Projection screen types and performance in ambient light environments
Here is an overview of main projection screen technologies. Screens in the market are often a combination of two- or more technologies and could thus be described as hybrid-screens. One example is the dnp Supernova Fresnel technology which is using both optical features, dark tinting features and silver reflector features. Scoring is based upon getting 15:1 in contrast.
Classic matte white diffusion screens
The perfect diffusor is reflecting incident light 100% equally in all directions and is the gain 1.0 reference. Classic matte white diffusion screens, also named “Lambertian screens”, are generally close to this, hence they have excellent spread of light, good uniformity, good color stability and gain close to 1. However, due to poor black level, these screens do not perform well in bright environments. The only way to improve the contrast is to boost the white level, which often results in excessive brightness and an uncomfortable viewing experiences. As there is no difference to how ambient light and projected light is reflected, this type of screen has no ALR properties.
Tinted screens, also known as grey-screens or high-contrast-screens, are made by adding dark (black) tint to the screen material, either directly in the material mix (most common) or as a separate layer if the screen has a multi-layer structure. The purpose of the dark tint is to lower the black level and thereby increasing the contrast. In principle, it works identically to the matte white screens, but the darker base-color is lowering the amount of reflection, hence tinted screens have a gain lower than 1. The darker the tinting, the more light is absorbed, the lower the gain. A downside with tinted screens is that they require a more powerful projector to compensate for the lesser reflection. Another downside is that the darker base makes it more difficult to reproduce colors correct. As there is no difference to how ambient light and projected light is reflected, this type of screen has no ALR properties.
Spherical glass-beads screens
Adding spherical glass-beads to the screen is a way to direct ambient light away from the audience. Glass-beads work by retro-reflection, meaning incident light is reflected back towards the light source. If the light source is from the ceiling, which is typical for ambient light, the light is reflected to the ceiling, away from the audience. This technology is vastly used for road-signs, so drivers easily see the signs, but can also be used for projection screens. This technology works best with table-mounted projectors as the projected light is then reflected towards a sitting audience and the ambient light from above is reflected towards the ceiling. Downsides: Narrow viewing cone, off-axis color change, significant speckle, hot-spotting, resolution limitation, not for ST/UST. This could be considered a semi-optical technology, as it is using the physical properties of a spherical glass-bead, but it’s not comprising any engineered lenses. This technology has ALR properties, but works by re-directing ambient light, not rejecting it.
Silver screens, or high-gain screens, work by so-called specular reflection, which is reflecting light like a mirror. This means light hitting the surface at an incident angle A is reflected in a similar but opposite directed outgoing angle A, like when a pool-ball enters and leaves a cushion on a pool-table. In normal cases overhead (ambient) light is reflected to the floor, where there is no audience, and light from a ceiling mounted projector is reflected towards a sitting audience. Silver screens are made by adding diffusion particles (“silver dust”) or shredded metal foil to the screen composition. The amount of diffusion particles and the structure of the surface result in how precise the (specular) reflection of light is. No real optical properties. A downside with silver screens is poor uniformity, which results in hot-spotting and narrow viewing cone, but other issues are speckle and off-axis color change. Not suitable for ST/UST projectors. This technology has ALR properties, but works by re-directing ambient light, not rejecting it.
dnp Supernova Black-Stripe technology
The dnp Supernova Black-Stripe technology is for standard throw projectors. The technology works basically like a black version of a window blind (a black louver), where it allows light in front of the screen to pass through the blinds/louvers and hit the reflector, whilst ambient light from above (or below) is absorbed by the black lens system and will thus not get reflected. This is a very effective way to eliminate/reduce ambient light and it increases the contrast in the projected image significantly. The reflector can be a matte white diffusor for perfect uniformity (dnp Supernova 08-85) or a silver reflector for high brightness (dnp Supernova 23-23). A limitation with this technology is that it doesn’t allow projectors to be installed with a steep incident angle, hence not suitable for ST/UST. Screens should not be installed high up because the lens system is blocking both incident light as well as reflected light. This is a true optical ALR screen technology. Ambient light is truly rejected by the build-in optical lens system.
dnp Supernova Fresnel technology
The dnp Supernova Fresnel technology is designed for UST projectors. It comprises a concentric lens system with center point outside (below) the image area. Each individual circular lens has a unique angle, which, together with the high-gain reflector, is engineered specifically to reflect incident light from a certain position (the projector) straight out from the screen, towards the audience. Light from other angles (ambient light) is redirected to the sides, ceiling or floor because the angle of the lens is “wrong” for incident light from other directions than specified by the design. This technology makes a bright uniform image in a narrow viewing cone, in front of the screen, but is not good for wide seating audiences. The optimal projector position is below the screen but above can be used. With the projector above the screen, the lens system is open for reflecting overhead ambient light, which can reduce the image quality. The dnp Supernova Fresnel technology is an optical ambient light re-directing technology.
dnp Supernova Black/White lenticular technology
The dnp Supernova Black/White lenticular technology is designed primarily for UST projectors, but ST can also be used. The technology comprises black, triangular-shaped, lenticular (linear) lenses with white reflector material on the lower part. The black upper part of the lenses is absorbing incident (ambient) light from above, whereas the lower white part of the lenses is reflecting incident (projected) light from below. All lenses are identical in shape and angle, contrary to Fresnel technology, but due to the white diffusor material the reflected image is very uniform in a wide viewing cone, even though the projector is not placed in a very specific position (hence both UST and ST possible). As the white part is not covering the entire downward facing area there is some loss of incident light from the projector and thus the gain of the screen is relatively low. Will not work well with the projector above. This is a true optical ALR technology, where ambient light is truly absorbed by the build-in lenses.