Projector Visual Artifacts
Screendoor and Rainbows
Two terms you will often hear to describe visual artifacts or motion artifacts in the display of digital video projectors are “Screendoor” and “Rainbows”. And to a lesser extent you might also hear the term “Silk Screen”.
LCD projectors suffer from the “Screen Door Effect”. The term is used to describe the mosaic appearance of pixels projected onto the viewing screen, essentially it looks as if you are viewing the image through a screen door because of the black space between pixels. As resolutions improve (eg WXGA), the “screen door” pixelisation effect becomes less of a problem. Different projectors exhibit this artifact to varying degrees. Increasing the viewing distance or defocusing the image slightly can help reduce the effect.
While a DLP projector don’t normally suffer from the “screen door effect” it may exhibit the “Rainbow Effect”. Rainbows are a motion artifact that can be perceived by some observers, and not others, to varying degrees, particularly with DLP technology projectors. DLP technology is based on a monochrome image that is tinted by a spinning color wheel. The wheel revolves very quickly as the image is drawn in each primary color, resulting in a composite image that appears to the eye to be a full color image. When objects displayed in the projected image move location, or the observer changes his viewpoint rapidly, the colored projections misalign briefly, giving the appearance of a rainbow trail. This artifact is troubling to some, and others can’t even detect it when they look for it. Projectors with color wheels of progressively higher rotational speeds are more immune to the effect. LCD technology projectors do not produce this artifact.
Another visual artifact that is sometimes noticed is one that is refereed to as “Silk Screen Effect“. This term is used to refer to an image that appears to be viewed through a thin piece of silk.