Ambient Light Rejection(ALR) Screens and the Laws of Physics

As we know, the ultimate goal of an ALR screen is to provide viewers brightness and excellent quality that competes or even exceeds 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 the field of projection screen, a matte white screen like the XY Screens WF1 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.

(Flexible White Screen fabric WF1)

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.

ALR

screens use a combination of optical filters and dispersion techniques

to accomplish their task. 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.

In

current models of ALR screens, there is both a horizontal and vertical

viewing angle that is designed as the “sweet spot” for the viewer. Once

again, we can’t break the laws of physics, so the techniques of

rejecting ambient light will reduce the viewing angle for the audience

to a varying degree. As with traditional gain screens, going beyond the

specified viewing angle will reduce the quality of the image on the

screen and the ambient light rejection capabilities… but within the

specified zone, the ambient light is directed (reflected) outside of the

viewer’s eyesight. In this process, brightness, contrast, and color

saturation are maintained in the presence of higher ambient light and in

some cases even enhanced.

All ALR screens are not created equal. I will use the newest ALR screen on the market, the XY Screens Black Crystal Screen as an example of the current state of the art. First of all, from a physical/external perspective, it is designed as a flexible screen in sizes up to 250 inch that can be placed in a fixed frame or put

in an electric screen enclosure. The visual acuity math and testing

shows that it is inherently 4K+ compatible (and beyond to 16K!) as we

continue to migrate to higher resolutions.

(Black Crystal Screen)

It

is a unity gain screen that facilitates the standards of system design

whereas competitors are in the 0.8 to 1.3 gain range. It has an even

directionality in the viewing cone in both the horizontal and vertical

viewing axis where others tend to favor the horizontal over the

vertical. It uniformly performs its ambient light rejection task across the full screen width and height.

What

this all translates into is the evolution of ALR screens and the

ability to produce surfaces that effectively restrain ambient light from

intruding on the image. As UHD and High Dynamic Range (HDR) become more

pervasive and 4K becomes commonplace with 8K and yes, even 16K on the

roadmap, ALR screens like XY Screens Black Crystal Screen will further enhance your viewing pleasure on the big screen.

For more details on the Ambient Light Rejection screens please  contact  xyscreen-august@foxmail.com