Chip Magazine - December 2006 Issue

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					 December 2006     Volume 4   Issue 1     Rs 100




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 HARDWARE         INFOGRAPHICS




Image Projectors
Projectors are not restricted to conference rooms anymore. A look inside to see
what makes them tick BY ASHUTOSH DESAI

B   y definition, a projector is an
    optical device that projects an
enlarged image onto a screen.
                                                   LCD Projectors                                                  LCD Panels
                                                                                                                    Each LCD panel blocks
   The projector is provided with an                                                                                differing amounts of light. An
input signal—which could be video                  Dichroic Mirrors                                                 LCD panel measures 0.9-inch
from a computer or a DVD player.                   A lamp acts as the light source in the LCD projector.            in size. Such a small area
This video signal is processed into                This light is split up into the primary colors – Red,            contains 786,000 pixels (for
an image that is projected onto                    Blue and Green. This is done with ‘dichroic mirrors’.            a regular XGA projector).
the screen. Before understanding                   Dichroic mirrors selectively reflect certain                      There are three LCD panels.
projectors treat a video signal to                 wavelengths of light. This is because the mirrors                Therefore over 2 million pixels
light up a screen with an image, let’s             are placed at an angle to the incident light. The Red            can be controlled to produce
simplify the process. A projector                  dichroic mirror reflects Blue-Green wavelengths                   an output.
consists of a circuit that converts                of light. In this manner, the mirrors reflect light and
incoming video signal to digital—if the            direct the RGB channels towards their respective
input is analog. The resulting digital             LCD panels.
signal is sent to another part of the
projector known as the ‘light engine’.
This is where a light source directs                               Microfine Grid
a beam towards a color splitting                                   Since the pixels are packed so closely, there is bound to be some ‘mixing’.
component, goes through color filters                               To prevent this, a microfine grid is used to absorb some of the incident light
and complex optical elements.                                      to prevent the pixels from ‘bleeding’ light into their neighbors. This results
   Different methods and technologies                              in a good quality image but with lower intensity. The grid also produces a
are used to transform this video signal                            ‘screen door effect’. When the image is sharpened, the pixels become more
into light.                                                        visible and appear to be within boxes.
   There are two types of projectors
that are used extensively today. They
are either ‘transmissive’ or ‘reflective’.                                                                                             R,G,B LCD
From these names one can gain an                                                                                                        Panels
understanding on the method used                         Dichroic
to treat light within a projector.                       Combiner Cube
                                                                                                                                             Mirror
   A transmissive projector contains
components that allow light to
pass through it. These components
might use CRT or LCD technology                            Mirror
to form the final image Here only
the LCD projector will be explained
                                                                                                                          Dichroic Mirror
in detail as it has grown more
                                                                                                                                (B)
popular due to its portable nature.                                                                            Dichroic Mirror
   A reflective projector makes use of                                                              Dichroic Mirror (G)
tiny movable, mirror-like components                                                                    (R)
to reflect light in the device. The
reflective projector usually uses
DLP technology to form the final                     Micro Lens Array
image. Projectors that use LCOS                     To prevent the screen door effect, a micro lens array is placed
technology use both transmissive                    before the microfine grid. This lens array ensures that the light
as well as reflective techniques to                  falling on the LCD panel is transmitted with almost the same
form an image on the screen.                        intensity through the grid, reducing this effect.


108 | INTELLIGENT COMPUTING CHIP | DECEMBER 2006
                                                                                                                   INFOGRAPHICS       HARDWARE




    DLP Projectors
     Digital Micromirror Device (DMD)
     A Digital Light Processing projector uses some
     sophisticated electronics. It is fi tted with a Digital
     Micromirror Device (DMD), which is a chip. A
     DMD is made up of tiny (microscopic) mirrors
     that can tilt in two directions so as to reflect light
     incident on it (devices that have microscopic
     mechanical systems built into them are known as
     micro-electricomechanical systems). These tiny
     mirrors move in a particular direction to either
     allow light to reflect or not. This leads to the typical
     ‘On/Off’ situation. The DMD gets its instructions for
     positioning each mirror – there can be more than
     two million of them – from the digital video signal.
     Information relayed to each mirror changes over a           Color Wheel
     thousand times every second. The light reflected              The light source in a DLP projector shines light through optical elements
     by the mirror (depending on whether it is on or off)         that converge a beam onto a spinning color wheel. The color wheel (usually
     produces 1024 shades of gray.                                made up of four segments – R, G, B and a clear portion) splits the beam into
                                                                  red, green and blue. The filtered light once again goes through another set of
                                                                  optics to finally reach the DMD chip. This filtering and the reflecting micro-
                                                                  mirrors are synchronized perfectly to produce the right shade. This setup
                                                                  does not produce the ‘screen door effect’ but is affected by something known
                                                                  as a ‘rainbow effect’. This is removed in high-end DLP projectors that have
                                                                  three DMDs – one for each color channel. Newer projectors also have 6 or
                                                                  7-segment color wheels, where the primary colors are repeated. This way
                                               Processor          the image reproduction is closer to the input signal and also reduces a faint
                                                                  multi-colored rainbow from appearing on the projector screen.
 DLP
Circuit
                                             Memory
Board
                                                               Projection
                                                                  Lens

  DMD




          Shaping Lens



             Color Wheel



                                          Condensing
                                             Lens                                  Light
                                                                                  Source




                                                                                                                  |
                                                                                                      DECEMBER 2006 INTELLIGENT COMPUTING CHIP   | 109

				
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