Short Optical Path: The Curse Of The CRT-based RPTV

By David Gibbons
Revised April 2007

Author's Note, April, 2007 - The CRT-based RPTV is a dying breed, but I will continue posting this revised article as a historical document. The new "micro-display"-based RPTV's (LED, SXRD, D-ILA, DLP) overcome many of the difficulties mentioned due to the radically improved optical system these TV's use.

Recent tune-up work I performed on a Toshiba 50HDX82 rear projection television (RPTV) has prompted me to point out a weak point inherent in CRT-based RPTVs.

The core of the problem is that the optical path length from the CRT/lens assemblies to the screen is quite short in comparison to the width of the (flat) screen. This short path means that the light beam has to spread out from the lenses at a very wide angle to cover the width of the screen. Check out the simplified illustration:

Here are the resulting difficulties that occur:

1. Inability to achieve accurate focus across the entire screen.
   Since the distance from the lenses to different points on the (flat) screen varies a lot due to the wide spread of the light beam for each color, you can never get the entire screen in focus. This could be corrected if a spherical (dome-shaped) screen was used, but you can imagine trying to make such a screen work. (Think of the glare problem!) Someone focusing a set of 3 RPTV lenses (red, green, blue) has to decide if they want accurate focus at the center of the screen, with the edges being well out of focus, or if they want so-so focus across the bulk of the screen area.
   When doing a mechanical focus ( a physical adjustment of the lenses' focus), one position of the focus adjustment will get a good focus at the center of the screen, and another position will produce good focus at the edges of the screen. Setting the focus midway between these two points will result in an "average focus." The person making the adjustment has to decide whether to go for an average focus or for best center focus...
2. Color imbalances across the screen.
   Again, the differences in distance from the three lenses to various parts of the screen will result in differing brightness ratios between the colors on the screen. Thus the color temperature (simply, the color of gray on your TV) will vary at each side of the screen, and the "lens striping" technique (which you may still be able to read about on the Keohi HDTV web site) comes into play. However, blocking part of a lens' light output will perhaps lead to a slight dimming at the edges of the screen as a cost of correcting color imbalances. No free lunch.
3. Varying line widths across the screen.
   The further each light beam travels from the lens assembly, the wider it gets, and therefore the picture elements in that beam get wider with those longer distances, even if the image is correctly focussed on the screen. Light from the gun on the right side of the cabinet has to travel less distance to get to the right side of the screen, while the light from the gun on the left side has to travel further. Therefore, lines of equal width in the video source signal will not be of the same width as they hit various parts of the screen. This can help make convergence interesting at the edges of the screen, as lines may not be the same width. Edges of supposedly white lines at the edges of the screen may show some color no matter how careful the convergence.
   (Problems 2 and 3 are interrelated, as brightness of the light beams falls as the beams spread. This affects color balance.)

CRT-based Front Projectors which sit back a long ways from the projection screen will suffer much less from these problems as the difference in distances from the 3 projector lenses to the various parts of the screen is much smaller. This is a definite advantage of front-projection systems over CRT-based RPTV's.

Today, the "microdisplay"-based RPTV's do a LOT better, as they have all the light coming from one lens in the bottom of the cabinet. Time has shown that good optical engineering in sets offered today from the better manufacturers has nailed the problems so visible in the CRT-based RPTV's.

For the average person, this improvement makes rear-projection RPTV's ever more competitive with front projection systems, in my opinion.

Good Viewing,
David Gibbons
Home Theater Tune-Up