Projection Displays

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Projection Displays

• OPT 696D

2
Contents

• Requirements
• Inputs and Outputs
• Sub-Systems
• Conservation of étendue
• Approaches and tradeoffs

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Requirements

• Cost
• Size and weight
• Noise
• Good looking image
• ANSI lumens
• Contrast
• Resolution
• Color gamut
• Subjective appearance
• Size and location of image

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Inputs and Outputs

• Inputs
• Power
• Video signal
• Focus/zoom inputs
• Image quality controls

• Outputs
• An image
• Audible noise

5
Sub-Systems

• Electrical
• Thermal
• Mechanical
• Safety
• Optical

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Electrical Sub-System

• Most inputs are electronic
• This sub-system overlaps with the optical at the
  light valve
• Includes the menu system for settings
• Often includes buttons on housing as well as a
  remote control
• Controls thermal sub-system

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Thermal Sub-System

• The light source generates significant heat
• The sources temperature can significantly impact
  its life and performance
• Some types of light valves are more sensitive
  than others to temperature
• Some liquid crystals turn into just liquids
  around 80oC

8
Mechanical Sub-System

• This is what holds everything in place
• One of the few areas where there is some control
  of cost
• How loose are the assembly tolerances?
• What compensators are appropriate?

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Safety

• Lamp needs high voltage to start
• Lamp gets hot enough to cause burns
• One failure mode of the lamp is explosive
• Many lamps emit a significant amount of UV

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Optical

• Light source
• Illumination system
• Spatial/angular control
• Color separation
• Polarization
• Light valve
• Color combination (in some cases)
• Projection lens
• Screen (in some cases)

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Conservation of Étendue

• Start with a differential area and differential
  solid angle (i.e. differential étendue ?A1 ??1)
• There will be some flux in this étendue
• Propagate the light through an optical system and
  the flux will be contained in a new differential
  étendue
• ?A1 ??1 ?A2 ??2

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Conservation of Étendue, Cont.

• The assumptions are that a deterministic ray
  trace can fully describe the system
• No scatter, diffraction or beamsplitters
• Essentially the same thing as the Lagrange
  invariant or conservation of radiance
• Each wavelength and orthogonal polarization state
  can be thought of as a separate source

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Throughput vs. Étendue

• I define throughput as the A? product for finite
  areas and angles
• The throughput for a F/2 lens and ½ in CCD is
  6.47 mm x 4.80 mm x ?sin2(14.5 deg)
• It is conceivable to not conserve throughput in a
  deterministic system
• Use throughput because it is simple to calculate

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Why Is This Relevant

• Some portion of the optical system will have the
  lowest étendue
• This should be set by the active area of the
  light valve and the NA of the projection lens
• You can only use the lumens from the source that
  are contained in the same amount of étendue
• Anything outside this étendue can not be used

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Light Source

• You need a source that has lots of lumens in a
  small étendue
• The most common choice is a short arc, mercury,
  high intensity discharge lamp

• Typical arc gap is approximately 1.2 mm for
  7,000 lm at 120 W
• 250 W lamps put out up to 15,000 lm

Image from http//www.lighting.philips.com
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Spectral Distribution

• Mercury lamps do not have uniform spectral
  distribution
• They typically are red deficient

Image from http//www.lamptech.co.uk
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LEDs for Projection

• Luminus makes LEDs specifically for the
  projection display market
• Phlatlight PT120 is largest LED set
• 12 mm2 area for each die
• Not quite Lambertian
• Over 3,000 lm from set

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Illumination System

• A 43 aspect ratio rectangle has about 61 of the
  area of the circumscribed circle
• A 169 aspect ratio rectangle has about 54 of
  the area of the circumscribed circle
• Must form a uniform, rectangular spot
• Spot must be aligned to the light valve
• How much should the spot be oversized?

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How To Get a Rectangular Spot

• Light pipe
• Source is imaged into input of rectangular
  lightpipe
• If lightpipe is long enough, output is spatially
  uniform
• Output surface is imaged onto light valve
• Flys eye integrator

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Lightpipe Approach

• For modestly large angles, small changes in the
  input location result in large changes in the
  output location
• For straight sides, the angular distribution does
  not change

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How Do System Requirements Drive Lightpipe Design?

• Size of reflector on lamp?
• Size of input and output?
• Length of lightpipe?
• Size of light valve?

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Flys Eye
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Flys Eye Description

• Input into two lens arrays is narrow angle beam
• There is one-to-one mapping from the first array
  to the second
• All of the light from one element in the first
  array goes through its matching lens in the
  second array
• The lens in the second array images the aperture
  of the first lens to infinity
• A monolithic lens images all of the apertures
  onto the light valve

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Schematic Layout of Flys Eye
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How Do System Requirements Drive Flys Eye Design?

• Size of reflector on lamp?
• Number of lens elements?
• Spacing of arrays?
• Size of light valve?

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Light Valves

• This is the device that spatially modulates the
  light to form the image
• Three technologies are currently viable
• High Temperature Poly-Silicon LCDs (HTPS or just
  LCD)
• Digital Micro-mirror Device (DMD)
• Liquid Crystal on Silicon (LCoS)
• Single most expensive item on bill of materials

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HTPS LCD

• Transmissive device
• Think of each pixel as an independently
  adjustable wave plate
• Polarizer on each side of device
• Illumination should not depart significantly from
  telecentric
• Switching times support video rates
• Up to 1.8 diagonals

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DMD

• Manufactured by Texas Instruments
• A MEMS device where each pixel is a mirror that
  can tilt to an on or off state
• In the on state, the mirror reflects light from
  the source into the entrance pupil of the
  projection lens
• In the off state, the mirror reflects the light
  out of the pupil
• Grey scale achieved with binary pulse width
  modulation
• Up to 0.9 diagonals

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DMD, Cont.

• Graphic of DMD concept

• TIR prism used to get light to and from DMD

Image from http//www.oerlikon.com
Image from http//focus.ti.com
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LCoS

• Reflective LCD
• Can put processing on LCoS chip
• Potential for high resolution
• Late to the market so not as mature

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Color Approaches

• Three panel
• Three separate light valves where the images are
  combined downstream
• Field sequential
• Each color image is shown sequentially
• The colors are switched faster than the
  integration time of the eye

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Three Panel

• Key feature is X cube
• A four piece prism with two different coatings

Image from http//www.oerlikon.com
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Field Sequential

• Color wheel is most common
• Rotating disk with different colors that is
  synchronized to video image
• With LEDs, turn them on and off as needed

Image from http//www.oerlikon.com
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Polarization Conversion

• LCDs can only use one polarization state
• If the source étendue is smaller than the rest of
  the system, it is possible to gain from a
  polarization conversion system (PCS)
• Adds cost and complexity, but gives you more
  lumens

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How Does a PCS Work?
PBS
Fold mirror
½? plate
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PCS and Flys Eye
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What About Étendue?

• You can not superimpose two identical sources in
  both space and angle
• The different polarization states are two
  separate sources
• When you separate them and switch the
  polarization of one, you double the étendue of
  the source

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Is a PCS Worth It?
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Projection Lens

• Typically fast
• F/1.5 for LCD and F/2 for DMD
• Large BFD through moderately high index glass
• Some longitudinal chromatic aberration may be
  acceptable in a three panel system
• Wide FOV
• Wider for optical keystone correction
• Zoom
• Cant depart from telecentric by too much

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Screens

• Screens are rated with a gain
• Gain is the ratio of the on axis radiance
  generated by the screen compared to a Lambertain
  diffuser
• Screen gain completely ignores angular
  distribution
• Front projection screens are easy
• Rear projection screens are hard

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Front Projection

• Relatively easy to get a white screen
• Uniformity with angle is usually good
• Typically viewed under well controlled lighting
  conditions
• People are used to turning off the room lights
• Resolution is typically not an issue

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Rear Projection

• Tradeoff between a white screen, uniformity,
  resolution and efficiency
• Lighting environment is typically not as well
  controlled
• Tinted substrate can provide contrast enhancement
  at the cost of lumens
• Lenticular screens often used to direct light
• High resolution applications can result in
  speckle