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WP: Properties and Performance of Rear Screen Materials
Stewart AeroGlas 100 vs. Da Lite DA 100 WA
One area of the audio visual industry that requires further study involves published specifications of products and how these specifications relate to the products as delivered and their actual performance in systems integration. The two areas of concern in our current studies involves video projectors and projection screens. For this analysis we began by purchasing samples of 10 of the most popular screens on the market and performed a technical evaluation of each one in terms of materials, construction, specifications, performance as delivered, gain, half gain, contrast, and color uniformity. We began by taking two of those screens, the Stewart Aeroglas 100 and the Da Lite DA 100 to see how independently purchased samples would perform versus their published specifications.
All screen companies that use large acrylic substrates purchase them from a single source supplier who is one of the few manufacturers of acrylic sheets that possess the required dimensions and image performance characteristics. The acrylic sheets from this supplier often contain a number of waves and visible artifacts that stem from the manufacturing process. These undesirable visual characteristics present in the acrylic can be remedied through a heat treatment process, but this treatment adds additional production time and cost, to an acrylic screen. Also, this heat treatment process does significantly change the structure of the acrylic, causing it to no longer be able to support its own weight or maintain flatness. Think in terms of steel that is not heat treated properly. To correct for this, large acrylic screens must be installed into a specific frame structure that supports the weight and allows the screen to be hung. An additional frame system is required to correct for screen flexing and keep the screen flat. These additional steps are not required with a glass substrate, as the clear float optical grade glass that is used does not contain any optical aberrations or waves, and the more rigid structure of the glass can support itself, requiring less mounting hardware.
As we know, we cannot break the laws of physics and acrylic by its very nature is hydroscopic, or water absorbing. It will, over time, absorb moisture from the surrounding air, even in a dry climate. This will produce the undesirable effect of visible changes to the screen properties, caused by noticeable warping or sagging of the acrylic material. Acrylic also exhibits a higher temperature sensitivity than glass. Acrylic demonstrates a much larger expansion coefficient than glass, approximately 8 times the level of expansion. For example, a 30° F increase in temperature would cause a 36 foot long acrylic sheet to expand by one half inch. Glass would only expand by 1/16 of an inch under the same temperature increase. Consider this from the perspective of a projected image and the resultant effect on the focus and visual acuity of the sourced material. Also, acrylic is much less thermally conductive than glass, acting more as an insulator than a conductor. This can cause problems with bending or warping due to temperature differences. Take for example, an 8 foot span of acrylic substrate. If an air temperature difference made the acrylic 9° hotter on one surface, the temperature difference would cause the screen to bend outward nearly 1 inch in the center, causing a horizontal cylindrical shape. The same conditions for glass would create a lesser temperature difference, by nearly a factor of 5, causing a deflection of only 1/16 of an inch at center of the span.
While the acrylic is “rigid”, it is still by its nature a semi flexible material, allowing it to be bent or curved into custom shapes which depending upon the application can be useful. This also means that acrylic screens can be subject to a “flutter” effect where the screen can move slightly under atmospheric pressure differences in front and behind the screen material. In addition, the acrylic material is not rated as a construction material, and does not rate under safety or building codes as a certified building component. It is not capable of supporting its own weight, let alone any additional structure. This must be borne by the mounting system. This means that care must be taken to ensure that any structure with a screen of sufficient size must be reinforced to meet code standards. Also, acrylic materials can be highly flammable, and must be protected from high heat, as they are subject to melting and becoming slightly cloudy as they cool down and heat up. Finally, the screen’s diffusion coating, which is the material that allows it to hold an image, is coated onto the front surface of the acrylic substrate. This can make it prone to damage, from environmental or human factors, either through impact or scratching. This sort of damage may be able to be repaired, but only in a very limited manner and typically this sort of damage can only be solved by replacing or recoating the entire screen surface.
The Stewart Aeroglas, being constructed from glass substrates, is inherently more stable. It carries a higher weight than a comparable acrylic screen, and is slightly more fragile to install, requiring more careful handling, but it’s stronger construction means it is more flexible in terms of applications as a building material, and thus can be built as part of a structure with less concern of compromise to the structural integrity. Aeroglas is a laminated screen, with a diffusion fabric applied to the front of a glass substrate. This gives it
Reflectivity must also be taken into account. The Da Lite acrylic screen presents with a more satin or matte type finish on its surface, and as such, also has the effect of diffusing glare from ambient light on the screen surface. Aeroglas, being a laminated fabric over glass, presents a very similar reflectivity profile. However, the two screen coatings differ in repair capability. The Da Lite diffusion material is applied directly to the acrylic substrate as a coating. This means that the coating, if scratched, cannot be repaired easily. It would
Stewart Aeroglas is a “white” screen surface, but not a true Lambertian diffuser in a purely scientific sense. It provides a half gain of 60 degrees whereas a true matte white surface has no half gain angle and would be considered is a perfect diffuser. The Stewart Aeroglas had a noticeably wider half gain angle than the samples of the Da Lite DA100 in our tests.
Screen Gain and Half Gain Angle:
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