Angles Of View
Dr. Richard Burrows is the Product Development Manager for Da-Lite's Polacoat Division, located in Blue Ash, Ohio. He was graduated from Muskingum College in 1988 and received his Ph.D. in Physical Chemistry from the University of Cincinnati in 1993. Dr. Burrows can be reached at firstname.lastname@example.org or through Da-Lite's website: http://www.da-lite.com. He is interviewed here on the subject of
Screening the Future - Da-Lite at the HorizonsDa-Lite: What were some of your first impressions of the technologies used to create projection screens?
Burrows: When I interviewed for the position I was privately wondering, do they really need a chemist? But then, when I saw the processes and grew familiar with how screens are made, I wondered why we don't have a larger staff of chemists. The chemistry that's going on here is wonderful chemistry and it is a particularly wonderful application of polymer chemistry.
Da-Lite: What do you mean by "Polymer" chemistry?
Burrows: If you stop to think about it, all projection screens are made up of two parts, a substrate and a surface coating. The coating is the part that does the optical work. The substrate is the part that provides the coating with mechanical stability. The science is to get the coating to lie flat on the various substrates and to get it to adhere. Polymers are typically carbon based molecules which can be formed into long chains. The chain structure enables great flexibility within the coating while at the same time fostering its ability to stick to a substrate.
Da-Lite: Is all of this chemistry pretty well finished business? That is, have all the questions been answered?
Burrows: By no means. I am continually researching ways to make our products flatter, harder, stronger, more flexible, and/or more durable.
Da-Lite: Those of course are all mechanical properties descriptive of substrates. Are there instances where alterations of the substrates can have optical significance?
Burrows: Absolutely. When the material in question is part of a rigid rear projection screen, the substrate itself has to be optically active. It needs not only to be translucent but as transparent as possible.
Da-Lite: Doesn't any clear material, glass or acrylic, accomplish that?
Burrows: Yes; but not with equal efficiency. Acrylic has a measurably higher optical throughput than glass in the visible spectrum.
Da-Lite: Does that mean that acrylic substrates are to be preferred to glass ones?
Burrows: You might have been able to say that in the past, but I've been working just recently on ways to increase the transmission of our Da-Glas. The results, while still a bit premature, are very encouraging. Stay tuned.
At the moment, however, a higher percentage of the light emanating from a projector will get through a Da-Plex screen than will pass through Da-Glas - even when the two screens have identical gains.
Da-Lite: Please explain the difference between Gain and Transmission.
Burrows: Screen gain is the ratio of a measured amount of on-axis light reaching the back of the screen to a measured amount of light radiating from the front. The angle from which both measurements are made should be the same and is usually 0o.
Transmission is a measurement of the total amount of light available from the projector compared with the total amount of light radiated by the screen. Transmission is generally given as a percentage and is reduced by phenomena such as back surface reflectivity, internal reflection, and absorption.
Da-Lite: Obviously you can vary the gain of a screen by altering the density of its diffusion. Can you also vary the transmission?
Burrows: Yes; the purpose of diffusion is to scatter light, not to absorb it. Often rear projection coatings contain colorants which serve to darken their overall hue. The "N" suffix in our Polacoat line, for example, stands for "Neutral Gray" which is a highly desirable color for a rear projection screen because the precise "Gray" we've developed significantly improves image contrast without altering the overall color balance between the Red, Green, and Blue portions of the image.
Da-Lite: But the gray colorant absorbs some light, doesn't it?
Burrows: Yes it does. And that's convenient if the light absorbed is from ambient sources, but less convenient if it is light from the projector. But, as it's not possible to have one without the other, transmission is reduced. The consequence of lowered transmission is that the screen's half-angle will be diminished, not its gain.
For example, our highest transmission screen is Video Vision which, in addition to containing no colorant, is a coating specially formulated to be highly non-absorptive to light rays. Therefore, since the resultant transmission is higher, there is more light available to be scattered over larger angles of view. This is what makes this screen so exceptionally uniform. Its molecular composition also enables nearly perfect chromatic fidelity - but details of that process are proprietary.
Da-Lite: What about front projection screens? Does your work include their consideration?
Burrows: Of course. Because front projection screens are not rigid, all sorts of chemical things need to be manipulated so that the mechanics of their substrates behave suitably. There are requirements that the screen not chip, fade, peel or crack, for instance.
Da-Lite: And what about the optical layer, the front projection screen's front surface?
Burrows: I'd like to answer that question this way: Of all the elements which make up a contemporary display system - computers, switchers, interfaces, projectors, lenses and screens - there is only one which performs anywhere near its theoretical efficiency. That one, of course, is the projection screen. The energy losses incurred at every other stage of the display are enormous. Yet here we have in this simple, unassuming flat surface coated with ordinary Matte White, or Pearlescent, or Video Spectra, an optical device which is capable of reradiating at least 98% of the energy it receives. I think that's quite remarkable. I also think finding ways to reduce that remaining 2% is very difficult. But we're trying.
Da-Lite: What else are you currently working on?
Burrows: We're making a screen for a laser system which needs a diffuser with an especially small particle size, in this case about 5m . That's way smaller than the human eye can resolve but it's not too small to be machine readable. In my opinion such specialty screens will become more and more prevalent as data capture technologies advance.
Another project which is particularly interesting is a rear projection domed screen which has to preserve polarization.
Da-Lite: Is this for some sort of 3-D application?
Burrows: Very probably. Polarization is relatively easy to preserve with a front projection screen: you just make sure the coating is metallized. To get a rear projection screen to be non-depolarizing is extremely difficult. As we're finding out, however, it is not impossible.
Da-Lite: Do you do a lot of work with curved screens?
Burrows: Yes; both simple (curved in just one direction) and compound (curved in two). Some of the screens we produce for customers in the aircraft simulation industry are quite large. They consist, for instance, of spherical sections which have radii greater than six feet and which extend horizontally for 220o. (That's more than 23 feet of arc length, by the way.) We have become quite expert in getting single pieces of acrylic formed into that shape and then coating them to very exacting tolerances.
Da-Lite: Do you coat on the outside or the inside of the curve?
Burrows: Either one, although the convex side is more usual. Obviously the chemistry of the diffusion for a curved surface needs to be different from flat screen diffusers. Getting the coating to apply uniformly across curved surfaces requires very careful formulation and a custom delivery system.
Da-Lite: Does your group do a lot of custom work?
Burrows: Indeed we do. The laboratory that I work in is extremely well equipped to develop and prepare a wide variety of coatings for virtually any display application. Sometimes the specifications we're given call for thousands of small screens to be included in a customer's OEM product and at other times we'll work equally hard developing special formulations for customers who maybe want only one or two big screens. Inquiries like those are what makes my job so fascinating. And it's always intriguing to see how imaginative some customers can be with applications for projection screens. One of our cardinal goals at Polacoat is to try never to say no to a customer.
Da-Lite: Does the work you do for your custom customers help you improve the regular product line?
Burrows: Of course; but the distinction between regular and custom is getting increasingly blurred. All of the tolerances we work with around here are getting tighter and tighter. A scant five years ago customers were perfectly comfortable with a gain tolerance of ą25%. Recently we've had to perfect ways of holding tolerances given by the second decimal place. And that's for our regular customers!
Advances in the collateral technologies, the computers and especially the projection devices compel us to upgrade our formulations continuously.
Da-Lite: Can you provide an example?
Burrows: Sure. Not so very long ago the best resolution you could find coming from a CRT was NTSC video. Today resolution of 1024 by 1280 is commonplace. Our job is to make sure that our screens can display that sort of detail with no degradation. That means that all of our coatings have got to be extremely smooth and uniform.
We are fortunate in this regard because the chemistry that we use to fashion our diffusers enables them to form molecular bonds with their substrates so that they cannot be peeled off. The only way to remove them is chemically. This means, among other things, that we can keep the thickness of our diffusion coatings to an absolute minimum.
Da-Lite: Why is that important?
Burrows: Because the thicker a coating is, the more chance it will have of degrading image resolution. And when projected displays are expected to match the resolution of work stations, it doesn't take much extra thickness to make a noticeable difference.
Da-Lite: Speaking of work stations and such, do you think that devices of that kind will ever supplant the projection screen?
Burrows: I think the future of visual displays splits into two paths: displays for small audiences and displays for large audiences. Wonderful technology is nearly upon us for displays which have an audience of one. Although the helmets and the other devices under development will perforce contain screens, they won't be projection screens as we currently define them.
But for large audience venues (and as projectors get brighter and more versatile there will be more and more of those) projection screens will remain the best solution. As such their future is sure to be bright.