• Mikes Tips - Contrast Enhancing (front) Projection Screens

    MyCockpit ģ Presents "Mikes Tips" by Mike Powell

    Contrast Enhancing (front) Projection Screens

    Contrast has a profound effect on both image quality and oneís sense of subjective wellbeing. Poor contrast can ruin an otherwise great image; and, particularly after dropping major coinage for a new projector, it can convert a pleasant upbeat frame of mind into a dark pit of despair. In such circumstances we can turn to contrast enhancing projection screens. Sometimes they work. Sometimes they donít.

    Hereís why.
    Contrast can be defined as the ratio of the brightest luminance in an image to the darkest: Lmax/Lmin. It has also been defined as the ratio between the largest change in luminance to the darkest luminance level: (Lmax-Lmin)/Lmin. Both definitions will get us to the same destination, but the first makes the arithmetic easier, so weíll stick with it. Good contrast starts with the projector. We wonít see more contrast than the projector can produce, but thatís rarely a problem given the performance of contemporary projectors. Projector contrast specifications are reported in several ways. For our purposes here, weíre interested in static contrast, the range of brightness a projector is capable of producing simultaneously within a static image. A static contrast ratio of 1000:1 is typical of newer, consumer video projectors. Of course, we donít view the image directly from the projector. We view it on a screen. To understand potential contrast limitations, we need to understand a bit about screens and image brightness. The industry standard reference screen reflects 100% of the light projected onto it and distributes the light so that an image appears to have the same brightness regardless of the viewing angle. No real screen performs this way, though some screens come close. When we talk about unity gain screens this is what weíre referring to. A screen can have a gain greater than one by trading off viewing angle for brightness. A high gain screen is brighter when viewed from the sweet spot and significantly dimmer from a large viewing angle. And of course, a screen can have a gain less than one. A low-gain screen has a grey appearance and produces images which is less bright, and when used properly can increase image contrast.
    The general formula for screen brightness is:

    Brightness in foot-Lamberts = (screen gain) * (light power in lumens) / (screen area in square feet).

    The contrast of a projected image is best measured using a special test pattern. The ANSI standard contrast test pattern is a four by four checkerboard scaled to the resolution and format of the projector being tested. The black areas are displayed at zero intensity while the white areas get full brightness. Dividing the average brightness of the bright blocks by that of the dark blocks yields the contrast. Weíll use the same conceptual approach.

    Imagine filling a 6í by 8í unity-gain screen with this pattern using a projector with a 1000 lumen output and a 1000:1 static contrast ratio. Half of the screenís 48 square feet will be illuminated by 500 lumens while the other half will receive 1/1000th of the remaining 500 lumens. If we were to do the arithmetic at this point, weíd calculate an image contrast of 1000:1.

    And sadly we would be wrong.

    Light directly from the projector is never the only source of light hitting the screen.

    A realistic calculation of image contrast must include all light hitting the screen regardless of source. As a first step toward realism, letís add a little fixed room light. Letís add half a lumen per square foot. This isnít much; itís a dim twilight level. The 24 square feet of bright blocks now receive 12 additional lumens for a total of 512 lumens. The dark blocks also receive an additional 12 lumens, bringing that total to 12.5 lumens. Image contrast has dropped to 41:1.

    This is the type of situation that would cause one to consider using a contrast enhancing screen. A contrast enhancing (front projection) screen has a gain usually between .7 and .8. No magic. Itís simply a neutral grey screen that reflects less light. Plugging a gain value of .7 into the above example gives a disappointing result: thereís no change in image contrast.

    However, we canít say there is no change in the image. Because of the lower screen gain, the image is dimmer. Even though the image contrast is the same, the viewing is slightly worse because the image does not stand out quite as brightly against the roomís twilight lighting.

    Clearly the contrast enhancing screen isnít a panacea.

    Another source of contrast woes is light from the screen being reflected back onto the screen by the room and its contents. Some 15~20% of light is reflected by a typically decorated room. A white ceiling reflects (much) more, while a dark carpet reflects less. Weíll use 15% for this example.

    If we had a perfect unity-gain screen, all of the light projected onto it would be reflected into the room. If we project the contrast test pattern, thatís a total of 500.5 lumens. Fifteen percent of that would be sent back toward the screen. How much will hit the screen depends on the room dimensions. If the room is twelve feet wide and eight feet tall, it has a cross section of 96 square feet. We can expect the bright blocks of the contrast test pattern, which occupy 24 square feet, to intercept a quarter of the light. The dark blocks will intercept another quarter. The remainder will bypass the screen entirely.

    Plugging the numbers in and turning the crank results in a 27:1 image contrast ratio disaster.

    If we were to use a contrast enhancing screen with a gain of .7, we would see an image contrast of 38:1.

    A 38:1 contrast ratio is not all that great, but itís a substantial improvement over 27:1. It also serves to demonstrate the usefulness of low gain screens for enhancing image contrast.

    Image contrast in a realistic projection system will be reduced by stray light from a number of sources. Your first steps in enhancing contrast should be to control that stray light. Make the room light tight. Reduce fixed light sources within the room. Shield the screen surface from any necessary lights like sim panel and cabin lights. Reduce room reflectance as much as practical or acceptable. If image contrast is still too low, consider using a low-gain contrast-enhancing screen only if the room reflectance is the limiting factor to contrast.

    Mike Powell, author of

    Building Recreational Flight Simulators and

    Building Simulated Aircraft Instrumentation.