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Digital Light Processing - DLP Panels

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DLP (single chip) - Digital Light Processing

DLP Logo         DLP color wheel

Digital Light Processing by Texas Instruments uses a Digital Micro Mirror Device (DMD) that has thousands of tiny mirrors (see below), each representing a single pixel. The mirrors move back and forth and deflect light to the screen to create the image. The estimated life expectance of DLP panels is 20 000 hours. Due to the use of one chip for all the primary colours (where LCD uses 3) single chip DLP is light and compact. The main advantage of DLP over LCD is its ability to run 24/7 without damage to the panels from the Ultraviolet light produced by the projector lamp.

 

Unfortunately the downside to the single chip system is that a colour wheel is used (see below) to project the primary colours sequentially (one after the other). This means that in any one instant, there is only one colour on the projection screen, unlike 3LCD systems that project all three primary colours in one hit. Single chip DLP relies on the slow response of the human eye to combined the colours to create the full colour image in the viewers mind.

 

 

 

The introduction of the colour wheel to the light path also reduces the brightness of the projector. This means that more powerful lamps must be used to achieve the same lumens as similar LCD machines. The increased lamp power subsequently produces more heat which is dissipate with faster fans - producing more noise.

 

To reduce the brightness drop of the DLP image through the colour wheel, many DLP data projectors have had a white (clear)segment added. Although this is fine for data presentations, Its creates a disparity between the brightness of the projected colours and the projected white content of the image. This disparity can be seen more when the data projector is used for video. DLP home theatre projectors do not have the white segment. If the white segment is passing through the full lumens of the projector the colour segments may only be passing 50% of this. LCD projectors do not have this disparity and subsequently colours look more vibrant.

 

Click here for flash demo

Digital light processing system  Below A human hair in front of a DLP mirror surface.

DLP mirror size next to a Human hair

 

Wheel Speed, Segments and the Rainbow Effect.

 

The first generation DLP projectors incorporated a colour wheel that rotated sixty times per second, which can be designated as 60Hz, or 3600 RPM. With one red, green and blue panel in the wheel, updates on each colour happened 60 times per second. This baseline 60Hz rotation speed in the first generation products is also known as a "1x" rotation speed.

 

Glass DLP color wheel 

Upon release of the first generation machines, it became apparent that a small but vocal percentage of the population were experiencing headaches from either seeing colour breakup more commonly known as the "Rainbow Effect" or from the sequencing of the colours. One point that must be considered with public projection systems using single chip DLP projectors is the possible effect on Epileptics. Although not visible to the naked eye the rapid flashing Due to the sequencing of colours produced by the single chip system, has been noted to make some epileptics feel ill.

 

To counter this effect in the second generation DLP projectors, the rotation speed was doubled - referred to as 2x Speed (120Hz, or 7200 RPM). The doubling of the refresh rate reduced the margin of error, and so reduced the visibility of rainbows. It also reduced the number of people who could detect the rainbows at all.

Today, many DLP projectors being built for the home cinema market incorporate a new six-segment colour wheel which has two sequences of red, green, and blue. This wheel still spins at 120Hz or 7200 RPM, but because the red, green, and blue is refreshed twice in every rotation rather than once, the industry refers to this as a 4x speed. This further doubling of the refresh rate has again reduced number of people who can detect them visibly. Yet there is still a small fraction of the population who are sensitive to them.

 

Two things that is critical to the reduction of the rainbow effect is the timing of the projected image through the colour wheel, This varies from manufacturer to manufacturer and the number of segments in the wheel and the number and order of the colours in the wheel.

 

Brightness drop

 

Using a four segment colour wheel, gives higher brightness than using six segments with the same wattage lamp. If you consider that the lamp switches on and off between each segment, more segments more off time and the percentage of the wheel that is coloured glass is higher. A six segment colour wheel can be 75% of the brightness of a 4 segment colour wheel, e.g the NEC NP4100 which has the option of 6 or 4 segment wheels. LCD manufacturers are making the most of this by quoting light output and colour light output which are the same for LCD machines. You may notice the difference in brightness on DLP projectors with white segments. Reds can look muddy and less vibrant.

Dithering Artifacts

The mirrors on the digital micro mirror device have two positions on or off. Unlike LCD which can project shades by varying the light transmission of the LCD panels DLP can not be partially on. In the on position the mirror reflects light. In the off position no light is reflected - black is projected. DLP systems uses Dithering to project Grey. The process involves increasing the DLP mirrors switching speed. So that only a partial amount of light is projected compared to the normal on time. As with the sequential colour projection, your eye is not supposed to perceive this rapid switching and only see the desired grey. The side effect of dithering is that it can produce some visible instability in solid colours especially in darker areas of the image. This instability is commonly referred to as dithering artifacts.

DLP lens  light path 

To see the rainbow artifacts spread your fingers and wave your hand in front of your face while watching a DLP image or move your eyes (see diagram 1) quickly from left to right as if watching a Tennis match. Picture 2 shows a bad case, this is created from the projection of a single white circle and using the eye movement technique. For those of you that don't have the classic white circle DVD, picture 3 shows an example of when the rainbow effect is most visible compared to an LCD projector. Are you sensitive to the Rainbow Effect? click here to take our eye test.

 

Diagram:1

digital light processing DLP rainbow effect 

Pic: 2

 DLP rainbow effect accross the screen 

Pic: 3

Rainbo behind fast moving image 

 

Comparison of the big Two

 

 

LCD projectors DLP Projectors (single Chip)
tick Best Value for Money   Although prices are dropping still higher than equivalent LCD
tick No rainbow effect   Rainbow effect experienced by small proportion of users. Virtually eliminated with new DDR chip.
tick No Dithering Artifacts   Inherent in all DLP systems, Due to the way DLP renders greys
  Small gap between pixels, have
larger panels to reduce this
.
tick Much smaller gap between pixels (better fill factor), resulting in smoother overall image
tick Sharper image than DLP especially with text.  
tick Better colour reproduction with up to 216 Billion colours   Less saturated colour, but improving significantly, especially with new DDR chip.
tick Top end Home Theatre projectors achieve up to 75000:1 contrast ratio using a dynamic iris with data projectors reaching 2000:1 tick Data contrast ratios average 2000:1 but top end Home Cinema up to 15000:1
tick Smooth movement easy on the eye   May see blurry images in rapid motion video
tick Higher Lumen output per Watt   The Colour wheel reduces the lumens
tick Brightness of Colours match the light output of White   Brightness disparity when a data projector has a white segment. i.e. colours have lower brightness than the white parts of the image.
tick Better Lens Systems - offer larger lens shift and larger zooms   DLP system restricts some lens options
  Filters are used to prevent dust and debris entering the projector, projecting circuitry LCD panels and Lamp.   Filter Free design - The DLP panel is a sealed unit. while this is touted as a design benefit of the DLP system the electronics and lamp are not sealed and may be affected by dust.
tick LCD Panels are Organic and can discolour over time. The latest iLCD panels are inorganic and don't have this problem and have a life span of 20000 hrs plus tick DLP image does not deteriorate - recommended for long running 24/7 applications. Estimated panel life is 20 000 hours

3 Chip DLP projectors 

The three-chip DLP has great looking images with the only competition coming from D-iLA(see JVC models). The three-chip system is much like the single-chip DLP only better. Eliminating the colour wheel allows for very high Lumens and no Rainbow effect.

 

Unfortunately it is also the most expensive and the projectors are bulky.

 

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