Projection Screen Tip Friday: Screen Fabrics Part I
Screen surface fabrics have particular characteristics that can help any application achieve the image quality you demand from your home theater. You’ll see that each fabric type is designed for particular applications, and each have specific benefits.
Each screen fabric has particular characteristics.
- Gain. This is simply a measurement of reflectivity of the screen. The higher the gain number, the brighter the image will appear. The increase in gain is based on how light is reflected by the fabric. Lower gain fabrics disperse light in a wide area, while higher gain fabrics reflect more light back along the projection axis. This causes higher gain fabrics to have a narrower viewing angle. When comparing these fabrics, the gain value is relative between each fabric type. The Glass Beaded fabric, for example, has a gain of 2.5, and is therefore 2.5x brighter than the Matte White fabric with a gain of 1.0.
- Viewing angle. This is a measurement of the angle at which you begin to lose picture quality. This is actually a “half angle”, but is expressed as an angle. It’s measured from the screen surface and along the projection axis. To determine the actual viewing area for the audience, the angle is measured out from the axis to all directions – left, right, up, down, and in between. This creates a “cone” of sorts. To determine the full viewing angle, take this value an multiply by two.
- Seamless vs. seamed. Some fabrics are listed as “seemless”. This means that the fabric is one continuous piece. When fabric sizes extend past the standard width for a fabric, two pieces must be mated together. A seam may affect the image quality, but is not normally discernable. In most cases, this is only an issue for very large screen sizes.
- High contrast. Some fabrics are listed as “high contrast” fabrics. The purpose of these fabric is to increase the contrast ratio of the image. Contrast is the difference between the darkest black and the whitest white projected on the screen. The contrast ratio is increased with the use of a gray fabric base, which enhances the black and darker colors in the image.
- High gain or High Power. High gain is a term for fabrics that are generally listed over 2.0 gain. Note that high gain is not necessarily the best for most applications, and it’s not true that a higher gain means that the fabric is “better”. High Power is a type of fabric offered by Da-Lite that has the highest possible gain – 2.8.
So what should you look for when choosing your fabric? It depends on your application and environment. Here’s some things to consider about your application when preparing to choose a fabric:
- Ambient light conditions. Ideally, you would want to have a completely dark room. However, you may be traveling with your screen and are unable to know what the lighting conditions are for the venue, and an elevated fabric would be acceptable. Or, you may not be able to block out sunlight from the room. You can define your ambient light conditions as “controllable” or “uncontrollable”.
- Projector output power. This is typically expressed in lumens. Higher output power projectors would require lower gain fabrics. Typically, LCD and DLP projectors would not need elevated gain fabrics, unless the ambient light conditions are such that a brighter image was necessary. Older projectors, such as CRT-type projectors, slide or 8mm movie projectors would benefit from elevated gain fabrics with a gain of 2.0 or better.
- Display Brightness. Related to your output power, this determines how much light is reflected back towards the audience. A display brightness of under 25 foot lamberts would require a fabric with a slightly elevated gain (1.5-2.0). The calculation for this is:
[Output Power (lumens)] / [Viewing Area (square feet)] = Display Brightness (foot-lamberts)
- Projector contrast ratio (2000:1 or better). Typically, projectors with a listed contrast ratio of 2000:1 or better would not need the High Contrast fabric, and would instead benefit from an elevated gain fabric. These would include:
- Projector contrast ratio (less than 2000:1). For those projectors with lower contrast ratios, the high contrast fabrics would be a good selection, especially for home theater use. Note that lower output projectors may benefit more from elevated gain fabrics than from high contrast fabrics. High contrast fabrics would include:
- Projector mounting location. Projectors can be located in several locations – even behind the screen itself.
- Front projection – ceiling mounted. In most cases, this is the ideal location for your projector. However, issues with higher gain fabrics and their narrow viewing angles may arise. It is recommended that a 1.5 gain or lower fabric is used for ceiling-mounted projector applications.
- Front projection – table mounted. Since the light from the projector is reflected back at the source, the projection axis is not perpendicular to the screen surface, and people sitting at a “normal” distance may have issues with image quality with fabrics that have a narrow viewing angles. As such, it’s recommended that the projector is mounted as close to eye-level as possible when using high gain/narrow viewing angle fabrics.
- Rear projection. There are a few fabrics that are designed to be used for rear projection. These fabrics have to be tensioned, so they’re not available with manual or standard electric screens. Da-Tex is a rear projection only fabric, while Dual Vision can be used as a rear or front projection applications.
- Cleaning required? Most screen fabrics are able to be cleaned, with the exception of the Glass Beaded fabric. This fabric has glass beads that are impregnated into the fabric. Rubbing or rough handling can dislodge these beads, creating dead spots and ultimately affect the image’s uniformity with respect to brightness. If a high gain fabric is required, and it’s possible that cleaning may be required in the future (kids, dusty location), then the High Power fabric would be a better choice (it can be cleaned). Cleaning is done with warm water and mild soap.
When you choose your fabric, be sure that they are fire retardant and mildew resistant, as all of Da-Lite fabrics are.
So now you know the basics about projection fabrics. Ok, so now what? I’m glad you asked. Using all the information you’ve gathered about your application, see Part II next week to help choose the best fabric!
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