Video Systems in AV
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Basic and Advanced Theory Part 2
So far we’ve discussed the following:
Origins of Projection Technology
Projection Evolution
Projection Screens.
This article will focus on the basics of different types of projection technologies, this being the following:
CRT
LCD and 3LCD
DLP
LCoS
LED
Laser
Lumens vs ANSI Lumens
Video Projector Technology
CRT
A CRT projector is a video projector that uses a small, high-brightness cathode ray tube (CRT) as the image generating element. The image is then focused and enlarged onto a screen using a lens kept in front of the CRT face.
The first colour CRT projectors came out in the early 1950s. Most modern CRT projectors are color and have three separate CRTs (instead of a single, colour CRT), and their own lenses to achieve color images.
The red, green and blue portions of the incoming video signal are processed and sent to the respective CRTs whose images are focused by their lenses to achieve the overall picture on the screen. Various designs have made it to production, including the "direct" CRT-lens design, and the Schmidt CRT, which employed a phosphor screen that illuminates a perforated spherical mirror, all within an evacuated cathode ray tube.
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| CRT Projector |
LCD
In 1968, a John A. van Raalte at the RCA-Laboratories, experimented with liquid crystals to generate a video image.
His concept was based on e-beam-addressing to generate an electronic charge pattern corresponding to a video image, which in turn controlled the LC layer of a reflective LC cell.
E-beam-addressing requires a CRT with a modified faceplate to generate a charge pattern on its surface. No practical application of this concept for projection purposes is known. However, a similar concept was used for print heads without an LCD.
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| Early example of an LCD Projector |
How does an LCD Projector display Images?
Liquid Crystal Display (LCD) projectors, typically send light from a metal-halide lamp through a prism or series of dichroic filters that separates light to three polysilicon panels – one each for the red, green and blue components of the video signal.
As polarized light passes through the panels (combination of polarizer, LCD panel and analyzer), individual pixels can be opened to allow light to pass or closed to block the light. The combination of open and closed pixels can produce a wide range of colors and shades in the projected image.
Metal-halide lamps are used because they output an ideal color temperature and a broad spectrum of color. These lamps also have the ability to produce an extremely large amount of light within a small area; current projectors average about 2,000 to 15,000 American National Standards Institute (ANSI) lumens.
3LCD
There have been advancements made with LCD technology and 3LCD is the name and brand of a major LCD projection color image generation technology used in modern digital projectors. 3LCD technology was developed and refined by Japanese imaging company Epson in the 1980s and was first licensed for use in projectors in 1988.
A projector using 3LCD technology works by first splitting the white light from the lamp into its three primary colors of red, green and blue by passing the lamp light through special dichroic filter / reflector assemblies called “dichroic mirrors.” Each dichroic mirror only allows specific colored wavelengths of light to pass through while reflecting the rest away. In this way, the white light is split into its three primary color beams and each is directed toward, and subsequently through its own LCD panel.
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| 3LCD Logo |
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| Epson 3LCD Projector |
DLP
The DLP®, or Digital Light Processing, chip was invented by Dr. Larry Hornbeck of Texas Instruments in 1987.It is probably the world's most sophisticated light switch. It contains a rectangular array of up to 2 million hinge-mounted microscopic mirrors; each of these micromirrors measures less than one-fifth the width of a human hair.
When a DLP® chip is coordinated with a digital video or graphic signal, a light source, and a projection lens, its mirrors can reflect a digital image onto a screen or other surface. The DLP® chip and the sophisticated electronics that surround it are what we call DLP® technology.
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| DLP Logo |
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| Single Chip DLP Projection Technology |
Differences between 1-Chip and 3-Chip DLP Systems
The major advantage of the 3 chip DLP is that it eliminates the color wheel. The 3 chips represent each colour Red, Green, and Blue.
This eliminates the rainbow effect that occurs with some single chip DLP machines. Cost is significantly higher as these machines are usually the high end of each manufacturer.
LCoS
LCOS stands for 'Liquid Crystal on Silicon' and is a type of digital image processing Chip that is able to produce Projector images that are superior to both LCD and DLP images.
The problem is that LCoS Projectors are more expensive than either LCD or DLP projectors.
LCoS Chips are very similar to LCD chips.
LCoS also uses LCDs to process the light signals, but reflects the processed light rather than letting light pass through the chip as in an LCD chip. Because of this some call LCoS a 'Reflective LCD'. The DLP chip also uses reflective technology.
LCoS can be considered as a combination of the two technologies: LCD and DLP
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| LCoS Projection Technology |
LED
These types of video projectors make use of one of the above-mentioned technologies for image creation.
The difference is that they use an array of Light Emitting Diodes as the light source, negating the need for lamp replacement.
The typical lamp life of most projectors is 1,000-5,000 hours, while LED projectors have a lamp life of upwards of 20,000 hours.
LED Video Projectors can also be very small and compact making them suitable for portable applications
Laser
Laser diode video projectors have been developed by Microvision and Aaxa Technologies. Microvision projectors use Microvision's patented MEMS laser beam-steering technology, whereas Aaxa Technologies uses laser diodes + LCoS.
Laser projectors are now available from most projector manufacturers.
These units use a monochrome blue laser light source to excite a yellow phosphor medium creating a broad spectrum light (instead of incandescent or mercury vapor lamps used to create the broad spectrum light).
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| Laser Phosphor Light Path with 3 x DLP Chips |
This broadband spectrum light is then split into the primaries with a color wheel and are used with most common projection technologies, including single & 3-Chip DLP, LCD and LCoS.
They offer numerous advantages, including eliminating the high cost and downtime of replacing lamps, variations in brightness and color that occur as lamps age and improved color fidelity.
Typical laser light sources used in projectors are rated for 20,000 hours before the light output is reduced to 50%, whereas lamps lose brightness quickly and need to be replaced after as little as 1000–2000 hours.
So, Laser Video Projectors are the best available video projection technology available today, as they offer better colour and brightness, lower maintenance costs and operate more efficiently when compared to other projector types.
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| Example of a leading brand 4K resolution, 50,000 lumen laser projector |
Lumens vs ANSI Lumens
What is a Lumen?
A lumen is a measure of the amount of visible light emitted by a light source. Lumens are important to understand and evaluate when choosing a portable projector. The higher the device’s lumen measure is, the brighter the projected screen output will be.
This measure is similar to the way Watts measures the energy output of a light. Lumens just measure how high the brightness levels reach. The definition of the word "lumen" is "light" which makes it an easy term to remember.
Light source lumens are also known as lamp lumens. These measures the amount of light output emitted from the light source. This measurement was born around the same time as the ANSI lumens and LED lumens.
What are ANSI Lumens and LED Lumens?
The American National Standards Institute (ANSI), founded in 1918, is a non-profit organisation that works with the U.S. Voluntary standards and conformity assessment system. This organisation works to create standards for efficient and accurate industry measurements.
ANSI created this measurement and defined it in the ANSI document in 1992. The LED lights came about in 1993, shortly after the ANSI lumens.
Later, in 2006, the LED lumens reached a level of 100 lumens per watt
The ANSI lumens' purpose is primarily to rate how bright a data projector is and is the best method to use for comparability.
You will commonly use LED lumens to measure household LED lights and large commercial fixtures such as Stadium Lights. You can use light source lumens to measure a wide range of light sources. The greater the brightness and intensity, the greater the light source lumens.
Quick Recap
So far we’ve learned the following:
Since the CRT Projector, projection technology has improved immensely over the past 50 years.
Selecting the correct video projector for your project depends on several factors, such as cost, maintenance and the environment where the video projector will be used
DLP, LCD, LED and Laser Diode projectors are the most common types of projection technology used today and can vary in cost.
Laser Diode projectors present the best image available from a projector and is mainly used in the rental or integration markets, and makes use of the most common projection technologies, including single & 3-Chip DLP, LCD and LCoS.
Next Up…
We discuss the basics of Video Projection Mapping
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