ANSI Contrast And Your Room In Front Projection

When designing a home theater one of the questions that comes up is what color should I paint the walls.  In general you want the walls as non-reflective as possible.  The big question is how much will it really matter.  In general the answer is a great deal.

The primary thing the walls will do to the image is reflect light back on the screen and reduce the contrast and therefore the color saturation of the image.  In other words making it look more washed out than it should. The degree to which this happens will depend on how far the reflective surfaces are from the screen, how much they can reflect and how much light the screen casts onto the walls.  The impact on the image will depend on how great a difference there is between the dark and the bright parts of the image along with how bright the image is on average.  In other words a very bright image on average with some very dark elements is going to be impacted the most while very dim scenes will have little impact from room reflections.

I recently conducted some experiments in my own theater with this by hanging drop cloths over my dark walls and then measured the impact on the image with the standard ANSI checkerboard test pattern which contains alternating full white and black boxes.  Unfortunately, photographs demonstrating this do not work well because the contrast of the projected images that show these problems are much greater than my camera can capture.  The following are the results that I measured with my JVC RS20, Carada Cinema White screen and Photo Research PR-670 in a very darkly colored room with flat paint and dark furnishings.

Native Projector ANSI contrast: 235:1

On screen ANSI contrast will all dark brown and black surfaces: 150:1

On screen ANSI contrast with light ceiling and everything else dark: 83:1

On screen ANSI contrast with light ceiling, side walls and everything else dark 44:1

On screen ANSI with light ceiling, side wall, back wall and furnishings: 41:1

As these numbers show the room itself is likely to be the limiting factor when it comes to inter image contrast and bright scenes.  Most front projectors today deliver ANSI contrast greater than 100:1, but it will take great care to even reach that level much less the 500:1 or greater provided by some products.  This test confirms what I have seen and measured many times.  The room itself is extremely important when it comes to delivering an image with good contrast on bright scenes and few rooms do not limit the projector’s performance in this area.

W. Jeff Meier

ISF and THX Certified Home Theater Consultant

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Cory

Hi Jeff,
Wondering if you have any different screen material to test this with…I’ve specifically been wondering if a retro-reflective screen (like dalite high power) would help retain ANSI contrast by limiting the amout of light that bouces onto the ceiling, side walls, etc. It would be much easier to talk the family into painting the back wall than the entire theater!

W. Jeff Meier

Cory,

To do this test with another screen material would require a whole screen which I do not have. Screen gain will help with side reflections, but it is less helpful with the wall behind the projector. The closer and more reflective this wall is the greater the problem.

Using a retroreflector adds another problem in that some projectors have much higher ANSI contrast with lens shift than without. One of these are the JVC DILA products.

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