Digital Projection is deeply interested in screens and screen technology, given that applying the right screen in an application helps the overall image quality, while the wrong screen can result in a reflected (front screen) or transmitted (rear screen) image that dramatically sacrifices the projector’s native performance. The same goes for the mechanical relationship between the projector and the screen.
Here is a bit of insight from DPI’s Application Support team as to how Digital Projection approaches screens and screen material.
Screens with Black Level Enhancement
Black level enhancing screens, as offered by many screen manufacturers today, can add a lot with respect to black level performance in an ambient light venue. However, it is important to keep in mind that dark screens and ambient light venues swallow projector lumens, and as a result, much higher foot lambert targets should be set when designing projector/black level enhancing screen combinations for such venues.
As an example, in a theatrically dark venue, attaining 20 Ft Lamberts will result in an exceptional image on a unity gain white screen. However, 100 Foot Lamberts or more is generally required to attain exceptional image quality on a black level enhancing screen in a high ambient light venue. Stated another way, for the same screen size, the projector in the high ambient light venue would need to deliver 5 times the lumens as the projector in the theatrically dark venue – so a 2000 lumen projector in the theater translates to a 10,000 lumen projector in the ambient light venue! The lumen ratio can be even higher if the dark screen’s reflectivity is less than unity gain.
Screens with Optical Characteristics
Some front and rear projection screen surfaces, and some black level enhancing screens, feature optical properties that help enhance performance – actually directing more light toward a horizontal and vertical "sweet spot" within the viewing area. If the mechanical and optical relationship between the projector and screens of this type are correct, viewers within the sweet spot can be treated to an image that demonstrates higher dynamic range than would otherwise be enjoyed from more traditional screens. Contrarily, viewers outside of the vertical or horizontal sweet spot see an image that is typically less dynamic.
Luminance uniformity can also be compromised, sometimes significantly, for viewers outside of the sweet spot. If the application requires the use of a screen of this type, it is vital to assure the screen’s sweet spot is broad enough to cover the entire horizontal and vertical viewing area. We are often asked to look at applications where customers believe the projector is not performing properly, only to discover that a screen with insufficient sweet spot geometry was employed in the venue.
Projector and Screen Relationship — Throw Ratio
The mechanical and optical relationship between the projector and screens with optical characteristics is vital. Typically, screens with optical characteristics are either designed to be employed with projectors at a very specific focal length, or they have recommended minimum focal lengths. This simply means that the projector must be equipped with a lens that satisfies the focal length requirements of the screen. Failing to do so can sacrifice image brightness and / or luminance uniformity – sometimes dramatically so.
Again, over the years we have been asked to look at many systems where the customer believed the projector was not performing properly, only to discover there was a mismatch between the optical performance of the screen and the throw ratio of the lens being employed. This is especially common in rear projection applications, where a high gain, optical rear projection screen is employed to compensate for ambient light, but matched to a projector equipped with a very short-throw lens due to rear projection space limitations. The result is always compromised display system performance.
Whenever we hear that luminance uniformity is extremely poor in an application, our first suspicion is a mismatch between a high performance screen and the throw ratio or position of the optics of the projector.
Projector and Screen Relationship — Projector Position
The position of the projector with respect to the screen, in both rear-screen applications and front screen applications, where high gain or optical screens are employed, can also have a significant impact on the image the viewer sees. To optimize brightness and luminance uniformity, and minimize hot-spotting, the rule of thumb is to install the projector with respect to the screen, such that a line drawn from the projector lens and through the center of the screen, will intersect the average viewer eye position within the venue.
Of course, employing a lens with a throw ratio that matches the requirements of the screen is still necessary. If it is not possible to place the projector in the optimum position with respect to the screen, it may be possible to coincidentally tilt the projector and screen, in order to move the line that extends from the projector lens through the center of the screen, such that it intersects the average viewer position.
Most screen manufacturers provide very detailed performance specifications for the screens they offer. Understanding those specifications, especially with respect to black level enhancing screens and screens with optical characteristics, is vital, as it provides the integrator with the criteria needed to select and position appropriate projectors and lenses, in order to optimize display system performance in the application.