Front Projection Screens

Definition of Gain

The gain (or relative brightness) of a screen is a ratio of the screen reflectivity in comparison to a perfect matte white (lambertian diffuser) standard such as barium sulfate (BaSO4) or magnesium carbonate (MgCO3).

Some optical laws that apply to front projection screens

The amount of hot spotting is dependent on the angle of projection, the screen positioning, the viewer position and of course the gain of the screen surface.

On these 3 pictures, you can see a flat screen surface with a gain of 1 (left); then with a gain screen (middle) and a high gain screen. The hot spot is shown on each picture. Remember that these are flat screen surfaces; you will see  the effect of the curved screen, a little further in this document.

These images show the difference in the gain graphs for the matte screen and a gain screen. These graphs are only to help you understand and are not the actual gain curves of the MDI screens.

One optic law states that: Light is reflected from a surface at an angle equal to its incident angle. In the case of a screen, this law still applies. The predominant ray is reflected at an angle equal to the incident ray.

These 2 pictures show how the light behaves on a mirror (left) and on a more diffusing surface such as a screen (right).

This means that if you want to use a maximum percentage of light that hits the screen, it must be directed to the audience. The more light directed toward the audience will help you have a bright picture with less chance of  a low contrast image. Low contrast images come with a screen receiving light from another source than the projector, including light reflected on walls from the screen itself.

The curved screen

The importance of curving the screen is demonstrated below. You can see that on picture #1 there is an amount of light that is lost. This light will be reflected by the screen toward the walls and wasted; or if the walls are not sufficiently absorbent, this light will bounce back and some of it will go on the screen again and affect the contrast of the image; so every precaution should be taken to avoid this.

Picture #1

On the contrary, the picture #2 shows a curved screen; you can see that most of the light will be directed toward the audience, thus improving the brightness of the image and also avoiding wall reflected light, so better contrast.

Picture #2

These are the basics to help you understand what is the gain and why the curvature of a screen. If you want more details or need more information you may contact STRONG / MDI; it will be a pleasure to help you better understand.