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Understanding Contrast Ratios in Video Display Devices

Part II

May, 2006

Darin Perrigo


Why Are Contrast Ratios Important?

One reason contrast ratios are important is because separation between intensity levels in an image help us differentiate things. That separation can also be a factor in how 3-Dimensional the images look. When lights in a room are turned on and the image is washed out to the point that it is hard to tell what is happening on the screen, this is basically because the simultaneous CRs off the screen have dropped very low with the extra light added. On/Off CR is important both because it indicates the absolute black level for a particular white level and so gives a good idea of how realistically the projector can produce blackout conditions, and because On/Off CR is usually the limiting factor in maintaining good simultaneous CR in the darkest non-blackout images. ANSI CR is important because it is often the limiting factor to simultaneous CR in brighter mixed scenes and it also has some relevance to visibility of shadow detail in mixed scenes. We can experience both blackouts and good simultaneous CR in real life, so in a way these determine how realistically the display can provide those things.

I should mention that while I am discussing that a higher CR is desirable, a person can rightfully prefer less CR for whatever reason, just like they can prefer that colors be different than standards call for, prefer much brighter or dimmer images than the standards call for, watch with lots of lights on in the room that change the images from the standards, etc. I see the advantages of higher CR, and the standards call for video black to be no light (or high CR), so that is what I will discuss. The bottom line is that if the projector is capable of a very high contrast ratio, you will have the option of seeing a better image, because if the movie scene has black shadows, you will see them as black, and not dark gray.

Contrast Ratios and the Room Environment

One thing that is nice about On/Off CR is that the colors of the walls have basically no effect on it, as long as there aren't lights on in the room, light leaking out of the projector where there shouldn't be light, or light coming into the room, and the only light is that going from the projector to the images. The reason is that the amount of light reflecting around the room and back onto the screen will be the same for the full screen 100%stim image as for the full screen 0%stim image. Even if 20% of the light came back, 1.2x divided by 1.2y is still the same as x divided by y. *1

Now, as to *1, I did simplify things a little bit here, as 100%stim images and 0%stim images can have different color balances, and so there can be minor differences in how reflections affect them (like if the image is stronger in red and the walls are red), but that is a complication beyond the level I want to discuss here and likely to fall into the margin of error for most people doing measurements.

ANSI CR is much more room-dependent. It is different than On/Off CR as far as the effect of reflections around the room, because reflections from white areas that go around the room do fall on the black areas, whereas with On/Off CR, the full white image and full black image are only raised by their own reflections. If 20% of the light comes back and splits between the white and the black rectangles, each will be raised by about 10% of the original level of the white rectangles. For a projector providing 100:1 ANSI CR, raising both the white rectangles and black rectangles by 10% of the white rectangle's intensity would result in the brighter rectangles being 110% of their original values, and the darker rectangles being 11% (from 10% + 1%) of the original intensity of the white rectangles, for an ANSI CR of 10:1 just considering the first reflections that return.

Even a projector with the best ANSI CR can have its contrast killed by a bad room, and the best room for ANSI CR retention (basically black velvet everywhere) can only do so much for a projector weak in ANSI CR. But ANSI CR can be helped by things like gray screens when the projector is bright enough to maintain a good white level with that screen. Gray screens basically help ANSI CR by reducing the effect of secondary reflections. For instance, ignoring directionality, a screen with 0.5 gain will kill half the light hitting it compared to a standard 1.0 gain white screen. That won't help the CR for the light going from the projector to the screen the first time, but only half the light will go toward the walls with the 0.5 gain screen and any reflections that come back will get reduced by 50% again, and so on. So, ignoring 3rd order and beyond effects, where the darker screen continues to help, this is similar to 0.5 gain for the initial light from the projector and 0.25 gain for the reflections coming back from the room, or a 2:1 ratio in favor of the light you generally want to see and against the light you want to reject. The result is less washout effect than with a standard 1.0 gain white screen where the initial light from the projector and the reflections both get 1.0 gain, or a 1:1 ratio.

Screens with directional layers can also help ANSI CR off the screen for a viewer in less than perfect rooms by giving more gain for the projected light for a viewer at a particular location compared to the average gain for the secondary reflections. For instance, a person sitting at a 2.0 gain position for a high gain screen should only get about 1.0 gain average at most for reflections around the room that come from random directions and then fall back on the screen.

I expect to see more advancements in screen design over the next couple of years to help ANSI CR retention in less than perfect rooms and both ANSI CR and On/Off CR retention with external lighting. In looking back at the contrast calculator located at http://home1.gte.net/res18h39/contrast.htm, which I mentioned earlier in this article, one thing that these gray and other screens designed for ANSI CR retention could be thought to do is to decrease the "room gain (reflectivity)" value that a person should use for that calculator, even with no changes to the walls or anything else in a room. The reason is that the screens help reduce the effects of those reflections, as mentioned above.

Go to Part III.

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