While many think about projectors as just a presentation device, these also often work very well as a large TV low host home cinema solution. They also take up very little space and in fact none at all if mounted from the ceiling. Like with any other projector system such as cine-film and slide projectors, these need a suitable display for showing the picture. A matt white to light-blue wall will generally do fine, however a projection screen will generally give a sharper, smoother and brighter picture.
A front projection system is the equivalent to a â€˜separatesâ€™ Hi-Fi system. A complete front projection display consists of a projector, screen, speaker system and one or more AV sources. Very few if any projectors incorporate a TV Tuner and those that include speakers generally have very small speakers more suited for those sound effects in slide presentations rather than watching TV.
There are several things to look out for when comparing or deciding on a projector. The main specifications to look out for are native resolution, maximum resolution, aspect ratio, image forming technology, lumens, contrast ratio and lamp life.
Most projectors come in either SVGA or XGA resolutions. SVGA is generally 800x600 and XGA is generally 1024x768. The higher the native resolution, the more physical pixels the projector is capable of producing. Native HDTV projectors usually have a resolution of 1280x720.
If the projector will be primarily used for watching TV, DVD and other standard definition content, an SVGA would be ideal here to save on costs. However the SVGA resolution may be too low for general PC usage since many software applications and websites are designed for use with a 1024x768 resolution display.
An XGA capable projector would also be useful should the user consider viewing high definition content at a later stage. In this case, the display would show greater detail than with an SVGA projector.
Finally a native HDTV resolution projector with an aspect ratio of 16:9 would suit all needs as well as show 720i/p HDTV content without resorting to lossy compression on the resolution.
A few manufacturers may rate the resolution in mega-pixel similar to digital cameras. The mega pixel rating for LCD projectors is worked out by multiplying the number of pixels horizontally by the number of pixels vertically by three (one for red, green and blue). The most common ratings for LCD projectors are:
1.44M = SVGA (800 x 600)
1.55M = 960 x 540
2.36M = XGA (1024 x 768)
2.76M = 720p HDTV (1280 x 720)
3.93M = SXGA (1280 x 768)
6.22M = 1080p HDTV (1920 x 1080)
Pretty much all projectors are capable of taking in a higher resolution than its native resolution. When a higher resolution is provided as a source, the projector will resample the image to its native resolution before displaying. This may result in distorted detail or make small writing difficult to read.
Many widescreen projectors will accept both HDTV 720i/p and 1080i/p resolutions however these are resampled to its native resolution and will result in less detail shown than with a higher resolution display.
When comparing projectors, be sure to carefully look at the specifications when determining the native resolution. Some manufacturers place their maximum resolution in a large font, but its native resolution hidden in the specifications. I have seen many projectors advertised as â€˜XGAâ€™ which only had a native resolution of 800x600.
Image Forming Technology
There are several technologies used for developing the image. The main two are LCD and DLP.
The LCD panel in a projector works similar to a regular TFT monitor; however the LCD panel in a projector is generally around 1â€ in size with a powerful light source to project its image (similar to a slide projector). Some LCD projectors have three LCD panels using one for blue, one for red and one for green. A set of mirrors usually combine the three primary colour images to produce a full colour image.
LCD projectors are known for producing excellent colour rendering; however the maximum contrast ratio is generally no better than 800:1. LCD technology use to have an annoying â€˜updateâ€™ latency where a pixel would take some time to change to a new state; however this is generally not an issue with most modern projectors.
Some of the lower end and particularly older projector models suffer from a â€˜screen doorâ€™ effect. This is where a noticeable gap is present between each pixel, thus giving the appearance of a â€˜gridâ€™ present over the picture. Most of the newer LCD models, particularly the Panasonic models no longer have a noticeable grid effect when viewed beyond one to two metres from the screen.
DLP projectors use an array of miniature mirrors to produce the image. Each mirror represents a single pixel and can either be in an on or off state. When the mirror moves to its on position, the light is reflected onto the next stage, however if the mirror is in its off position, the light is reflected towards a black surface to absorb the unwanted pixel. The final stage usually consists of a rotating colour wheel to make up a full colour image. To display a colour for a given pixel, the mirror for that pixel is only set to the on position while the light is passing through the appropriate colour part of the wheel. To display different shades of colour, the mirror for a pixel is rapidly switched on & off (several thousand times per second) with on-time being varied to produce the intended shade.
The main advantage with DLP technology over LCD is the ability for the panel to handle greater heat. This allows the size of the DLP and weight projector to be reduced as well as allow for brighter bulbs to produce a brighter image. This is the main reason my most large venue projectors now mainly use DLP technology.
As DLP projectors reflect light using mirrors, they have virtually no update latency issues and also generally have a much higher contrast ratio than LCD projectors. Good DLP projectors generally have a rating of up to 3000:1 contrast ratio.
The one small downside to DLP projectors is that colour rendering is generally not as good as with LCD projectors. Some makes also suffer with displaying faint â€˜Rainbowâ€™ effects which some picky users have reported quite annoying.
Due to the higher lumen output and contrast rating of DLP projectors compared with LCD projectors, these would be better suited for use in lit-up areas.
The lumen rating of a projector is the maximum light it can output. The lumen rating is the equivalent to the rating of a lamp where 750 lumens is the light output of a 60 watt tungsten filament light bulb.
The rating becomes important when deciding on the intended use. If the projector will be used in a dark room with a picture size up to 100â€, then any lumen rating will generally do fine. However for images over 100â€ or when considering operating the projector in a lit up room, it is well worth going for a higher lumen output. In a lit up area, the projected image must be bright enough to significantly overcome the ambient light being reflected from the screen to show better contrast and colour rendering.
Note that as the lumen rating is the maximum output, this may be the rating with all contrast, brightness, gamma controls, aperture (if present), bulb brightness and other controls set to maximum as well as a new bulb. The actual lumen output with the controls properly adjusted is often less than half of the advertised rating and also gradually drops as its light bulb ages.
In the ideal environment where the room is pitch black, the contrast ratio is the number of times the brightest possible shade of white is brighter than the darkest shade of black in the projected image. Again like the lumen rating, the advertised contrast ratio is usually with the contrast, brightness and RGB controls (if present) tweaked.
The contrast ratio becomes important with images that have dark areas or the whole image is dark. With a low contrast ratio, dark areas can look rather â€˜hazyâ€™ and have poor colour rendering. High contrast ratio gives more realistic and darker night time scene images.
Note that the actual contrast ratio also depends on other characteristics such as if the room is lit up as well as any picture adjustments made on the projector. The more ambient light hitting the screen, the lower the contrast ratio gets. However in a lit up room, a higher the lumen rating means a higher actual contrast ratio.
Although LCD technology generally only has a contrast ratio of up to 800:1, some manufacturers tweak this by varying the bulb brightness depending on the image being displayed. For example if the bulb brightness is halved on a dark scene compared with a bright scene, this effectively doubles the contrast ratio. However the contrast ratio with images containing combination bright and dark areas is only as good as what the image forming technology can do.
Use in a lit up area
Just note that no matter how good the lumen or contrast ratio rating on the projector is, the darkest shade of black than can be present is the ambient light being reflected from the display. This means that a pitch black picture shown in a sunny room will always appear white on the screen.
For frequent viewing in a lit up area, I would recommend considering a rear projection or flat panel TV. For occasional viewing during the day such as checking the news or whating short programmes, a 2nd small TV would be ideal here.
Like cine-film and slide projectors, LCD and DLP projectors use a light source to project the image. These use a specially designed bulb with a short arc and high colour rendering properties. However just like a regular household light bulb, these have a limited lifetime of between 1,000 and 5,000 hours depending on the make of the bulb as well as how often the projector is operated in the â€˜low lampâ€™ mode. Replacement bulbs are not cheap however which range between â‚¬200 ($240) and â‚¬ 600 ($ 720) per bulb. :eek: