIR does(at a certain point, so you have to go past THZ into MMW for this benefit)
And also, so human's don't have peak emission in MMW- but surely they "glow" in it a little? Or does  that sort of effect only happen further in the spectrum

(here is a 90 ghz image)
https://www.researchgate.net/figure/Photographs-and-corresponding-90-GHz-images-taken-with-the-ABOSCA-imaging-radiometer_fig2_225000895

I feel like there is a lot of extreme potential for imaging further and further into the spectrum(yes,  get tricky as the sensor gets bigger , but right up to tha point, passive imaging has a few applications..)

That 90 GHz image, cool find! The ground itself looks as reflective as a mirror. Everything above the ground is reflected by the ground.

[Bud]

Interesting it is reflection with little to no bluring, same details as reflection from the object itself.

[CatalinaWOW]

As with everything there are trades.  Required aperture for a given resolution goes up linearly with wavelength, so you either have to have a honking big antenna or put up with lower resolution.  Look at the black body curves and you see that there is an awful lot less energy to play with.  Detectors are near thermal noise limited so the only other way to go is to reduce bandwidth (lower frame rate).  Atmospheric transmission in fogs and smokes is better, but just as in the infrared bands there are some absorbtion bands to watch out for.   

Penetration through materials is highly variable.  Imagine the difference between dry seasoned wood, fresh sappy wood, and wood soaked in salt water.  Or think about the foil back insulation used in many modern houses.  If you wear your sequined stage suit or one of those metallic high fashion garments into the airport imaging scanner I imagine you get asked to step aside for a pat down and wanding.

The military guys like it for the fog and smoke penetration and also for the hopes that there won't be as much CM gear.  The latter is at best a temporary thing, and few if any nations have any fundamental reason to think they are way in front.