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Let's begin by observing that both traditional screen measurements are made by pointing the light metering device at screen center and only at screen center. Everybody's published gain charts (including ours) provide no direct information about how bright other parts of the screen are as seen from any viewing angle. All they tell us is how bright a screen's center will appear from viewing angles between 0ø and something like 50 degrees. Back in the days of multi-image slide projection it was a generally safe bet to assume that the "area of principal interest" in a typical image was going to be located at its center. Today, however, when our customers are looking at video projected spread sheets (for example), neither they nor we can be sure if the most important data won't be in the cell way out at one of the screen's corners. And that can be a problem. Is it technically possible for a projection screen to have a perfectly uniform coating? Certainly. Does such uniformity guarantee an equally uniform display? Certainly not. The most uniform screen we make is our front projection Matte White. Theoretically a matte white surface is "a perfect diffuser." That is, the observed brightness of such a screen doesn't change with viewing angle. Thus the eye perceives as much brightness at the center of the screen as it sees at the corners which in turn is the same amount of brightness visible from any angle of view. Even though we come pretty close to this perfect uniformity (our gain is 1.1, for example, and not the theoretically required 1.0), the chance of finding one of our screens exhibiting completely even light distribution in the field is surprisingly small. Why? Three-gun video projectors have a serious limitation in that the light emitted from each of their CRTs, one Red, one Green, and one Blue (RGB), is decidedly non-uniform. When we measure the brightness at the center of one of these lenses and then compare it with a reading from an edge, the ratio is not 1:1, but 10:3. The effect of this disparity is that the cone of light shining out of one of these projectors is 70% dimmer at its edge than at its center. The human eye is normally tolerant of brightness differentials which are less than 50%, but is sensitive to larger discrepancies. It is ironic that screens are often blamed for the resultant "hot-spot" when in fact even a matte white can't disguise brightness discontinuities as gross as 70%. What happens when we use a screen that is not matte white and which has a gain greater than 1.1? Gain, we must always remember, does not imply amplification. No screen can add power to the display. All available brightness is created by the projector and only the projector. So when a gain screen exhibits increased brightness at its center, it is certain that it has robbed that extra energy from somewhere else. With all diffusion screens therefore the higher the gain, the lower the uniformity. This is the principal reason to recommend low gain screens whenever possible. If the diffusion is on a rear projection screen does it make any difference? Not really. Diffusion coatings always scatter light rays equally about their angle of incidence and this is true whether the coating is reflective (on a front screen) or transmissive (on a rear). Our Video Vision, for example, is an extremely efficient diffuser whose uniformity can be degraded only by projector limitations. When the diffusion coating contains more than a diffuser, however, uniformity problems particular to rear screens begin to arise. When darkening pigments are added to our rear screens we are improving two important attributes. 1) The darker hue significantly increases image contrast. 2) The pigment serves to absorb (and therefore not reflect) ambient light incident to the screen's front surface. This second virtue is all very well except that the pigment also absorbs light emanating from the projector and thus the overall transmission of the screen is reduced. Transmission should not be confused with gain. The latter is controlled by the diffusion and governs the degree to which light from the projector is scattered. The former is reduced by the quantity of darkening pigment and governs the total amount of light that gets through the screen. Obviously the balance between diffusion and pigmentation is a delicate one. We are fortunate to offer an exceptionally wide selection of Polacoat diffusion screens which range from Video Vision (which has no darkening pigment) to the DA-WA N HC (which has lots). Careful attention to our customers' needs is essential to ensure that they make the optimal screen selection. In addition to diffusion coatings we also manufacture profiled screens which are comprised of lenticulations and/or Fresnels. What effect can they have on the uniformity of a display? Lenticulations have no influence on uniformity. Although they are lenses, their only function is to scatter light about its angle of incidence. The difference, of course, is that lenticulations restrict their dispersion to the horizontal axis only. This results in excellent horizontal viewing angles but does not result in reducing brightness discrepancies between an image's center and its corners. There is only one screen element that can improve uniformity and that is a Fresnel lens. Of the billions of light rays that come out of a projector at any instant, let's look at the paths of just three. First there is the centermost ray, the one that's going to pass exactly through the middle of a screen. Call this the On-axis ray. Then there's the outermost light ray on the right. Let's call that one the East ray. And finally there's the outermost ray to the left, which we'll call the West ray. We remember from above that both the East & West rays start off life a lot less bright than the On-axis ray and now, when we consider the angular direction of their paths, we see that they are aimed far away from the direction of the On-axis ray. Therefore as we viewers sit in front of this projection beam, it will be particularly difficult for us to detect much brightness at all from these East & West rays because they aren't aimed anywhere near our eyes. The angles through which those outer light rays would have to be bent in order to reach our eyes are called Bend Angles. What a Fresnel lens does is reduce these bend angles so that each of the light rays emitted by the projector is bent back just enough so that its direction becomes parallel with the On-axis ray. We can see that at the center of the projection beam the Fresnel is not doing very much work. But by the time we move out to the edges of the beam the Fresnel is bending the rays through ever larger angles until we get right out to the East & West rays where the bend angle is maximal. Notice that our Fresnel has its greatest effect at the very places we need it most: at the extremities of the image. Many of us used to assume that a Fresnel was primarily used to increase screen gain. Although it does do that, it's no longer very important (high brightness projectors are now routinely available). But by making the corners and edges of an image less dim, a Fresnel significantly reduces the brightness fall off from the center and thereby serves to increase overall uniformity. The process by which divergent light rays from the projector are bent so that they are all parallel is called collimation. No other rear screen function is more important to the critical question of image uniformity. Another recognition of the importance of uniformity in a display is the new way in which many of the projector manufacturers are quantifying the brightness output of their products. When a projector manufacturer used to assert that his device was rated at 800 lumens "peak white," that meant that he could get a meter to read 800 lumens at a zero angle of view when he drove the projector flat out and in a way that was useless for displaying acceptable images. Furthermore, that specification said nothing whatsoever about how many lumens were available elsewhere across the field. We could be sure, however, that it was a lot less than 800; maybe 70% less. Just like the screen manufacturers, the projector people took their maximum reading at the center and conveniently ignored everywhere else. That is, until they created ANSI lumens. In 1992 the American National Standards Institute (ANSI) helped establish and promulgate a series of measurement specifications which were intended to evaluate "the actual viewable image which emanates from large screen display devices." At last people were judging the image, not just the projector or just the screen in isolation. It is the two in combination which make up the display. Prominent among the standards which evolved from the ANSI effort is a new way of measuring brightness. The display is now divided into nine rectangles each of which measures of screen Height by of screen Width. A brightness reading is taken at the center of each rectangle and then "the average of the nine readings in lux (lux = lumen/square meter) shall be multiplied by the number of square meters of the image at the plane of the meter reading. The result is the light output specification of the projector in lumens." The choice of units simply ensures international comprehension; the choice of method represents a momentous change in the way all of us in this industry think about displays. Internal to a brightness specification in what have come to be called ANSI lumens is a clear recognition of the vital importance of uniformity available from some candidate projector. Particularly welcome to our industry are the newly refined light valve and liquid crystal light valve projectors. In addition to their exceptional brightness, these machines are able to provide uniformity across their fields which may vary from center to edge by as little as 20% - a vast improvement over previous display technologies. When one of these projectors is rated at 2500 ANSI lumens we can be confident that overall brightness will be high and that any discontinuities across the image will be minimal. The emphasis on image uniformity in the display industry is certain to increase. Computer and video projector manufacturers will go on pushing as hard as they can to boost bandwidth and resolution. Our own on-going attention to the uniformity available from our screens and our understanding of the factors which create it remain essential components of our job as salespeople and as manufacturers.
M. K. Milliken, Jr.
CONTINUE: ANGLES OF VIEW |