Some ideas

[Stefano]

So, regarding phosphors and SWIR... I talked about "erasing phosphorescence", and I did a quick test of mine.

This is what I have. Two (one showed here) "sachets" of green glow-in-the-dark paint. They come from a game which I never finished and after 10+ years they dried up inside their sealed, unopened envelopes. They aren't particularly bright, but apart from some Geomag magnets they are basically the only phosphorescent thing I have (bread is also phosphorescent, but I can't use it of course. I actually saw it glowing very faintly for ~one second after turning off my UV flashlight, and I had my eyes closed while exposing it to make sure it wasn't an afterimage).

 

Of course, the setup. The screw is the imaged subject.

 

I charged the paint with a 365 nm light, making sure to also cover the area behind the screw. Then, I exposed It with a 10 W 660 nm deep red LED, with a 90-100° lens at ~34 cm, for like one minute or two. I didn't measure the exposure time because this was just a quick, rough test, as I usually do. Then, I photographed the scene.

 

Camera: full spectrum Panasonic DMC-F3, f-stop: f/2.8, ISO 80, 60 s exposure. No filters.

 

You can see it, right? The paint thickness is very uneven , so the image has an "hole" in the center. The exposed paint discharged more quickly that the paint in the screw shadow, because red photons allowed electrons in the metastable state to "fall down" and emit a photon, thus discharging the paint.

 

And, just for fun, this is an exposure with the fully charged paint.

f-stop: f/2.8, ISO 80, 60 s exposure. No filters.

[Andy Perrin]

That is pretty cool. What was the connection to SWIR again? I thought you said you just needed red light for this.

[Stefano]

Yes, it should work with SWIR (I hope...). I used a red LED because it is more powerful and faster. I tested it with a 946 nm LED and it works. Since photons must have enough energy to promote electrons to that higher energy state (and energy levels are discrete), I hope that SWIR photons carry enough energy to do that.

[Andy Perrin]

Oh, I see, I don’t know that I’m interested enough to test that one.

[Stefano]

Oh, I see, I don’t know that I’m interested enough to test that one.

You mean "erasing" with SWIR? I think that there is some potential in this, if you use a very good, long-lasting phosphorescent material. Surely not the fastest, easiest way to try SWIR (provided that it works).

[Andy Perrin]

You want to use it for imaging? So it would be something like making a camera body with a phosphorescent material coated on a piece of glass at the rear of the camera. Then:

A) expose the phosphorescent material to UV to charge it up

B) turn off the UV and focus a SWIR image on it

C) before the phosphor stops glowing, snap a photo from the rear of the glowing phosphor

 

The problem I see is that even if the SWIR makes the phosphor dark faster in places, your overall exposure is limited by how long the phosphor will glow.

 

ETA: what if you paint the phosphor material directly on the sensor?

[Stefano]

Yes, you got it.

 

You charge the screen (with UV, violet, blue, its your taste. You may used the wavelength of peak absorption of the phosphor), then you focus the SWIR image on the screen (you have to either use a filtered light source or use a filter on the main lens), you expose the screen, creating a negative image of your subject (bright=little light reached the screen), and then, as you said, snap a photo of the screen. Coating the sensor directly would be similar to the deep UV cameras with fluorescent coatings on the sensor to downconvert deep UV into long-wave UVA or visible light. Maybe coating a lens would be better than building the contraption above. In my example the phosphor screen should be thin (to avoid diffusion and soft images) and transparent enough to see through it (like image intensifiers for X-Rays).

 

Oh well, I uploaded a huge image. I may leave it like this.

[Andy Perrin]

You can’t coat the lens. The reemitted light does not keep moving in the same direction.

[Stefano]

You can’t coat the lens. The reemitted light does not keep moving in the same direction.

Right, you said a "piece of glass" (not a lens, a flat pane probably).

[Andy Perrin]

Right, you said a "piece of glass".

For the screen, not the lens!

 

This is all pretty close to the upconversion phosphor rig that I built awhile ago, except now it's even more complicated because you have to charge the screen first. Honestly I don't think this is the right way to go. Better to just coat the sensor with upconversion phosphor like several commercial products do. You can buy the upconversion phosphor from MaxMax even.

[Stefano]

 

For the screen, not the lens!

Yes, sorry, I didn't understand initially.

[Stefano]

Better to just coat the sensor with upconversion phosphor like several commercial products do. You can buy the upconversion phosphor from MaxMax even.

Can I simply use a brush to do that? I would need a very fine powder and avoid uneven spots.

[Andy Perrin]

Honestly I don't know! I bought my screen premade, by lurking on eBay for ages and pouncing when one of the Edmund tubes came on sale for like $150. It was the most expensive part of the whole thing (not counting the camera obviously).

 

https://maxmax.com/p...s-dyes-and-inks

https://maxmax.com/p...horsdyesandinks

 

I would email the maxmax guy before buying anything. His name will be Dan because everyone is a Dan in this business.

[Stefano]

An anti-stokes phosphor would be nice to have. Imagine a (very weak) blacklight effect with a safe, IR lamp. Anyway, a lot of time ago, I found this:

 

https://hackaday.com/2015/01/21/diy-single-pixel-digital-camera/

 

and now I found this:

 

https://www.ribbonfarm.com/2016/06/29/the-daredevil-camera/

 

You may want to give a look.

[Andy Perrin]

Very cool, the pan and scan method clearly can be made to work, although I am not sure I want to know how much time it took to get the bugs out. I guess the most interesting thing about that is that he can do mid-wave infrared, which is quite expensive by most methods. I would consider it for that purpose. For SWIR, there are lots of ways to do it that are in the "moderately expensive" category (as opposed to the "insanely expensive" category). TriWave is one, and a friend got an InGaAs for about $3k not long ago also, on eBay. I saw a little cellphone SWIR cam on eBay for $1k recently and posted it here. The price is coming down.

[Stefano]

He can change photodiode and select the wavelength range. With a mercury cadmium telluride photodiode he should be able to do LWIR. Making a matrix with them would be probably very expensive, but if you buy just one of them it can be way more affordable. Also in MWIR people start glowing. The thereshold between reflected and emissive photography at room or near room temperatures is about 3 μm. MWIR is cool because it is between this two worlds. Images in MWIR are usually very similar to those taken in LWIR, but glass is still slightly transparent (almost completely opaque), and shadows due to sunlight are still visible. Hot enough objects are brighter in MWIR than in LWIR because of their blackbody spectrums.

 

This video (probably shot in MWIR) shows very well both reflected and emitted light.

 

Note that SWIR characteristics (black skin and white hair) are maintained in MWIR, LWIR and maybe FIR and above. Water stays very black all the way up to radio waves (especially shorter ones). What happens if you invert a LWIR image of a human face? Guess what, you have black skin and white hair.

[Andy Perrin]

Images in MWIR are usually very similar to those taken in LWIR, but glass is still slightly transparent (almost completely opaque), and shadows due to sunlight are still visible. Hot enough objects are brighter in MWIR than in LWIR because of their blackbody spectrums.

LWIR shows shadows due to sunlight just fine. Reflections are quite visible in LWIR. You do see both the actual shadow (because the incoming light is not reflected) AND the effect of the cooling on the emitted light at the same time, so if the shadow moves you can see after-images from the emission difference. Just like in the video. I don't know why you think MWIR is different from LWIR in this respect.

[Stefano]

I got other three ideas, and I didn't think it was necessary to start a new topic only for them, so I am writing them here.

1. What does limit the range visible by thermal cameras at ~8-14 μm? The lens? The absorbing material on the microbolometers? Can a thermal camera be modified to see in a wider wavelength range? Can it see at, say, 30 μm? Of course diffraction will start to ruin images at longer waves, but still better than using an antenna or something similar.
   
2. Can a multispectral thermal image be obtained? Just like RGB for visible or the recent experiments by Andy and Bernard on three channels NIR vision. The problem would be to find the filters. I expect to see hot objects (like 100°C objects) as blue. Hot objects are brighter in MWIR than in LWIR.
   
3. This one really interests me. During the day, we can not see stars and planets because the sky is bright enough (Rayleigh scattering) to cover them, but if you point a telescope at the biggest planets in daytime you can still see them
   . In UV I cannot see the moon in daytime, and only managed to get some faint images. Now, in the NIR, Rayleigh scattering is much reduced, and this means that, maybe, stars could be seen in daytime. Can someone try pointing a camera directly to the sky, in daytime, with a long-pass filter (like 1000 nm), and see if he/she can see something? That would be crazy awesome.
   

[Andy Perrin]

Stefano

1.) the atmosphere! Air goes rapidly opaque outside the 8-14 micron window. I suspect the camera may be sensitive to at least some wavelengths outside that range, but the image may not be in focus outside those wavelengths. Definitely 30 microns is out because blackbody radiation from room temperature objects is nil there, the lens won’t focus (and I’m not sure if it’s transparent even) and that may be larger than the pixel size, so diffraction may matter too.

https://commons.m.wikimedia.org/wiki/File:Atmosfaerisk_spredning.png#mw-jump-to-license

 

2.) Probably a three band color image is possible but there may not be any point because the spectral distribution depends only on the temperature (Planck distribution) to first approximation. (There is some variation in emissivity but it’s not that significant for most materials.) This is another idea I had but dismissed because of cost of filters and the above reasoning.

 

Far more interesting would be to do the tri-band imaging in MWIR, where room temperature objects don’t glow strongly, but material reflectance spectra go nuts. This is the regime where traditional spectrography is done. This is on my “do it when I can afford a MWIR camera” list.

 

3.) Worth a try.

[Stefano]

About THz waves... should I start a topic dedicated to them?

 

I had an idea: how many transistors can I nest (like a darlington pair but with more transistors) to have super high gain? I know that this will likely increase noise, and that, anyway, one THz is a VERY high frequency for a semiconductor. Also I have to reduce parasitic capacitance...

 

But, anyway, apart from that, can I amplify a very weak signal from an antenna this way? Can it work (theoretically)?

 

[Andy Perrin]

I think you need an electrical engineering forum for this sort of question. It's definitely beyond me. I have just a basic understanding of how radio receivers work. Maybe someone else here knows more, but I suspect asking on a more EE-oriented forum would get you more results.

[dabateman]

Yes start a new topic in gear discussion.

I have no idea who most of the people are whom read this forum. So you never know what they know.

Also seems like most members don't post anything.

[Stefano]

Also seems like most members don't post anything.

Probably 20% of the members write 80% of the posts, use 80% of the server memory and write 80% of the private messages Andrea and Birna receive.