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DLP TV - Advanced Guide

The principles

DLP displays generate an image by manipulating thousands of microscopic aluminium mirrors in an rectangular array on what is known as a DLP chip, or ‘Digital Micromirror Device’ that is small enough to hold in the palm of your hand.

Light reflected off the mirrors can be directed on to the back of a screen in a rear-projection TV, or via a lens on to a big screen in a front-projection system.

DLP technology can also be used on a much grander scale, in full-sized cinemas.

How it works

The DMD houses thousands of mirrors, each of which is separated by just one micron. Five of the mirrors put together are only the width of a human hair.

Despite being so tiny, the angle of each mirror is independently adjustable, within a range of +/- 12 degrees. When the mirror is tilted in one direction it reflects light from a light source inside the TV on to a screen, creating a white pixel. When the mirror is tilted the other way it does not reflect the light and a black pixel is created.

The mirrors are able to switch between the ‘on’ and ‘off’ positions thousands of times in a single second. The more time the mirror spends reflecting light, the brighter the pixel – the less time, the darker the pixel.

A digital signal is delivered to the DLP chip, telling each mirror how to behave, switching on and off as many times as necessary to create the desired level of brightness for the onscreen pixel. In this way, a greyscale is created, with a possible 1,024 shades of grey for each pixel.

To add colour to this greyscale image, a ‘colour wheel’ is used. This is a rapidly spinning wheel with transparent sections of red, green and blue – the primary colours that make up a TV picture.

The wheel is positioned between the light source and the DLP chip. Pinpoint timing is employed to determine when a mirror reflects light, so that either red, green or blue light (or combinations) can be reflected. If this seems like an impossible task, it is only elevating the same concept that saw World War One bi-planes have machine guns that actually fired through the propeller, being carefully timed so that the bullets missed the blades.

Use of the colour wheel allows a single-chip DLP device to display 16.7 million colours, enough to make a full-colour image.

It might appear that this technology would suffer from its complexity. The tiny mirrors needing to move so many times a second must surely put them under tremendous strain. In fact, DLP technology is surprisingly robust thanks to some advanced design concepts.

The ‘hinges’ that each micromirror rests on are actually a thin film, rather than a solid structure that would break given the number of times it would be expected to move. The DLP chip itself is also a sealed enclosure, so no dust particles can get inside to disrupt operation.

The technology is taken a step further for use in full-scale cinemas. A single-chip device would not be able to produce a good enough image to stretch across a cinema screen, so DLP projectors in cinemas use three separate chips.

A prism between the light source and the three DLP chips splits the white light into red, green and blue. Each chip receives one of the primary colours and delivers an image to a lens, which focuses the three light beams on to the screen. This creates a picture with a breathtaking 35 trillion possible colours.

DLP technology is also constantly being developed. A relatively new advancement saw the inclusion of a ‘dark metal’ coating in the interior of the DLP chips. This coating absorbs stray light photons, which could previously find their way on to the screen. It is estimated that the addition of dark metal coatings to DLP chips boosted onscreen contrast ratios from 1:800 to 1:1500.

The +/-12 degree swing on a micromirror also introduced an estimated 20% increase in brightness compared with the original +/- 10 degrees movement.

DLP TV Glossary

DLP chip – An array of microscopic mirrors that are used to generate a TV picture in a DLP projector or rear-projection TV

Front-projector – A projector that works just like one in a cinema, throwing a potentially huge image onto a screen at the other end of a room

Pixel – A ‘picture element’, the little dots that make up an image on a screen

Rear-projection TV – A TV that uses a projector inside a case to deliver an image on to a screen

Article from HomeCinemaUK.com

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