By Alan C. Brawn CTS, ISF, ISF-C
Ambient Light Reflective Screens and the Laws of Physics: Introducing Phantom™ HALR™
As we know, the new kid on the block in terms of projection screens falls under the heading of ambient light rejecting or ALR. The ultimate goal of an ALR screen is to give viewers brightness and picture quality that rivals or even surpasses what other display technologies can provide with the lights on or off. The key issue is the management of light falling on a screen and ultimately, what is reflected back to the eyes of the viewers.
Surfaces tend to reflect incoming light in all directions. 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. In projection screen parlance, a matte white screen like the Stewart Filmscreen StudioTek™ 100 is a near perfect diffuser, spreading the incoming light at nearly 180 degrees for extremely wide viewing angels. From a purely technical point of view this would be called a Lambertian surface. Lambertian reflectance is the property that defines an ideal “matte” or diffusely reflecting surface. The apparent brightness of a Lambertian surface to an observer is the same regardless of the observer’s angle of view.
A “perfect” diffusion screen spreads the incoming light out evenly and for this reason, there is little natural ability to control the incoming ambient light that interacts with the projector’s illumination other than by some means of lighting control. Any unwanted ambient light falling on the screen surface will degrade the contrast and color saturation to some extent. In high ambient light situations this can render the images unacceptable.
Another type of surface that reflects light is known as a specular reflector. Specular reflection is the mirror-like reflection of light from a surface. Incoming light from a single direction is reflected into a single outgoing direction. Such behavior is described by the “law of reflection, which states that the direction of incoming light (the incident ray), and the direction of outgoing light reflected (the reflected ray) make the same angle with respect to the surface normal, thus the angle of incidence equals the angle of reflection.”
With specular reflective aka angular reflective screens, we have a degree of control over the general ambient light hitting the screen. In systems design, we can calculate where the incoming light is hitting the screen surface and then know that the reflection is going to equal that. The reflected light can then be taken into consideration such that it does not interfere with the viewing cone of the audience. Since angular reflective screens reflect light more in one direction than in others, it tends to make the image brighter for people sitting in a pre-determined area. These screens are typically higher gain where the incoming light is focused on a narrower viewing angle. The concept of controlling incoming light and reflecting it at predetermined angles is the basis for most ALR screens we see today
By selectively reflecting the projector’s light, you can position the screen in such a way that the projector’s light is directed towards the audience’s eyes. The ambient light in the room, that can negatively affect the viewing experience, is absorbed to a degree and then reflected in some other direction away from the viewing cone. The solution is to match the light coming from other angles and to reflect as much away as possible, reducing image degradation. Keep in mind that ALR screens only work if the ambient light and the projector’s illumination are coming from different directions. Examples might be reflections from light colored walls, light from general illumination room lighting or task lights, or lights coming in through a doorway, or the sun coming through a window. Continue reading