Angles Of View
Not so very long ago, gain was the only specification anybody wanted to know about a diffusion screen. As the display world has become more sophisticated and quite properly more demanding, however, other criteria informing projection screens have come to be appreciated. And, outward appearances perhaps to the contrary, some diffusion screens have been significantly both improved and upgraded. To see how, we ' ll need to give
Diffusion Screens - A Closer LookFirst off, it might be worth reviewing exactly how a diffusion screen works. That is, just how does a diffusion screen form an image and just how does it thereby enable us to see a picture?
From any given viewing position we are only going to be able to see an entire projected image if (and only if) at least some light from every single portion of the screen reaches our eyes. Thus, we may think of the screen (the "Big Picture" ) as actually being made up of many, many little screens each of which is dutifully spreading its mini portion of the projected light throughout the entire viewing area.
If light from any one of these "micro-screens" failed to reach our eyes, then, of course, what we would "see" is a dark spot from which no information could be derived [perceived].
But now let's turn the magnification up a notch or two farther and look to see what's actually making up the "micro-screens." Because, sooner or later, we've got to get down to some lowest common denominator which is the level at which something is actually being done optically with light rays. When we examine a screen microscopically, we do indeed see that its surface is made up of an array of tiny particles which, one way or the other, must be the entities which make the whole thing work.
If the screen is a front projection surface, its particles will principally be made of miniature chunks of a white, non-absorptive, chalk-like material, generally magnesium carbonate (MgC03). Like a huge expanse of irregularly shaped pebbles on a beach, these particles scatter light rays falling in them in every conceivable direction. If there are no other sorts of pebbles on the beach, the screen will have a Matte White surface.
If, here and there among the pebbles, however, we litter the beach with shards of small mirrors, some of the light rays falling on the beach will not be scattered by the pebbles and will instead be reflected back by the mirrors in specific directions. When this happens, the screen begins to have gain. And the more bits of mirror we strew among the pebbles, the more gain (directionality) the screen will have.
Rear projection diffusion screens also rely on particles for their optical action, but not, of course, reflective ones. On the microscopic rear projection beach the pebbles have to be transmissive. That is, light rays have to be able to pass through them and thus each acts as a refracting lens, bending light rays incident to its back surfaces so that they exit from its front at a large variety of new and near random angles.
In both types of diffusion screens, then, it is the jumble of particles deposited on one surface which disperses the projected light throughout the viewing area. Now, a possibly interesting question might be, what would happen were we to vary the size of those particles making them either significantly larger or smaller?
As the average diameter of the particles in enlarged, the image they are collectively displaying will begin to exhibit an additional characteristic which, quite properly, will be called graininess. As the coarseness of this grain is increased, the diameter of its constituent particles will start to become some meaningful percentage of the magnitude of a pixel and, when that happens, the resolution of the overall image quickly collapses.
To see how this is so, we need to remember that each particle is redistributing light in some direction that is random compared to a neighboring particle but is quite specific relative to itself. If the particle has become too large to have lots and lots of closely packed neighbors, light rays reaching its part of the screen will be processed only by a few particles and not by many.
When that event, in fact, happens, the continuity of the diffusing effect is (literally) coarsened and the dissemination of the information contained by the light rays becomes similarly non-continuous. Note that as the average particle size is increased, the spaces between the particles must also become larger. These spaces, of course, can =t refract light with anything like the same efficiency as the particles and thus they can become (literally) holes in the data.
Now let's ask what happens when we make the particles smaller? Obviously the graininess we observed through large particles will diminish as the "fineness" of our diffusion is increased.
But, since we're thinking about particles which get measured in microns (1ľ = .00003973"), even the large ones can't be all that big. So when we make them smaller, can we tell the difference between a 4ľ screen and a 10ľ screen? Let's see.
Figures 1 and 2 are representations of two carefully made images. Figures 1 and 2 are not, however, the images themselves. They are printed illustrations of a single, United States Air Force target slide which was placed directly against the diffusion surface of two different diffusion screens and photographed. The position of the camera and the projector were identical and fixed for each shot. The only light source in the laboratory was the projector.
After the high contrast copy film was developed, its negatives were scanned into a computer at a resolution of 300 dots per inch. Those images were then copied by the computer used to write this article and printed by an ordinary laser printer. The size of the two reproductions is nearly, but not exactly, identical to the original target slide which is precisely three inches square.
It is essential, therefore, that the reader appreciate that much of the differences clearly apparent to observers in the laboratory have been obscured by the numerous iterations the data have endured prior to appearing here. That disclaimer aside, however, there are, even here, some clearly discernable differences between the two figures.
The diffusion surface used to make Figure 1 is a commonly available rear projection screen made by a prominent manufacturer in our industry which has a specified gain of 1.8.
Figure 2 was made with Da-Lite's new, DA-180 high resolution coating which also has an on-axis gain of 1.8. The average particle size of the Da-Lite screen is very much smaller and thus, for instance, the characters "Edmund Scientific" are very much more defined in the second figure than they are in the first.
Unfortunately the resolution available to this document is not adequate to display the series of increasingly fine line pairs located at the center of each slide. Here is where, of course, the high resolution standard is properly to be tested. On actual screens the differences between the two surfaces are instantly apparent. Here they can only been simulated. (And, if you are reading from the web, the resolution you get from the Internet is even worse, maybe only 72dpi.)
In a world increasingly dedicated to the display of high resolution data and graphics, the impact that screen surfaces can have upon the perceptibility of the projected information can no longer be safely ignored.
One diffusion is in fact no longer equal to another.