Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!husc6!panda!genrad!decvax!tektronix!tekgen!kurtk From: kurtk@tekgen.UUCP (Kurt Krueger) Newsgroups: comp.graphics Subject: Re: CRT technology? Message-ID: <745@tekgen.UUCP> Date: Thu, 15-Jan-87 16:46:15 EST Article-I.D.: tekgen.745 Posted: Thu Jan 15 16:46:15 1987 Date-Received: Fri, 16-Jan-87 21:35:35 EST References: <2029@batcomputer.tn.cornell.edu> Reply-To: kurtk@tekgen.UUCP (Kurt Krueger) Organization: Tektronix, Inc., Beaverton, OR. Lines: 48 One point that hasn't been touched upon are some inherent limitations of color CRT's. Most of the current high resolution color CRT's are built in much the same manner as TV tubes, that is they use three electron guns and use a shadow mask to insure that the red gun (for instance) only lights up the red phosphor dots. The trick comes with that shadow mask. A 1024x1024 display requires a mask with 1024x1024 holes that are big enough to pass enough electrons to get a reasonably bright display and yet is strong enough that it doesn't buckle under stress (it gots hot where the beam strikes it). It also has to be precisely aligned (and stay that way) otherwise the red beam may light up a little green. This is a rather tall order, and it is why color displays are typically little better than 1024. The phosphor dots presents a lesser problem, but remember for each pixel you need three dots (one each red, blue, green). Black and white displays don't have the problems with a shadow mask. The technology is a bit more advanced with monochrome displays as higher resolution displays are available. This same technology can be applied to color except for the fact that no one (that I know of) has been able to build a shadow mask with the required resolution and size (it is possible to build a small shadow mask at some high dots/inch but getting much more than 1024 on a terminal size display is where the hitch is). It is possible to build color CRT's without shadow masks. The techno- logy has been around a long time (projection systems - three monochrome tubes projected onto one screen, penetrons - three layers of color phosphors excited by varying the beam energy, liquid crystal devices, + ???) but they have their own sets of problems. The fact that every currently marketed high resolution color CRT that I'm aware of uses a shadow mask must indicate something. Another issue that I will only touch upon is grey scale. A typical color TV only has around 320 'pixels' of horizontal resolution. However, a continuous tone scene can render better on the TV than on a high (1024) resolution terminal. Why? The TV has a lot of grey scale while the terminal is limited. The better displays with at least 8 bits of color map information can do a respectable job but they are still lacking. Note that 8 bits only allows 3,3,2 bits for each red, green, blue. You only can get 8 levels for two of the colors and 4 levels for one. 256 colors sounds good until you look at it this way. If you divided your color space in this manner, a blue portion of your image can only have four levels (and one of those is black!). Techniques such as dithering can make improvements but the basic display is still limited. There is no technological limits that restrict a display to 8 bits of color information (indeed, 24bit frame buffers are available, the Tektronix 412x maps 256 colors into a 24 bit palette) but cost considerations enter - a 1024x1024 display needs a mega-bit of memory for each bit of color information. I expect this problem to be solved before the resolution problem is solved.