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Comment by diacritical | Hacker News Reader

From ~04:10 till 05:00 they talk about sodium-vapor lights and how Disney has the exclusive rights to use it. From what I read the knowledge on how to make them is a trade secret, so it's not patented. Seems weird that it would be hard to recreate something from the 1950's.

I also wonder how many hours were wasted by people who had to use inferior technology because Disney kept it secret. Cutting out animals and objects from the background 1 frame at a time seems so mindnumbingly boring.

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The lights are relatively easy to get. iirc (it's been a bit since I watched their full video on the subject[1]) the hard part to find was the splitter that sends the sodium-vapor light to one camera and everything else to another camera.

1. https://www.youtube.com/watch?v=UQuIVsNzqDk

Yup, I wanted to say that the prisms are hard to recreate, not the light itself.

It would seem to me to be relatively easy to build something like that if you're okay shooting with effectively a full stop less light (just split the image with a half-silvered reflector and use a dichroic filter to pass the sodium-vapor light one one side.

The splitter would have to be behind the lens, so it would require a custom camera setup (probably a longer lens-to-sensor distance than most lenses are designed for too), but I can't think of any other issues.

At the end of this video they link to another video from a year ago [1] (this is the same link as the comment you were commenting on, whoops), where they recreate the sodium vapor process with a rig with a beam splitter, one side had a filter to reject sodium vapor light and the other has one to reject everything but sodium vapor light, and then a camera on each side.

The Disney process had the filter essentially built into the beam splitter, but afaik, nobody knows how to make that happen again (or nobody who knows how, knows it's a desirable thing). Seems like the optics might be cumbersome, but the results seem wortwhile.

Also, you need still need careful lighting, you don't want your foreground illuminated by sodium vapor, but I wonder if you could light the background screen from behind (like a rear projection setup) to reduce the amount of sodium vapor light that reflects from the foreground to the camera.

[1] https://m.youtube.com/watch?v=UQuIVsNzqDk

We know how to make dichroic prisms (Technicolor used them when filming, as did "3 CCD" digital cameras), but I imagine that to have a sufficiently narrow rejection band for the sodium-vapor prociess, you would need to be smart about where you place the prism, since the stop-band of a dichroic filter changes with angle of incidence.

It is a well known process, Not a lot of general use so costs are not low, but not nearly as high as the original disney prism, I would guess around a 1000 USD for one. As far as I can tell any well equipped optics laboratory could make a beam splitter with whatever frequency gate they want.

https://accucoatinc.com/technical-notes/beamsplitter-coating...

I have no idea about that specific company I just picked it after a search for "beam splitter"

After I saw that video on the sodium vapor illumination process I was curious as to what if you could instead use near-IR light as the mask illumination. In theory you would have a perfect mask(as in the disney process) and no color interference. I found that frequency gated beam splitters are a fairly common scientific instrument.

Thanks for the info.

As for the IR idea, I wonder if there's something like a crowdfunding/crowdsourcing site for ideas where the person who had the idea doesn't really want to do it, but leaves it open to others to try. You said you "don't really have the budget to try it out", but let's say even if you had the money, it wouldn't be a priority for you, as you're not an expert or you have better things to do or whatever. Is there a place to just shout ideas into and see if any market-oriented entity would take it upon themselves to try doing it? Besides forums full of ideas like "tinder but for X" and such crap? Because, imagine if your idea really is a great one. A couple hours from now it would be buried in HN.

Even if it had been patented, patents from the 1950s would have long expired. In fact, patents from 2005 would have expired - the US patent term is only 20 years.

Didn't know that, thanks. Although 20 years seems too much for some things, especially computer-related fields that move much quicklier than other fields. But now pretty much everything depends on or runs on computers so 20 years seem too much. I don't know if I phrased it correctly, but I mean to say that before computers, things moved much more slowly. Even a century for a patent would've been fine 500 years ago, but now almost every field has been changed by computers in the past few decades and will change even more rapidly. Letting 1 company have the advantage for 20 whole years now is much more impactful than it must've been before.

Yeah, that's just nonsense. We used sodium vapor monochromatic bulbs in my high school physics class to duplicate the double slit experiment.

I suspect the real reason is that digital green screen in the hands of experienced people is "good enough" vs the complication of needing a double camera and beam splitting prism rig and such.

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