Talk:DLP

From Academic Kids

Actually, red, blue and green are not primary colors. They are base colors for subtractive color mixing, but by definition, the primary colors are red, blue and yellow. -Anon

The anon has it the wrong way round. Theresa Knott (Not the skater) 16:23, 24 Oct 2004 (UTC)

Double Anon April 27th, 2005

Actually, neither is right. Red, green, and blue, (RGB) are the base colors for additive color mixing. Additive color mixing is commonly used when the light source is integral to the system (think television, computer monitor, LCD screen etc.).

Cyan, magenta, and yellow are the base colors for subtractive color mixing commonly used when an external light source is utilized (think paintings, printed documents etc.). It helps to think of Cyan, magenta, and yellow as being anti-red, anti-green, and anti-blue.

Yellow is often included in the list of primary colors due to a fluke in synthetic pigments. As described here: http://science.howstuffworks.com/light7.htm

I personally dislike the term 'primary colors' when it is used in reference to red, green, and blue in discussions of color science. It's use implies that color mixing is property inherit to the nature of light when in reality its root lies in the nature of human physiology and the nature of the human eye.

-DoubleAnon

The "Rainbow Effect" and three-DMD systems

The article makes the claim:

Three-chip projectors do not suffer from the "rainbow effect", since all three components are present at the same time.

While it's true that three-DMD devcies are far less prone to the "rainbow effect", it's not true that they are completely immune. Because the DMDs are (fundamentally) binary devices, they are operated as Pulse Width Modulators and the red, green, and blue beams are each individually turned on and off. This means that for certain colors, there are definite times when a (say pink) displayed object is illuminated solely by the red beam and at other times by all three beams. If your eyes are in motion, such an object could still be perceived as being striped in red and white, even in a three-DMD system. The reason the effect isn't seen as much in a 3-DMD system is that the pulse-width modulators operate a lot faster than the frame rate of the color wheel in a single-DMD system so there's less distance separating the colors when your eyes are in motion. Also, the produced color artifacts are (usually) a lot less distinguishable than the obvious red, green, and blue artifacts.

This isn't so important that I'm going to edit the article, but we should be aware of this.

Atlant 17:34, 28 Mar 2005 (UTC)

I've actually been to a DLP presentation by the head of Texas Instruments UK (who's personally quite heavily involved in DLP products) and they actually have a clever way of doing the modulation that means (in your example) the blue and green would modulate very very quickly while the red stays on, meaning the interval is so small that no one could possibly see it - unlike the color wheel problem, which is (or used to be) only just outside the bounds of normal human vision, hence some people can. --Dtcdthingy 20:29, 28 Mar 2005 (UTC)

You're correct that the DMD mirror isn't simply switched on and off once per frame for a single variable-width pulse, but the switching rate of a DMD mirror isn't all that high; it's only about 5 kHz tops ([1] (http://www.dlp.com/about_dlp/about_dlp_FAQs_technology.asp)) so a given mirror won't switch more than (say) 82 times per field for about 41 "pulses" of light. (The switching time isn't zero and there is a wear-out mechanism in the torsion beams that suspend the mirrors.) Your eye can sweep across an entire screen width in one field, so do the math and then tell me that you wouldn't be able to see the banding for certain well-chosen colors.

As I said, the effect is far less noticeable than with a single DMD device, but it's still there.

Atlant 01:05, 29 Mar 2005 (UTC)

Let's go with your numbers. The eye moves across the frame in 1/60th of a second, during which time 41 pulses of light are sent. The other number we need is the "shutter speed" of the eye - let's assume it's also 1/60th of a second. What you'd get is 41 images printed on top of each other on the retina during the exposure, equivalent to taking a photo, moving the film 1/41st of a frame, and repeating 40 times. If we imagine that film, the motion blur from the movement of the film would appear in 41 separate steps rather than being completely smooth. I really don't think the brain would be able to tell the difference, to be honest. --Dtcdthingy 02:01, 29 Mar 2005 (UTC)

(For ease of discussion, let's assume there's a single line of 50%-saturated pink displayed from top to bottom on an otherwise-black field.)

You don't need the "shutter speed" of the eye at all. If your eye sweeps left or right across the field in 1/60 of a second, then what you will see (thanks to persistence of vision and the like) is a screen that has 82 stripes. Half of the stripes will be white and half of the stripes will be red.

If you don't think you'll be able to see this (a pattern of 82 white-and-red stripes occupying the full field), then perhaps you need to get your eyeglass presrciption checked. :-)

The point is that your eye, in moving, "breaks up" what would otherwise be perceived (through persistence of vision and the like) as a static object. Again, the effect is nowhere as obvious as with a single-DMD system, but it is still there. Why don't you call your TI guy and ask him to contribute to our discussion?

Atlant 13:35, 29 Mar 2005 (UTC)