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Cinema Screens & Motorised Curtains: Reflecting Brilliance
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Think back to when you were in school and your teacher brought in the 16mm projector for an instructional movie or perhaps when your family had friends over and Dad pulled out the old 8mm projector to show movies of your last vacation. Depending on the time frame in which this occurred, it is a pretty good bet that the screens used in those situations were made with a glass beaded surface on the front. one of the reasons that a glass beaded surface was the popular choice back then was because the projectors were not very bright and, as such, we wanted to get as much light directed back to the viewing audience as possible.




Also, keep in mind that we normally needed to view these images with the lights off and the curtains drawn in order to observe an acceptable image. So, along comes the CRT projector. They had a bit more brightness but because of uniformity issues inherent in a CRT projector, we used screens that were a bit lower in gain and had other means of creating reflectivity. Here too, we had to have the room dark in order to view an acceptable image.

Then in the early 90s came the first LCD and DLP projectors. These were different in not only the brightness output and uniformity; they were a great deal brighter and much more uniform, but also in the way in which their image was presented. Before that time, nearly every projector on the market created its image by either an analog scan line or through changing the film piece in the projectors gate. When these new digital projectors came along, we in the screen business were challenged to make a few changes to our products. What had occurred was that we were now faced with images that were made up of thousands of tiny little pixels with small gaps between them. In addition, due to the brightness of these projectors came the desire of many technology users to place them in applications where the room was not completely dark. So as this occurred the demand on fabrics that had a moderate amount of gain to them and at the same time did not create resolution issues with this new pixel structure became greater. Where we were once only concerned about creating really bright images, we were now faced with making sure that the screen did not create resolution problems.

Okay, so fast forward to the present and the ever increasing resolutions of todays high output projectors. Are we facing a similar situation that we did when the LCD and DLP projectors were first introduced? For some, the answer is yes, while for others it is no. In order to determine if there is a potential problem with a given front projection screen material, let us look at the size of the particles used to create gains higher than 1.0. However, before we look at particle sizes we need to have an understanding of how these particle sizes are measured. In this case, they are measured in what is called microns. A micron is the abbreviation for micrometer and is a unit of measure that is equal to 1 millionth of a meter, or 1 thousandth of a millimeter. It can also be expressed as 0.001mm, 1 or sometimes 1 m. For reference purposes, a human hair is 100 microns in diameter, a human cell is typically several microns across and on a DVD, the track pitch is 0.74 microns and the pits are 0.4 microns wide. So, as you can see, these are very tiny measurements. Regardless, they are very important to the overall performance of a projection screen.

Now that we have an understanding of the measurements, let us begin to look at different screen materials. Traditional glass beaded materials, like the one your father used, has a typical particle size measuring 65.

So what does this mean? To answer that question, let us go back to the issue of pixel size for a given screen size and determine how many particles will be in each pixel. We would naturally assume that the smaller the particles, the higher the concentration per pixel, hence the better the image will appear. Let us use our 45" x 80" screen size again with a 1080p projector from our favorite manufacturer. Given our pixel structure is 1080h x 1920w; we can determine that each pixel will be 0.0417" in height by 0.0417" in width (only slightly larger than 1/32" x 1/32"). Therefore, each pixel has a surface area of 0.0017 in. Since our measurements for screen size are represented here in inches, let us convert our particle size from microns to inches as well. From the conversion we find that 1 micron = 0.0000394 inches (a very smallish number). After determining that, we can say that if a particle is 15 that it will consume 0.000591 inches of space. In this case, if we were to place these particles side by side in an orderly fashion, we can fit more than 4900 of them in one single pixel. Doing the same math for a screen surface such as the traditional glass beaded material reveals the disparity between the two types of surfaces and yields only 264 optical particles per pixel. The improvement, therefore, is increased by a factor of more than 18. That is one of the reasons why traditional glass beaded surfaces are not a good choice for todays high resolution projectors.

So, what does all of this tell us? Plain and simple, the particle size of the materials used to make the screens reflective surface can cause problems with resolution if those materials are not small enough. However, even more important to us in todays applications is the issue of scintillation. Have you ever looked at a screen with either a moderate or high gain and thought you saw a bad pixel or a tiny bright spot on the screen? There can be a couple of different types of phenomena causing this. In order to better explain why that occurs, let us examine more closely how these screens work. In order to increase the gain of a screen, we must introduce some type of material that either refracts or reflects the light that is incident to the front surface.
The materials that reflect the light are the ones we are most concerned about for this particular discussion. These types of screens have a diffusive base with platelets of mica strewn across its surface in a regular fashion. These crystals are also coated with Titanium Dioxide (Ti02), which then makes them highly reflective and behave like thousands of tiny little mirrors. We also learned through this article that these materials reflect light incident to their screen surface in a fashion that is equal but opposite the angle of incidence. Keeping that in mind, if one of those particles land on the screen surface at a very severe angle, this is one potential cause of a bright spot or sparkle, depending on your viewing position.
The second potential issue is if the particles are too large and do not allow at least a portion of the light to strike the diffusive surface behind the reflective particles. This too can be a source of a bright spot or scintillation.
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