How Do Light Rejecting Screens Work?

This guest technical primer on light-rejecting screens was written and submitted by Dave Rodgers, marketing manager for Elite Screens Inc., a 20-year professional in the audio/video and wireless communications industries. Elite offers several ambient light rejecting screen materials for different applications, including its popular Aeon Cinegray.

Ambient light allows us to see the world. From the moment of our first breath to our final sigh, the waking hours of our lives are bathed in ambient light. Think of your best days under the sun; the warm rustic glow of candles and firesides, or the countless light bulbs that have illuminated your path through all these years. Of all the phenomena in this great universe, it has contributed so much to the quality of our lives. That is, unless you are trying to get a good picture on any large video display—then it is just too darn bright. Once again, the old adage proves correct that “you can have too much of a good thing.”

The term “ambient light rejecting,” or ALR, is now part of the AV industry nomenclature as a screen feature that prevents ambient light from washing out a projected image. There are AV purists out there who hate the use of the word “rejecting” and, technically, they're right. ALR screens actually do the opposite of rejecting—they either absorb or divert ambient light away from the viewer’s field of vision.

So why say ALR?

Simply, “ambient light rejecting” gives customers an easy mental image of a screen that gets rid of ambient light and its washout effect. Besides, “angular divergent reflectivity of indirect light sources within an absorbent active matrix to mitigate the intrusive effects of luminous inundation onto projected images” is too much of a mouthful, and every marketing team out there hated it. And, “ambient light absorbent” sounds like something you clean your kitchen floor with.

What is Ambient Light?
To AV installers, ambient light is any atmospheric lighting that originates from an indirect source of luminance that can be inside, outside, naturally or artificially occurring. Basically, it’s any light source other than your projector that threatens to wash out your image or otherwise impair its visual aesthetics. This includes but is not limited to light coming through windows and skylights from outside, track lighting, in wall/ceiling lights, lamps, candles, fireplaces, or even the light of your own video screen reflecting off light-colored surfaces in your room.


An undoctored photo showing ambient light from an outside source washing out a matte-white projection screen.


An undoctored photo of an ambient light rejecting (ALR) screen in action.

It is surprising It is surprising but true: Light colored walls, ceilings and flooring can all contribute to ambient light problems and negatively affect your video performance. When projected light bounces off your screen, it turns every surface in your room into a potential reflector that will in turn contribute to washing out the fine qualities of your projected image. So yes, dressing up like Mr. Roarke and watching Fantasy Island episodes in a room lined with white marble floors, alabaster walls and a whitewashed ceiling is far from ideal for getting the best out of your projector screen. Next time, think Knight Rider instead.

“Rejecting” Ambient Light?
The bottom line is that most rooms do not have 100 percent control over environmental lighting. As cool as a fully contained home theater room is, most households just don’t have them. Instead, homes are trending toward having large projection screens in multi-use living rooms, dens, and media rooms as a means of replacing the household television set with a larger than life +100-inch screen. This is entirely feasible as long as you take the needed steps to counter the washout effects of ambient light.

To some degree, a brighter projector does help. The extra brightness drowns out a little more of the milky white glare of indirect room lighting, but it does bring with it a negative effect as well. As much as you want a bright, colorful image, you don’t want it too bright or your contrast suffers. An excessively bright screen can also cause eye fatigue and discomfort, so a more practical solution is necessary. As an alternative to a brighter projector lamp, you can consider the ALR specialty projection screen.

ALR screens come in two basic formats: "retro reflective" or "angular reflective." According to the Law of Reflection, “…the incident ray, the reflected ray, and the normal to the surface of the mirror all lie in the same plane and the angle of reflection is equal to the angle of incidence.” ALR surfaces may not fully react in accordance with the Law of Reflection because they add their own variables, which are either microstructures or multi-layered optical filters. Normal white projector screens have something called "diffusion uniformity," which means they scatter the projected light to provide uniform brightness over the full 180 degrees of the projection angle. However, it scatters all light—not just the projected signal—so high levels of ambient light will wash out the image on a standard white screen.

Angular Reflective
Angular reflective means that light reflects off the surface at precisely the opposite angle of incidence, exactly as the laws of reflection describe and as the diagram below illustrates.

This type of screen is the preferred model for commercial applications but is also widely popular with residential applications as well. The material prevents full diffusion from occurring to provide greater control over the washout effects of ambient light. The focused reflectivity rewards viewers with a bright projected image that achieves a superb color temperature. At the same time, off-axis light is diverted away from the viewer’s range of visible perception. The screen's multiple layers of diffusion material further negate the washout effects of ambient light while also enhancing black levels for sharp image clarity.

Retro Reflective
Retro reflective means that the material will reflect back the image direct to the light source. This limits the options for projector placement but performance levels for retro reflective screens are so superb that this format is typically the overall favorite of home theater aficionados.

A retro reflective ALR screen essentially absorbs ambient light away from the viewer’s field of vision (as shown above). The screen does this by means of an active microstructure design. In English, this means that the actual reflective surface of the screen has been structured down to a microscopic level in order to reflect the projected image directly into the viewer’s field of vision while absorbing the majority of all other indirect light.

When a cross-section of the active retro reflective material is viewed, it is revealed that the reflective surface actually has a serriform or “saw-tooth” microstructure (shown at right). This structure acts as a funnel to concentrate the direct light of a projector back in a narrowed angle of reflection that is comparatively a small “pie-slice” of the 180° brightness of diffusion uniformity. At the same time, indirect light is either absorbed in the dark saw-tooth formations seen in the picture or otherwise cancelled out by the diffusion layers.

With retro-reflective materials, light coming from the projector must hit the material in direct alignment with the screen’s level center without being off by more than a 5 degree angle. Any vertical off-axis angle of projection greater than that will be reflected off at so extreme an angle that it will not come into the viewer’s field of vision.

Why You Should Love ALR Screens
Today's ALR screens enable the viewer to maximize the effects of optimal contrast and even ISF Certified Color Temperature.in some materials. It enhances brightness while filtering out ambient light to create a realistic image that allows 4K content to be appreciated in its full glory with the lights on or off. Color is more defined, and the sharpness and image clarity of high resolution (UHD and 4K) content is more defined. For modern lifestyles, ALR screens offer the opportunity for huge, impactful images without a huge impact on projected image quality.

COMMENTS
canman4pm's picture

Yes the two screens show different levels of "washedoutness" but so do the walls, windows and the view outside the windows. Suggesting that if both images are undoctored, they were taken with either different cameras, of the same camera with different settings, or at different times of day with different lighting conditions. I also suspect they were taken in different rooms/places as in the second picture you can see a couple of things on the walls, below the windows, not seen on the first image. Not a convincing before/after.

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