When buying a projector - whether for a classroom, conference room or sports bar - “brightness” will certainly be one of the specifications at the top of the list to consider.
However, it's not only the projector that determines the perception of brightness for the viewer. While the lumen output of the projector is a key variable, the reflective properties of the screen have a significant bearing on how “bright” the images appear.
In addition, for spaces where blocking ambient light is either not possible or not desirable (eg. some light may be necessary for note taking in a classroom), the burden on projector light output and optimized screen technology to offset the impact of ambient light increases.
This post will focus on how the correct projector screen can play a significant role when trying to maintain high brightness for the viewer in the face of ambient room light.
In the last few years, a new type of projection screen has appeared on the market – ALR, Ambient Light Reflective screen. As the name implies, this screen technology addresses head on the issue of how to maintain the viewer perception of image brightness even in a space with significant ambient light.
The key to the ALR screen technology is how reflected light from the screen is managed.
A matte white surface, such as a wall, will reflect light equally in all directions. According to Alan Brawn on a StewartFilmScreen.com blog post, “You can take a look at a picture on a wall and from most angles it looks the same because the incoming light has been diffused or distributed evenly. These surfaces are called diffuse reflectors.”
You can purchase projection screens that, like the matte white wall, are diffuse reflectors. In a dark room, these screens provide all viewers with a similar image brightness even at wide angles. While great for a darkened space, these same screens will quickly lose contrast and color saturation when ambient light is present. The more extraneous light, the more washed out and unacceptable the images will be for the viewer.
ALR screens can mitigate the impact of ambient light by reflecting the incoming light from the projector into a restricted viewing angle.
ALR screens use optical filters and dispersion techniques to control both the projector light and the extraneous light. The idea is to shunt the extraneous light away from the main viewing angle, while optimizing the projector light within the viewing angle.
Alan Brawn explains that “Each ALR screen has unique characteristics that define it. It is typically multi-layered, consisting of a molecular layer that act as a form of optical filter to eliminate indirect light and this is positioned below a contrast-enhancing layer. These microscopic optical filters actively reflect a projected image while diverting indirect light from the viewer’s field of vision.”
The effectiveness of the ALR screens depends somewhat on the direction of the ambient light. Projectorcentral.com has done testing to show for example that for a certain ALR screen, black light level was fine when the ambient light came from the side, but was not good when the ambient light came from above.
Knowing the direction of the extraneous light in a space is important for buyers of ALR screens because they can then focus on only those models that address their situation. For example, there are ALR screens designed for spaces where ceiling light may be the primary extraneous light, eg. ultra short throw projection applications.
ALR screens are not perfect, they will still have image degradation outside a certain viewing angle. As the viewing angle moves away from the center, the screen dims and the brightness uniformity degrades quickly. ALR screen specifications will usually include a viewing angle within which the image brightness and contrast is acceptable.
Bottom line – ALR screens, properly specified, can be an effective solution for spaces with levels of ambient light that standard diffuse screens cannot tolerate.
For additional information, see detailed study on ALR screens done by ProjectorCentral.
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