Demoing is the most important thing.
The two most common types of digital projector are the DLP and LCD ones.
Each has its' own good and bad points, and there are two ways to look at this - to be ignorant of the bad points, and judge the pj on the picture you see, or know what to look for and be more critical of the images, and look for things you wouldn't have noticed before.
Sometimes ignorance is bliss - not knowing about something means you won't be looking for it, and probably won't notice it.
But if you do want to know, here are the basic differences:
LCD:
screendoor - the gaps between pixels are bigger on LCD than DLP, so seating distance is recommended to be 2 times screen width. Can be fixed by a slight defocusing of the lens, and in some cases, with a different screen or a lens filter. The latter two really only tend to be noticable as a comparison against a white screen.
Contrast ratio - the difference between the darkest 'black' and whitest white seems limited to around 1000: currently on these machines.
Black level - all bulb projectors pass light in one way or another. Black looks dark grey.
Green push. Some LCDs can tend to have a green tint to the image. Can be fixed with an FLD lens filter & recalibrating.
Banding - vertical lines within the image.
Dead pixels - having a bright blue, red or green pixel (or combinations of) staying stuck on, slap bang in the picture (center is worst place) can ruin the image. LCDs can come with these from new or develope them after a period of time.
DLP:
Screen door from less than 1.5 viewing distance,
Black level - generaly better than LCD but still not true black.
Rainbow effect - the biggest deal killer for this technology. Because the image is built up one colour at a time via a colour wheel and thousands of tiny mirrors reflecting light on and off at a very fast rate, you generaly just see the mixed resultant colour. Some people unfortunately can see the colours seperately and see a rainbow tiype artefact around certain parts of the image; normaly where there is a change from lighter to darker parts of the image.
Headcahes/ eye strain. Another effect of the colour rendition process.
Temporal dithering - IIRC the mirrors of the DLP chip trying to make better contrast over a period of frames causes a mosquito effect or noise in the image - mainly in darker background areas I believe.
Some older DLPs appear to produce orange reds and washed out colours (due to white section in colour wheel).
These issues make either technology sound unfit for purpose, but in fact they are rarely issues at all, otherwise nobody would buy them. other factors to look/listen for:
Fan noise. Less than 30dbs seems acceptable to many (depending on how close you are of course)
Throw length. Distance the pj lens allows the pj to be to the screen for the required screen size.
In built scaler/deinterlacer. Some (older) pjs aren't good at these, so show stair-stepping on edges like round objects. An external processor will do a better job in these cases (like a suitably equipped PC - an HTPC).
That's some basic things (I've probably missed some I'm sure), but you'll find other things the more you read, so demo as many as possible to get a feel for the technologies.
In most cases though, you'll fiind they're all pretty good, but depending on your own preferences, some will be better than others (to you).
There are currently a lot of home theater specific pjs that have addressed a lot of these problems to one degree or another, and are becoming increasingly cheaper. You'll see some of them being mentioned, along with the prices, so you can get some idea of what models to try to demo.
Pictures of screen shots don't show the true picture, especialy for black level, but can be used as a guide. Nothing beats seeing one for real though.
HTH
Gary.
Linear Mode
