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Projection Displays
OPT 696D
Contents
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Requirements
Inputs and Outputs
Sub-Systems
Conservation of étendue
Approaches and tradeoffs
Requirements
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Cost
Size and weight
Noise
Good looking image
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ANSI lumens
Contrast
Resolution
Color gamut
Subjective appearance
Size and location of image
Inputs and Outputs
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Inputs
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Power
Video signal
Focus/zoom inputs
Image quality controls
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Outputs
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An image
Audible noise
Sub-Systems
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Electrical
Thermal
Mechanical
Safety
Optical
Electrical Sub-System
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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
Thermal Sub-System
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The light source generates significant heat
The source’s temperature can significantly
impact its life and performance
Some types of light valves are more sensitive
than others to temperature
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Some liquid crystals turn into just liquids around
80oC
Mechanical Sub-System
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This is what holds everything in place
One of the few areas where there is some
control of cost
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How loose are the assembly tolerances?
What compensators are appropriate?
Safety
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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
Optical
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Light source
Illumination system
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Spatial/angular control
Color separation
Polarization
Light valve
Color combination (in some cases)
Projection lens
Screen (in some cases)
Conservation of Étendue
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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
Conservation of Étendue,
Cont.
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The assumptions are that a deterministic ray
trace can fully describe the system
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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
Throughput vs. Étendue
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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
Why Is This Relevant
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Some portion of the optical system will have
the lowest étendue
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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
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Anything outside this étendue can not be used
Light Source
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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
Spectral Distribution
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Mercury lamps do not have uniform spectral
distribution
They typically are red deficient
Image from http://www.lamptech.co.uk
LEDs for Projection
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Luminus makes LEDs specifically for the
projection display market
Phlatlight PT120 is largest LED set
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12 mm2 area for each die
Not quite Lambertian
Over 3,000 lm from set
Illumination System
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A 4:3 aspect ratio rectangle has about 61% of
the area of the circumscribed circle
A 16:9 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
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How much should the spot be oversized?
How To Get a Rectangular
Spot
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Light pipe
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Source is imaged into input of rectangular
lightpipe
If lightpipe is long enough, output is spatially
uniform
Output surface is imaged onto light valve
Fly’s eye integrator
Lightpipe Approach
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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
How Do System Requirements
Drive Lightpipe Design?
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Size of reflector on lamp?
Size of input and output?
Length of lightpipe?
Size of light valve?
Fly’s Eye
Fly’s Eye Description
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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
Schematic Layout of Fly’s Eye
How Do System Requirements
Drive Fly’s Eye Design?
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Size of reflector on lamp?
Number of lens elements?
Spacing of arrays?
Size of light valve?
Light Valves
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This is the device that spatially modulates the
light to form the image
Three technologies are currently viable
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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
HTPS LCD
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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
DMD
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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
DMD, Cont.
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Graphic of DMD
concept
Image from http://focus.ti.com
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TIR prism used to get
light to and from DMD
Image from http://www.oerlikon.com
LCoS
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Reflective LCD
Can put processing on LCoS chip
Potential for high resolution
Late to the market so not as mature
Color Approaches
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Three panel
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Three separate light valves where the images are
combined downstream
Field sequential
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Each color image is shown sequentially
The colors are switched faster than the integration
time of the eye
Three Panel
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Key feature is X cube
A four piece prism with two different coatings
Image from http://www.oerlikon.com
Field Sequential
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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
Polarization Conversion
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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
How Does a PCS Work?
PBS
Fold
mirror
½ plate
PCS and Fly’s Eye
What About Étendue?
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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
Is a PCS Worth It?
5000
4500
4000
24 mm^2
212 mm^2
589 mm^2
1155 mm^2
1909 mm^2
2851 mm^2
0.7", F/2
0.9", F/1.5
0.7", F/2 PCS
0.9", F/1.5 PCS
Lumens
3500
3000
2500
2000
1500
1000
500
0
0
50
100
throughput (mm^2 sr)
150
Projection Lens
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Typically fast
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Large BFD through moderately high index glass
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Some longitudinal chromatic aberration may be acceptable
in a three panel system
Wide FOV
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F/1.5 for LCD and F/2 for DMD
Wider for optical keystone correction
Zoom
Can’t depart from telecentric by too much
Screens
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Screens are rated with a gain
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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”
Front Projection
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Relatively easy to get a white screen
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Typically viewed under well controlled lighting
conditions
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Uniformity with angle is usually good
People are used to turning off the room lights
Resolution is typically not an issue
Rear Projection
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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