Hmm, why don't people do this with near infrared? It would allow using full spectrum lighting, and would be easier to selectively filter.
I'm guessing the answer is "they do, it's just uncommon and you haven't heard about it"
I'm guessing the answer is "they do, it's just uncommon and you haven't heard about it"
They did that to get a 'perfect' mask.
If you use infrared, which is generated by random environmental objects, how would you prevent random spill?
Practically (my understanding) is if you got random spill on the surface, you'd get random alpha in the mask.
The point of using the sodium light is that it's a single wavelength that you really really would not get naturally, so you can totally control the emission sources of it.
Could you do that with a single frequency infrared? I guess it's possible, but I'm not sure how you'd do it technically, and it's unclear what the benefit of using a different frequency would be?
It may be generated by lights but could be filtered out. It is generated by the Sun, but so is the sodium wavelength. The technique would work indoors just as well as the sodium one, but with important advantages. I think it could even work pretty well outdoors if you used a wavelength that is mostly filtered out of sunlight by the atmosphere, or maybe two different wavelengths in two channels simultaneously.
> but with important advantages
What's the benefit over using any other single frequency visible light?
Even if the atmosphere absorbs most of a particular band, if any of it gets through, your mask is going to be messy, which kind of ruins the point; I don't know anything can really fix that for outdoor purposes?
I mean, broadly speaking, all you need is a way to emit a sharp frequency band and avoid having any environmental pollution in the same band.
If there's a good way to do that in near-IR, it would work; but I feel like the same could be said about pretty much any frequency.
I'm not aware of anything specific about near-IR that makes it particularly attractive.
(but, clearly, I'm no expert in near-IR)
> It would allow using full spectrum lighting, and would be easier to selectively filter.
Making a broad spectrum IR filter is far easier than a notch filter for a single visible wavelength. In general it's harder to make more selective filters and an IR filter can be much less selective because the IR band is so large and far away from any visible light.
Also the infrared version would allow using any color background including black, and the on set lighting would appear to the eye exactly as it does to the camera. With the sodium vapor process the bright yellow background would make everything on set look different in person because the eye wouldn't be able to subtract it the way the camera does.
Yes, you would need a filter on any incandescent light. But this is also true of the sodium vapor process. Actually I'm not sure anyone makes a sodium vapor wavelength notch filter suitable for putting in front of an incandescent light. Consequently, as seen in the video, they restricted themselves to special LED lights that emit essentially only three wavelengths. These are very much not full spectrum and will not produce natural colors for some materials. And they had to completely block all windows, whereas for IR you could just install some IR blocking film on the windows and still use natural sunlight.
You're basically proposing something completely different here.
So... while I agree you could do something as you describe, and I'd be interested to see the results of it, I'm not sure what the results would be like.
> The advantages I stated were
>> It would allow using full spectrum lighting, and would be easier to selectively filter.
All I can say is the CC results in that video were pretty great, despite the obvious caveats, compared to green-screen.
If you (or anyone) can build on it to do better, people will be interested in it; but there's obviously a difference between us idly speculating on how/if it might work, and this, where they've actually done it and shown that it does work.
> "they do, it's just uncommon and you haven't heard about it"
I've never heard of it being done.
If anyone has, please post a link or something.
Yes, Corridor's results were great. Fewer limitations would make the process even better!
Alternatively you could use a solid state laser or narrow band LEDs if you need the band to isolate to be in the visible spectrum too.
One advantage might be to pulse the IR and detect it via that, if the detectors are sufficiently fast. Maybe have a filter that's pulsed also (Pockels cell?)
Not with a non-visible spectrum, but along the same lines. I can't seem to directly find anything with NIR though;
My guess though is that using a visible wavelength makes it easier to set up the lighting and ensure it's even and so on (remember, this was before electronic technologies that would make it possible to live-preview the result). Also, sodium lights and sodium band filters had other uses in industry so they might have been more available than NIR/NUV equivalents.
I know in IR still photography you're supposed to add a focus offset marked on your lens to adjust for that. A regular achromatic lens quickly fails outside of the wavelength range it was designed for[1].
[1] https://en.wikipedia.org/wiki/Chromatic_aberration#Minimizat...
EDIT: actually, since it's outside of the visible spectrum, typical lens design correcting longitudinal chromatic aberration and doing the spherical to rectlinear projection simply might be off, resulting in soft edged and general artifacts unwanted in a clean mask
It seems possible to produce a four-channel camera sensor specifically for this use case, if it was popular enough. Given how common green screen is in Hollywood I'm surprised I haven't heard of anyone doing it. Maybe Hollywood just doesn't care because they can do regular green screen and just hire someone to fix it in post.
I am guessing that that is dangerous to human health because infrared fails to elicit the flinch reaction that too-strong visible light does.
That's one of the things that makes it suitable for thermal nano cancer therapies. You
(1) Create a nanoparticle with a gold core of the right size that it excites when exposed to near-infrared radiation (the far end of "near" is ideal),
(2) Coat it with something that's hopefully less toxic than the cancer because the gold nanoparticles are pretty reactive (silica was popular last I checked, though I'm not convinced that's actually safe, and there hasn't been much testing on it),
(3) Coat that in something that binds to the right antigens,
(4) Inject that into the patient so that you'll eventually have a tumor rich in these nanoparticles while the rest of the body has a very low concentration,
(5) Shine an intense near-infrared beam at the nanoparticles. It safely penetrates body and causes minimal heating, instead depositing all its energy at the tumor, selectively cooking just the bits of tissue you don't want anymore.
It's yet another way to make mice immortal, but human testing is a long ways off. The hard parts are (2) and (3), along with the (4a) I didn't mention where most of these things don't want to stay "nano" in most chemical environments, and the solution they're suspended in is usually also not ideally suited to being injected into living mammals who would like to retain that "living" property.
Glowing-hot steel produces enough IR that the workers that work with it have 7 times the rate of cataracts as the general population.
NIR is much safer than the visible spectrum at the same power levels. The threshold for a few minutes of high-intensity NIR increasing your chance of eventual cataracts is staring at an NIR source 10x brighter (w/cm^-1) than the sun integrated across all its wavelengths. The threshold for sun-colored wavelengths causing eventual cataracts above the baseline from a few minutes of staring is under 1x.
Also worth noting, very very near IR (basically red, though we can't see it) doesn't quite enjoy those same properties, and during their time working with hot metal you'd expect a lot of energy in that band.
Also worth noting, many steel workers are exposed to dangerous amounts of visible light too. I absolutely believe that they can get enough NIR to cause problems, but if I wanted to try to prove that NIR specifically causes their problems to somebody else then I'd want to try to account for that fact (and for incidental welder exposure, ...).
Another alternative I was thinking about is just to use one of those monochrome astrophotography cameras, something along the lines of this[3]. The monochrome version doesn't have the IR-cut filter and sensitivity is pretty ok down into near-IR.
edit: The IMX492 sensor in that one has a 40% response at 850nm, and 850nm IR diodes are plentiful and shouldn't emit[4] much at all below the 700nm of the IR-cut filter the color camera should have.
Not sure how mixing sensor sizes and such would affect things, or if it's better/easier to just run two of the same camera.
[1]: https://s1-dl.theimagingsource.com/api/2.5/packages/publicat...
[2]: https://www.lifepixel.com/
[3]: https://www.zwoastro.com/product/asi294mm-mc/
[4]: https://lumileds.com/wp-content/uploads/files/DS191-luxeon-i...
Building 2 nigh identical cameras, but one with a sensor outside of the frequency range of the other, is probably a very expensive custom job.
It seems like the cube with mounts is the most expensive part, and there's no way it can cost more than one of your cameras, let alone the lens, right?
Now that I think about it, Disney must have done something pretty special for the lighting in the foreground in their version of the process. They didn't have single wavelength LEDs back then. I wonder how they did it?