The reflection factor for front projection screens is most commonly known as the gain factor. The term ‘gain’ should be used with care because it implies the screen amplifies light if the gain is great than 1.
As described in the Light section, a pure white Lambertian surface has the reflection factor of 1.
By adding a screen coating which has is more reflective, it is possible to reflect more projected light at the viewers. This increases the reflection factor measured perpendicular to the centre of the screen (if the projector is on-axis), but at the cost of reducing the light reflected to the sides and reducing the brightness uniformity. This can be an advantage if the viewers are seated within the angles where the screen displays a higher reflection factor – or ‘gain’ – than 1.
Screen gain is defined as the reflection factor measured perpendicular to the centre of the screen compared to a Lambertian surface.
A screen gain of 2 means the screen surface reflects twice as much light as the Lambertian surface. The screen gain perpendicular to the centre is called the “peak gain value”.
Again chart shows how the screen gain varies as a function of the dispersion angle.
For non-optical screens, as gain increases the viewing angles and the brightness uniformity decreases (which is the so-called ‘hot-spot’).
For non-optical front projection screens only moderate screen gains are normally used, between 1 and 2. Normally the gain is the same in both the horizontal and the vertical plane. This is not necessarily the case with optical screens, such as dnp.
As gain increases, the relative position of the projector becomes important. Above gain factors of 1, the screen acts more and more like a mirror, so the angle of peak gain will depend on the angle of projection. A table mounted projector, for example, shining onto a high gain front projection screen of, say gain factor 2, will have its angle of peak gain (i.e. most of the light) pointing at the ceiling!
Although rear projection screens are transmissive, the definition of gain relates exactly to the front projection reference methodology. The reference is still the front projected image using a Lambertian surface with gain 1. By using the same projector (i.e. with identical luminous output to the reference front projection test), and where the luminance transmitted through the rear projection screen equals the reflected luminance of the Lambertian surface (reference front projection material) – then the gain factor for the rear projection screen is said to be 1. If the luminance from the rear projection screen is two times greater than the reference Lambertian front screen surface, then the gain is 2 – and so on. This gain system allows front and rear projection screens to be compared on a like-for-like basis.
As the eye has a logarithmic way of adaptation to luminances, a drop in luminance of up to 50% will normally be acceptable, depending on the application.
As a supplement to screen gain charts the half-gain angle is often part of the screen specification, i.e. the viewing angle in which the gain has dropped to 50% of the peak gain measured perpendicular to the screen centre.
As a general rule, viewing angles within the half-gain angles will ensure an even perceived brightness of the image.