Here goes – my description of Dolby 3D Digital Cinema.
Buckle your seatbelts – this is going deep.
Wave Basics
Your eyes and your ears both pick up waves. Ears pick up sounds waves, which are compressions of air molecules, and your eyes pick up light waves, which are sort of like oscillations of magnetic fields.
Waves are described by a few basic factors… the ones that are important for our discussion are wavelength and frequency. In reality they describe the same thing just from different perspectives.
Wavelength is how long it takes for a wave to peak and valley. If you follow the wingtip of a bird flapping its wings, and the bird moves about 1 meter for each flap of its wings, then you say the wavelength is about 1 meter.
Frequency is the element of time. Using the above example, if it takes about 1 second per flap, you’d say the frequency of the bird flapping its wings is about 1 time per second.
Assuming bird moves at the same speed no matter what (not true for birds but basically true for light waves and sound waves), then the faster a bird beats its wings, the shorter the distance between peaks and valleys. In other words, the higher the frequency, the lower the wavelength.
So just keep that in mind. For most waves, if you double the frequency, you half the wavelength.
Sound Waves and the ear
Think about the sounds your ear can hear. It can pretty much pickup any frequency of sound between certain ranges.
So your ear knows exactly the difference between a sound wave with a frequency of 500 Hertz (just a fancy way of saying there are 500 peaks and 500 valleys per second), and a frequency of 600 Hertz.
Your eye can’t do that.
Light waves and the eye
Your eye really sucks at figuring out what wavelength light is composed of. (But on the flipside, your eye can pinpoint where light is coming from much more accurately than your ear can pinpoint sound, and your eye creates enough information to generate images. Human ears cannot do that. Sound waves actually are good enough to create images – think of sonar and ultrasounds – but the human ear just isn’t good enough).
When it comes to figuring out the color of light, your eye relies on 3 types of cones. None of the cones have any ability to determine wavelength by itself.
What a cone does is this… if you blast with light of the exact right wavelength, it’ll signal strongly. If you blast it with the same amount of light but with a slightly different wavelength, it’ll signal less strongly. It’s almost like that rattle in your car that shows up at just the right speed –my old Honda Civic rattled really bad right around 78 mph. At 80mph, or 76mph, the rattling was a little less, and less than 70mph or more than 90mph, the rattling was gone entirely.
So if you hit your red cone with light at about 550 nanonmeter wavelength, it’ll signal pretty strongly. 540 nm or 560 nm light signals a little less strongly.
But if your brain doesn’t know if it’s seeing bright 550nm light… or it’s seeing not very bright 540nm light. That’s why the cones in your eye are limited. Your ear can tell the difference between 2 wavelengths perfectly – your eye cones cannot.
BTW 540nm is roughly yellow. How does your brain figure out that when the blue cone is mildly stimulate, that 540nm light is bright yellow, not weak red light?
Because there are 3 cones. Pure yellow light 540 makes both your Red and Green cones fire… 550nm makes your red cones fire more than your green cones. That’s how your brain knows. It figures out the ratio of stimulation between the 3 cones.
This is really interesting for a bunch of reasons. First, there are combinations of colors that you can’t tell apart. If I blast you with an equal combination of 540nm and 560nm light, your brain thinks it’s just looking at 550nm light.
The other thing is that your brain can theoretically see colors that do not exist in nature.
http://www.scientificamerican.com/article.cfm?id=seeing-forbidden-colors
How all this is relevant to 3d
Okay, now that we’ve slogged through the basic biology of the eye… how does it all relate to 3D?
We know that 3D is all about isolating different images for each eye. Most technologies alternate images and rely either on high speed shutters, or polarization, to separate a left and right video.
Dolby3D uses a different technique that’s fairly similar to polarization.
If you look really close at an old TV screen, you’ll Red, Green, and Blue areas, or RGB. By varying the intensity of the Red, Green, and Blue light, computer monitors can reproduce just about all the colors a human eye can see (though not all for a bunch of reasons).
What Dolby3D does is, for the left eye, it’ll give an RGB picture that’s some combination of 3 red, blue, and green that’s shifted just a touch up. I am making this up, but call it 441nm (blue), 531nm (green), and 561nm (Red). For the right eye, it’ll do the opposite, say, 439nm, 529nm, 559nm.
Put on some glasses that, for the left eye, only allow in 441nm/531nm/561nm, and for the right eye, allow in 439nm, 529nm, and 556nm. Voila. You have image separation.
As with RealD’s polarized solution, the frames still alternate between left and right. And the glasses are a little more expensive. But the movie screens can be plain old white canvases, and not the special silver screens that RealD needs (the silver preserves the polarization).
Long post. Thanks for slogging through. Post up any questions you have.
