In a former professional life, I was dealing with some very high-end front and rear projection, CRT, LCD and DLP based. In rear projection, because the projected image is on a short-throw basis, the rear projection screen usually consists of two or three layers. The first layer the light passes through is a Fresnel lens consisting of many fine concentric rings, each ring just slightly different from its neighbors, to focus the original spreading light beam(s) consistently forward. Among other things, this helps avoid, as much as possible, the center ‘hot spot.’ The next layer is usually a black-stripe lenticular lens meant to spread the image horizontally, so that the viewing angle is improved and the brightness is as consistent as possible horizontally. Because these lenses are often large and easily deteriorate with abuse (even skin oils transferred from your fingers onto the Fresnel or lenticular lenses can be seen as bright or dark spots, depending on the viewing angle), there is usually a third protective layer closest to the viewer that can take standard touching and cleaning during normal use and, should something scratch it, this protective layer can be replaced at about 20% of the cost of the other two.

In any pixel based light controlling mechanism, the pixels generated do create their own grid, whether obvious or softened. The sizing of this projected grid can interact with the Fresnel rings and/or lenticular striping lens, creating a minor irregular moiré pattern. Because the two interfering grids are separated by some distance, just slight changes in viewing angle and/or light source can influence what is perceived.

The advantage that CRT based RP has in this area is that the projection is a series of horizontal lines, and the red, green and blue colors overlap rather than being neighboring pixels or sequential pixels. Only ‘three-chip’ LCDs and DLPs have overlapping color pixels, and these are more expensive because of the number of chips and the optical mechanism/alignment needed to properly overlap the three colors. Three chip LCDs still have a ‘screen door’ grid pattern. Three chip DLPs are expensive, which is why most are single or dual chip with ‘time sharing’ of pixel time divided by a color wheel. The increase in DLP standards usually means faster switching between colors more often to try to eliminate the ‘rainbow’ effect. Because the CRT lines are horizontal, the Fresnel lens is ringed and the lenticular lens striping is vertical, there are almost no ‘nearly aligned’ patterns to deal with. The downside is that these overlapping horizontal scan lines must be kept in proper alignment both in reference to the outside world and in reference to each other. The more critical the screen viewing, the better this alignment must be. Also, the phosphors fade out more gently than pixels turn off – sometimes an advantage.

Bottom line: each method has its advantages and pitfalls. This is certainly not a complete discussion in that regard.