Digital Displays FAQ - Projection System Questions...

Generally speaking your viewing distance should be no closer than 1.5 times the width of your screen i.e. you should be looking at an 8' wide screen from around 12 away. Projector Central has a very interesting article on this subject

http://www.projectorcentral.com/screen_size.htm

Asides from specific requirements such as rear projection systems or having acoustically transparent screens there are two main schools of thought on projection materials with the aim of getting blacks as dark as possible without jeopardising white levels. The first is to go for a gain of above 1. This means the screen material will reflect back more light than is projected on it. Generally we keep to below 1.5. The higher the gain the lesser the useable viewing angle and the more likely for the picture to suffer "hot spotting". This where a sweet spot of the image reflect back perfectly, but outside edges of the image are reflecting at a different rate and angle - essentially a large reflection of the projectors lens itself. This is the common and recommended approach with todays technologies which are able to achieve phenomenal black level and detail in a light controlled environment.

The second approach is to go for a gain of below 1 typically on a grey material. This has the appearance of blacker blacks with light being sucked into the grey projection material (often referred to as "High Contrast Material" because of this effect). However remember your clean whites are now projecting onto a grey surface too... The higher quality grey screens (Stewart Filmscreen's "Firehawk" for example) use a grey material with high reflectance properties. Up close it actually looks like it is dusted with glitter. This has the ability to strengthen blacks and retain a decent reflectance for whites. Expensive though...

In almost all cases we recomend a 1.1-1.5 gain or similar and controlling room light as best you can (thick curtains, black-out blinds etc)

Projector brightness is a tricky area. Obviously the larger the ANSI Lumen figure, the brighter the picture is going to be. However, and certainly in the case of Home Cinema, the general trend is the brighter the picture, the more colours are washed out and the more blacks become grey!! For a Home Cinema projector brightness should be around 600-1200 lumens. Some are a little brighter since they have “Lounge Modes”, but they will run at low brightness in Cinema conditions.

A projector's “Lamp Life” gives an idea of how much usage you will get between bulb changes. This information is available in the manual, and on product specification sheets. To extend the lifetime you could run in low brightness mode, and make sure you follow the correct procedure (as detailed in the manual) for allowing the lamp to cool down after use. Today's metal halide lamps won't simply pop and not work anymore, the output of the lamp will begin to deteriorate once it is near the end of it's life – see here for more information on replacing lamps…

DLP and LCD are the two most common technologies behind digital projectors. LCD has been around for much longer the DLP and as such is available at lower prices with high resolutions. In the sub-£2000 market LCD is King of the Roost with projectors from Panasonic, Sanyo and Epson giving hugely detailed and colourful images – the absolute latest models actually proving to be quite bright under “lounge room” lighting conditions too!

DLP is the superior technology but in terms of resolution for the money is more expensive. However, the colour and contrast of the technology is significantly better – so much so that many Cinemas are now changing over to this technology. To sum up DLP in a sentence – daytime looks like day, nighttime looks like night! Perfect colours without the washed out feel of LCD make daytime scenes feel like you're looking out of a window (!) and immense contrast ratios mean a DLP projector can happily pick out a black button, on a black lapel, on a black suit, worn in the dark! (Where an LCD projector would just about make out the guy in the suit against the dark background)

This also relates to LCD and DLP technology. In LCD projectors, light is passed through an LCD chip (displaying a tiny version of the picture) and then zoomed up onto the screen. Where dark areas border the pixels there is a “chicken wire” or “screendoor” grid pattern on the screen. This is only noticeable up close, but on older lower resolution (hence bigger pixel) projectors this was quite a problem.

DLP technology is based on reflecting light off a chip rather than transmitting through one, this means far less pronounced gaps between pixels and hence no “screendoor”. However, with the large cost of DLP chips, manufacturers cannot afford to install one for each colour (red, green and blue) in order to project all three primary colour images over one another as they do in LCD projectors. Instead all light is passed through a colour wheel that has these three colours as sections on the wheel and the single chip beams the same image three times – once for each colour. (Confused yet???)

The rainbow effect occurs when you dart your eyes quickly across the screen (can happen when you are sitting too close to a large screen and the action moves swiftly from side to side, or more commonly when you're not really looking at the image and you catch a glimpse of something quickly). When this happens your eyes see all three colour streams as a mini rainbow across the light beam from the projector! Many people quickly adjust to this once they have installed their DLP projector but some are more susceptible than others. For an interesting article on the rainbow effect click here