Vision is one of our most prized senses, and a
picture is the oldest form of recorded
communication. Long before mankind learned to write, artists drew animals and hunters on their cave walls to
chronicle their lives, or bring the depicted hunt
luck.
The first machine patented in the United States to show
animated pictures or movies was a device called the "wheel of life" or "
zoopraxiscope". Patented in 1867 by William Lincoln, a spinning circle of drawings or
photographs was watched through a slit, creating an illusion of movement via the
strobe effect. The first real movie projector was the
Vitascope invented by
Edison in 1896, and the world hasn’t been the same since.
There are many ways to generate an
image with a projector,
with the only constant being the need to generate a bright enough light, almost
always processed in the
primary colors of red, green, and blue (
RGB). The only
non-CRT solution to a
lamp-based device is a laser projector, and that has been
covered in excellent detail by
erias in the
Laser Projector node.
A projector can be placed in front of or behind the
screen.
Rear-projection systems are often used for
mainstream wide-screen
TV sets, as
they are
cabinet-based units that are easy to set up and operate.
Front-projection units have the advantage of being able to create a larger
image, as the distance between the projector and screen can be greater than
that allowed by a rear-projection TV's cabinet. The disadvantage to a
front-screen projector is the device's placement. This is being remedied to a
great extent by newer
lens designs with
keystone correction that allow for
off-axis placement of the projector.
- Film
projector – this type of projector works by shining a bright light
through a piece of film that is then focused upon the screen through a
lens. Sequential images recorded on individual frames of the film create
the illusion of movement. A light-interrupting device blocks the light so
that it only shines through the film when an individual frame is properly
positioned.
- CRT
projector – this type uses cathode-ray tubes like the one in a TV set,
one for red, one for green, and one for blue. These tubes project the
portion of the image for their color through lenses onto the screen, where
they mix to create a color-balanced image.
- LCD
projector – this type uses one or three LCD screens to create the
image. Either there is one LCD and a color wheel rotating through the
colors to build up an image on the screen, or there is light engine with
dichroic mirrors or prisms splitting the lamp light into the three primary
colors, routing each to a dedicated LCD. Single-LCD units are smaller and
cheaper than the triple-LCD units, but have a lower image quality and
produce 2/3rds less light.
- DLP
– a digital light processing projector uses micromirror technology. A
silicon chip has a field of microscopic mirrors representing every pixel
in the image. A light passes through a color wheel and the reflected image
is created by all of the mirrors needed to reflect each active pixel in
the picture. The key advantage here is that the light used can be much
more intense, as it is reflecting off of the device instead of passing
through it, as with an LCD. Also, non-polarized light can be used,
increasing the projector’s efficiency, in contrast to an LCD, which needs
polarized light to operate.
- LCoS
– liquid crystal on silicon technology is reflective like DLP, so it
has the advantage of being able to handle a higher intensity light as
well. It still requires polarized light, however, so pound for pound, it
isn’t as bright as a comparable DLP device. The advantage with this
technology is that current technology enables them to have a higher pixel
density than DLP, so LCoS chips can be made smaller with the same
resolution. In addition, since LCoS devices are made by multiple
manufacturers (DLP is the sole property of Texas Instruments), price
competition should bring lower prices faster.