Game Development Reference
market: CRT, LCD and DLP. However, a fourth technology, called LCOS, though not
fully mature as yet, is in the course of being developed.
The CRT (Cathode Ray Tube) video projectors are the oldest and the most com-
monly used to date, mainly because they were the first ones to offer bandwidths that
made it possible to attain refresh rates that were sufficiently high to be compatible
with the active stereoscopic applications at sufficient resolutions. These projectors use
three cathode tubes, one for each colour of the additive synthesis (red, green and blue).
These three tubes are connected to an optical device that is used to adjust the merging
of light beams, i.e. the exact superimposition of the images issued by each tube. This
procedure of calibration is a meticulous task. Besides, the CRT projectors are subject
to a deviation in their adjustment time, which requires providing for regular mainte-
nance. Once in a month would be a good calibration frequency, which of course needs
to be adjusted as the projectors' age increases. Other disadvantages of the CRT pro-
jectors are their price and their moderate luminous power (120 to 500 lumens), which
makes it necessary to use them in almost complete darkness in order to get good results.
However, their main advantages are their image quality and their high contrast ratio;
the black and white produced by the CRT are much clearer than those produced by
other technologies. Their bandwidth allows high resolutions: up to 3200
at 240Hz in high-end products. When used in a stereoscopic environment, the tube
projecting the green component of the images has to be a special tube, called “rapid
phosphorus'' tube, to avoid generating ghost images. In fact, the standard green phos-
phorus tubes are not sufficiently reactive to be refreshed completely at the frequency
of 120Hz. As a result, the green component of the images leads to a sort of persisting
afterglow creating an effect of a ghost image.
The LCD (Liquid Crystal Display) video projectors produce three primary colours
using the same optical path. Adjusting the merging of beams is thus not required, which
simplifies their maintenance to a great extent. The light coming from the projector lamp
passes through the liquid crystal modules mounted on glass plates. The liquid crystals
then make it possible to modulate the quantity of light transferred. This technology is
known as a “transmissive'' technology. Being more light and compact than the CRT
projectors, they are more appropriate for mobile immersive systems. Their luminous
power is much higher than the CRT projectors (about 2000 to 5000 lumens), but
it comes at the cost of expensive high power lamps with a rather short service life
(1000 to 3000 hours) and noisy fans required to disperse the surplus heat generated.
Compared to the CRT, the LCD projectors generally have a lower purchase price,
but higher maintenance cost. The quality of colours rendered is inferior to the CRT
technology and even the images are less contrasting (contrast ratio ranging from 500:1
to 700:1). This in turn degrades the purity of whites and blacks displayed using the
LCD technology. We cannot ignore that the image refresh rates are not sufficient to be
used in active stereoscopy. Even the resolutions of images that can be displayed by an
LCD projector are very low, the current maximum being 1920 ∗ 1080 pixels. Moreover,
the main disadvantage of the LCD video projectors is that the images are shown with
rectangular pixels which display spaces between them, which in turn degrades the
image quality (of course, if the application requires observing the screen closely, which
is often the case in VR). Remember that the luminous power needs to be high when we
need to display the images to a group of observers in an immersive room. However, in
the case of a single observer in VR, the desired power is generally low.