Click2Screen - Dynamic Digital Signage

In summary, todays projectors can fall into two categories. Liquid Crystal Display (LCD) projectors and Digital Light Processing (DLP) projectors. In shopping for projectors, you may have already noticed the acronyms DLP and LCD. There are technical differences between the two projection technologies. Before making a projector purchase it may be worth investing some time to understand the advantages and disadvantages of the DLP and LCD projection techniques.

LCD Projection Technology

NEC Lcd Projector

LCD projectors typically contain three LCD panels, one for each of the 3 primary colorants of RGB - Red, Green and Blue. An LCD panel contains arrays of liquid crystal based pixels. Light is allowed to pass or be blocked while traveling through the liquid crystal, creating different levels of intensity of the primary colorants. 256 levels of intensity for each colorant implies 8 bit detail. 65536 levels of intensity for each colorant implies 16 bit detail. This means the number of colours that can be produced for each pixel (or projected picture element) equals the levels of intensity for each colorant to the power of the number of colorants. For example, if there are 3 colorants, say RGB, and there are 256 levels of intensity for each colorant, the number of colours can can be created for each pixel (or projected picture element) is 256 X 256 X 256 = 16777216 colours.

LCD technology is "transmissive". Advantages and disadvantages of LCD technology are listed below.

Advantages of LCD

LCD is typically brighter than DLP for the same lamp white light intensity.

An LCD picture seems sharper when compared beside a DLP projection, however may be offset by the screen door effect (see disadvantages for information about the screen door effect)

Better colour saturation than DLP. No colour wheels.

Disadvantages of LCD

Suffer from the "screen door" effect. That is, it looks as if you are viewing the image through a screen door because of the black space between pixels. As resolutions improve (eg WXGA), the "screen door" pixelisation effect becomes less of a problem. A larger dot pitch makes the projected image seem pixelised.

Blacks are not black. Implies low contrast ratios. Because LCD technology is transmissive and not all white light is blocked, blacks are not as black as they could be.

Possible degradation of LCD panel over time, particularly for the blue colorant. This may cause colour balance problems and loss of contrast over time.

DLP Projection Technology

Infocus DLP Projector

DLP technology, developed by Texas Instruments, is somewhat different to LCD technology. Instead of pixels allowing light to pass through them, the DLP chip uses a reflective surfaces, or mirrors, for light to reflect off. DLP projectors have arrays of mirrors rather then arrays of liquid crystal pixels. Each mirror can be considered a pixel (or a projected picture element).

In a DLP projector light from the lamp is projected onto this array of mirrors. The mirrors move to reflect the light into the path of projection or away from the path of projection, turning the pixel on or off respectively.

There is usually one mirror array reflecting red, green and blue colours. In this method, a colour wheel is used. It sits between the array of mirrors and the lamp and is comprised of 3 light filters. White light from the lamp is filtered to produce the red green and blue primary colours by the colour wheel's corresponding light filter. Colour intensity is achieved by rotating the mirrors by the appropriate angle. In other more expensive DLP designs, there is one mirror array for each colorant.

Advantages of DLP

Blacker blacks than LCD meaning great contrast ratios. Mirrors can reflect light away from the path of projection, not allowing white light to pass through as in LCD. Contrast ratios of 3500:1 have been achieved! (NEC HT1100 DLP projector) Increasing the angle of reflection of each little mirror has the effect of making blacks blacker. Newer DLP projector have greater reflection angles.

Compact. One array of mirrors for all colorants rather than one array for each colorant (like LCD). Great for mobile projectors.

"Screen door" effect not as evident. As resolutions improve (eg WXGA), the "screen door" pixelisation effect becomes less of a problem.

Disadvantages of DLP

"rainbow effect". Colour wheels spin quickly between the primary colorants to create a colour, with each primary colour only remaining for a fraction of a second. This is usually fast enough for the human eye not to notice, however some do and it can be very irritating. In fact many report headaches! This is not a problem for LCD projectors.

LCOS Projection Technology

Canon LCOS Projector

Liquid Crystal On Silicone (LCOS) is a newer technology. It is similar to both DLP and LCD designs. In LCOS projectors, liquid crystal placed on a mirror substrate is used to reflect incident light. Like LCD projectors and high end DLP projectors, there is one LCOS chip for each colorant.

LCOS projectors are quite high in resolution, typically greater than XGA! As such, they are not cheap. LCOS projectors do not sell in quantity like DLP and LCD projectors. They have not reached economies of scale and thus are generally more expensive. This is not to say the technology is inferior, but it just has not reached mass market yet. As LCOS borrows technology from the LCD and DLP designs, it offers a neat mix of advantages. See below.

Advantages of LCOS

Smoother image. The LCOS pixel edges are not as sharp as DLP pixel edges.

Little "screen door" effect. Mostly due to the higher resolutions and smaller dot pitch.

No rainbow effect because there is no colour wheel.

Disadvantages of LCOS

Limited lamp life

Not so good contrast ratios.

Expensive