Film-Tech Forum ARCHIVE: How does Dolby 3D Work?

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Posted by Mitchell Dvoskin (Member # 751) on 01-28-2013, 04:40 PM:

I saw my first Dolby 3D movie last weekend, and I noticed that the glasses have a redish/cyanish tint. Clearly not anaglyph, I was wondering how the process worked.

I'm not asking the mechanics of the projector, I'm asking what is happening with the light to isolate each eyes image. Is it polarized? Does it require a silver screen? Why the tinting? etc.

Posted by Carsten Kurz (Member # 5396) on 01-28-2013, 07:23 PM:

Basically, electronic imaging works with color separation. The sunlight has a continuous spectrum of wavelenghts. But our eyes have receptors that are only sensitive to parts of that spectrum. A combination of only three primary colors, e.g. Red, Green and Blue, will create all(...?;-) necessary other colours, including white light, as well. It's simple economics. Why need a very complicated receptor, when three simpler receptors can do the same job?

So, if our eyes have only receptors for Red, Green, and Blue, electronic imaging devices do not need to capture the whole spectrum of light, but only those same parts of the spectrum that ours eyes are sensitive in.

Dolby 3D, or, Infitec, which Dolby licensed, makes clever use of that perceptual aspect of our vision.

They create very THIN bands of RGB, and use very narrow filters, so that a separate RGB triple reaches each eye. Both eyes see full color, but one triple is shifted against the other. You can see that shift on the glasses coating. Unlike anaglyph glasses, they do not filter ONE color of light, but three very narrow bands slightly apart.

It will create a slightly different color balance for both eyes, but the server/projector playing back the 3D movie will electronically compensate for this slight shift in color balance.

The projector has a colour wheel rotating in front of the light source, more or less with the same filter characteristics as the filters in the glasses. So the projector will show one RGB triple to the left eye, the other RGB triple to the right eye, in a very fast sequence (typically 144Hz). It will appear to most people that both images are perceived at the same time.

Watch this Infitec video:

http://www.infitec.net/index.php/en/ueber-uns/infitec

No polarization, no silver screen. But the filters cut out, or better let through, a very narrow part of the light spectrum that the Xenon lamp emits. This reduction to 3/6 narrow bands throws away a lot of light that would otherwise brighten the image. So, Dolby 3D has the highest light loss of all 3D systems - nearly 90% is wasted/cut out in the filters.

- Carsten

Posted by Jim Cassedy (Member # 4115) on 01-28-2013, 08:26 PM:

Carsten gives an excellent explanation.

If you want a little more 'in depth' technical info, check out
the Wikipedia page on Infitec: Infitec Wiki

It gives the exact optical filter wavelengths used and the
links at the end of the article can take you further, if
you're interested.

Posted by Mitchell Dvoskin (Member # 751) on 01-29-2013, 09:21 AM:

Thank you very much for the info.