Projector Technologies

Digital projectors broadly fall into two different technologies, DLP from
Texas instruments and LCD with its various derivatives (LCOS and D-ILA).

DLP (Digital Light Processing)

DLP uses a DMD (Digital Micro-mirror Device) chip made up of thousands of
micro mirrors, each of which corresponds with a pixel of the finished
image. Each of these mirrors can be independently tilted to either reflect
light towards the lens or away from the lens towards a light absorbing
baffle. The easiest analogy is to think of each mirror being a light which
is switched either on or off. The mirrors can tilt several thousand times
per second so when an individual mirror is “on” more often than it is “off”
we get more light reflected and when it’s “off” more often than “on” we get
less light reflected. This results in a lighter or darker shade of gray
pixel.

Adding Colour The light reflecting off the DMD is – at this stage – in shades of gray
and the next stage is to turn this image from grayscale to full colour.
Between the DMD chips and the lens array is a wheel made up of coloured
filter segments (red, green and blue in the simplest design). The colour
wheel spins rapidly (50 times per second for a single speed wheel on a 50Hz
supply) and the tilting of the mirrors is timed to allow light to either
pass or not pass through each filter as the wheel rotates, producing a
colour image.

The “Rainbow” Effect The result of this process is that DLP projectors build a colour image by
display a series of static monochrome image. The rainbow effect – sometimes
referred to as colour separation – occurs when the eye can detect these
monochrome images in parts of the overall image produced. Most people are
not susceptible to this problem but if you are then it can make single chip
DLP projectors uncomfortable to watch. The problem has been somewhat
alleviated recently with the development of faster colour wheels with more
segments, causing the image to be refreshed more often each second.

LCD

LCD (Liquid Crystal on Silicon) is currently the most widely used
projection technology in the world. Generally the systems used are made of
3 monochrome lcd panels, one each for red, green and blue. Light from the
lamp passes through two dichroic mirrors, which separate the light into its
primary colours. Each colour then shines on an lcd micro-screen, each pixel
of which can either be “on”, blocking light, or “off” allowing light
through. The light that passes through enters a dichroic prism which
re-combines the red, green and blue to produce a colour image which passes
on to the projector lens.

The “Chickenwire” or “Screen door” Effect

Until fairly recently people using LCD projectors have been able to see
fixed pattern noise when viewing an image. This is the grid pattern made up
of the gaps between individual pixels. As LCD technology has improved and
resolutions have increased this artifact has become less and less of a
problem. With high resolution lcd projectors it is reduced to the point
where it can rarely – if at all – be seen from normal view distances.

D-ILA and LCOS

D-ILA (Direct-drive Image Light Amplifier) is a development by JVC and is
sometimes referred to as LCOS (Liquid Crystal On Silicon). D-ILA operates
on a reflective rather than transmittive principal. In other words, the
polarized light (red, green and blue) is reflected by D-ILA chip’s rather
than be transmitted through the LCD chips. The D-ILA devices reflective
technique involves laying out the pixel address selection section and the
light modulation section liquid crystal in three dimensions. The entire
surface, except for the insulation section between pixel electrodes, is
used as a reflective surface, so a very high aperture ratio is possible
(making D-ILA more efficient than other technologies). The primary benefit
to consumers of D-ILA is the ability to produce high light output whilst
retaining high contrast without relying on lens iris adjustment.

Ken Davies Twitter: http://twitter.com/ivojo Ivojo Multimedia Ltd. Projection and Visual display systems. web: www.ivojo.co.uk

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