SIM University 2014

• We will learn about projector technology by exploring some of the essential inner components of digital projectors.
• We will discuss how video signals are processed by projectors with an emphasis on the operations that are important for simulation.
• We will discuss how light is transformed into an image on a screen, including a discussion of different kinds of light sources.
• • We will cover the basics of the three main projection technologies in use today: LCD, LCoS and DLP.
SIM University 2014

Light Engine w/ Micro-display Panels
Lens
Low-Voltage
Power Supply
(LVPS)
Lens
Mount
Frame
Light
Source
Input Panel
Fans
Processing
Electronics
Light Source
Power Supply
Skins
SIM University 2014

• A projector’s primary function is to input video signals in a variety of formats and output a focused image onto a projection screen.
• In order to do this it has to perform a number of supporting functions:
• Turn on/off and regulate the brightness of a lamp or other light source.
• Focus the light in a uniform manner onto the micro-display panel(s),
• Accept and process video signals into red, green and blue component images in the pixel format of the panel(s) and send them to their respective panel(s) with defined timing.
•
Modulate the light hitting the panel(s) with the image data.
• Combine the red, green and blue images and focus an enlarged image onto a screen.
• Accept commands for configuration and operation.
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• Image related
• Pixel format (H x V)
• Light output (brightness)
•
Contrast ratio (black level)
• Color gamut
• Others
• Power
• Size, shape and weight
• Appearance (Industrial design)
• User interface
• Projector environment
• Reliability
• Regulatory compliance
• Cost

• Pixel format
•
Horizontal x Vertical pixels
• Pixel aspect ratio (square or rectangular)
• Display aspect ratio (width
÷ height)
• Grayscale
• Dynamic range: The range from black to white
• Brightness or luminosity (light level of peak white)
• Black level (light level of black)
• Contrast ratio (brightness
÷ black level)
• Resolution: How finely the range from black to white is divided
• Number of bits (digital) or signal-to-noise ratio
(analog)
• Transfer curve, bits or voltage in to light out (“gamma”)
• Color of white (“color temperature”)
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• Color
•
Color space (e.g., RGB, YCbCr)
• Co-ordinates of primary colors (defines the gamut)
• Display-dependent (only) parameters
• Resolution
→ this is independent of the projector’s pixel format
→ could be lower!
• Area uniformity (black and white, color)
• Image refresh method (raster-scanned or other)
• Refresh rate (At what rate are images redrawn?)
• Duty cycle (ratio of On time to Off time during each refresh)
• Persistence (nature of image decay)
• And for moving images...
• Update rate (How fast can the content change?)

• Video signals need to be processed for output to the micro-display panels.
• Processing can be divided conceptually into front-end and back-
end portions, which may circuit boards.
• All video input is ultimately converted to red, green and blue image components for display.
Video input signals
Front-end processing
Back-end processing
SIM University 2014

• Every color a typical display produces is a combination (sum) of red, green and blue light – the “primary” colors – in controlled amounts. For example:
• Red + green = Yellow
• Red + blue = Magenta
• Blue + green = Cyan
• Equal parts Red + Green + Blue = White
• • Typical (three-color) digital projectors derive red, green and blue light by filtering a white light source (e.g., Xenon or UHP lamp).
• Newer light-source technologies for projectors (e.g., LED, laser) can produce red, green and blue light directly.
• Some projectors also filter out (or directly produce) what are normally considered “secondary” colors – cyan, yellow and sometimes magenta for a five-color or six-color system – or add other wavelengths of light such as infrared (IR).
SIM University 2014

• A “horseshoe”-shaped edge contains the “pure” spectral colors (as in a rainbow).
• The visible colors are those within the area bounded by the “horseshoe”.
• As colors get closer to the
saturate towards white.
• The black line passing through the central region is called the “black body” curve and contains a range of colors for white called
Color Temperatures.
SIM University 2014

• The color gamut of a display comprises all the colors within the triangle defined by its three primaries (or polygon for more than three primaries).
• Different color points for the primaries give different gamuts.
• Examples of industry-standard
EBU for SDTV, and ITU-R
BT.709 (Rec 709) for HDTV.
• The closer the primaries are to the “horseshoe”-shaped edge the larger or “wider” the gamut, allowing reproduction of more deeply saturated colors.
SIM University 2014
The Color Gamut Menagerie

• When using a white light source
(lamp) the red, green and blue primaries are created by passing the white light through filters, resulting in fairly broad spectral regions.
• Producing highly saturated colors requires avoiding overlap between the regions, which is done by filtering out more light
→ a tradeoff.
• LED spectral emittance curves are narrower (purer), which places the primary color points closer to the
“horseshoe” edge (more saturated).
• Laser light sources are nearly monochromatic (a few nanometers wide), which produces near-fullysaturated RGB primaries.
Typical Lamp-based Spectrum
Typical LED-based Spectrum
Typical Laser-based Spectrum
SIM University 2014

• The path from video input up to gamma correction is called “frontend” processing.
• Transmitting the processed RGB digital video to the panels is considered “back-end” processing.
Analog
Digital
Video
Switch
Digitize
Video
Decoding
Deserialize
Switch
De-
(if needed)
Frame Rate
Warping &
Edge
Blending
(Optional)
Gamma
Correction
Panel(s)
(Back-End Processing)
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• In its simplest form resizing converts incoming images with
X pixels by Y lines to M pixels by N lines.
• The process of mapping source pixels to display pixels inevitably reduces sharpness (fine image details) due to the requirement to spatially filter (interpolate).
• A practical requirement to perform resizing at the display’s frame rate limits the allowable complexity of the algo-
• Resizing algorithms differ in the quality of the final result and hence the number and severity of artifacts they produce.
• For the sharpest possible image avoid resizing altogether (NOT possible for edge-blended, curved screens).
Aliasing Blurring Edge Halo
SIM University 2014

Analog
Digital
Video
Switch
Switch
Digitize
Video
Decoding
Deserialize
Deinterlace
(if needed)
Resize
Convert
Frame Rate
Edge
Blending
(Optional)
Gamma
Correction
Panel(s)
(Back-End Processing)
SIM University 2014

• Warping is a sophisticated form of resizing that allows the incoming rectangular pixel grid to be transformed into a different shape.
• Most projectors lack this capability but some have it built-in; others allow it to be added as an option or use an external processor box.
• The simplest version allows correction only of keystoning (horizontal and/or vertical), not orthogonal to a flat screen.
• More advanced warping allows curved shapes, which is typically required when projecting onto curved screens.
• The most advanced systems include cameras or sensors that form closed-loop feedback systems for automated set-up and/or automated maintenance of alignment.
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• Edge blending allows the edges of projected images
(which may be warped into non-rectangular shapes) to gradually fade to black.
• The purpose is to allow adjacent images in a multi-projector mosaic to blend into one another without visible seams.
• Typically, the width of the blend region is adjustable. Advanced systems may allow the blend intensity function to be modified.
• • If black levels are not sufficiently dark, optical edge blending will be required in place of or in addition to electronic edge blending.
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Original Overlapped Images
Poorly Blended
Well Blended
SIM University 2014

Analog
Digital
Video
Switch
Switch
Digitize
Video
Decoding
Deserialize
Deinterlace
(if needed)
Resize
Convert
Frame Rate
Edge
Blending
(Optional)
Gamma
Correction
Panel(s)
(Back-End Processing)
SIM University 2014

• This converts front-end processed image data into a form compatible with the micro-display panel(s) and transfers the converted data to the panel(s).
• The circuitry and exact functions are highly dependant on the technology used by the micro-display panels:
• LCD (Liquid Crystal Display)
• • LCoS (Liquid Crystal on Silicon)
• DLP (Digital Light Processing)
• May also control the time duration over which each frame of the source video is displayed in order to improve motion rendition of the system
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LCD
LCoS
DLP

• For sample-and-hold type displays (LCD, LCoS and
DLP) motion in the image can appear blurred at frame rates lower than around 60fps.
• This is caused by the eye sweeping across each motion while the frame is being held.
Impulse Response Sample & Hold
Impulse Response (CRT) Sample & Hold (LCD, LCoS, DLP)
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Time

• AccuFrame is a Christie technology for DLP projectors that greatly reduces the blurring of fine features during fast motion (other DLP manufacturers do it too).
• It compresses the active display time into a shorter period, which emulates more closely an impulse-driven display such as a CRT.
• Also introduces flicker in the image because of the and reduces brightness in direct proportion.
• A mechanical shutter achieves the same effect for LCD and LCoS projectors.
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• Regardless of the micro-display panel technology used projection requires a common series of optical processing operations.
• Some operations may take place in a different order than shown.
• Some operations may take place in the same component (e.g., a
TIR prism does both color separation and color combining).
Production
(e.g., lamp)
UV and IR
Control
Light
Shaping
Color
Separation
Image
Modulation
Screen
Image
Projection
Light
Output
Control
RGB
Image
Combining
Eye/brain
SIM University 2014

• Most projectors today still use highintensity discharge (HID) lamps.
• Mercury vapor and Xenon are the most common HID technologies today.
• Mercury vapor is typically for smaller projectors, although using multiple lamps (up to six) is becoming popular:
• • Low power: 100 – 450 watts
•
Long life: 4,000 – 10,000 hours
• Red deficient and spiky spectrum
Relative
Strength
1.0
0.8
• Xenon primarily for larger projectors:
• Higher power: 500 – 7,000 watts
• Shorter life: 500 – 3,000 hours
• Wide, relatively flat spectrum
0.6
0.4
0.2
400
Lamp Spectral Comparison
Natural Sunlight
Xenon
UHP
500 600
Wavelength [nm]
700 800
SIM University 2014

• Solid-state LEDs replace lamps when at least three (red, green and blue) are used.
• Some projectors use additional LEDs to increase gamut (e.g., yellow, magenta) or to provide additional spectral channels
(e.g., infrared LED in Christie Matrix StIM).
• Primary advantages over lamps:
• • Purer RGB primary colors resulting in a wider gamut
• Longer life (>>20,000 hours)
• Fast turn-on and warm-up
• Less light drop per unit time
• Allow much faster switching between component colors in single-chip systems
• No ultra-violet (UV) light to deal with
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Gamut Comparison

• Separate red, green and blue lasers
• “Pure” laser projectors scan the laser beams to produce an image
→ few examples (e.g.,
Microvision SHOWWX+ pico-projector).
• Most laser projectors use lasers as light sources to illuminate micro-display panels.
• The primary advantage of a laser over an HID efficiency of light collection.
• A large color gamut (even larger than LED) is a secondary benefit.
• Challenges facing laser projection include high cost, the need for efficient cooling, and the visibility of “speckle” from coherence.
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• Hybrid illumination combines LEDs, lasers and phosphors to produce red, green and blue light.
• Several methods are used today:
• LED or laser-illuminated phosphor for red
• Laser-illuminated phosphor for green
• • Laser or LED for blue
• Green (and red in one system) is produced by a phosphor-coated wheel illuminated by numerous blue lasers similar or identical to the kind used in Blu-ray Disc players.
• Hybrid illumination currently achieves up to ~10,000 lumens.
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Method A (e.g., Casio)
Method B (e.g., Viewsonic)

Light
Production
(e.g., lamp)
UV and IR
Control
Light
Shaping
Color
Separation
Image
Modulation
Screen
Image
Projection
Light
Output
Control
RGB
Image
Combining
Eye/brain
SIM University 2014

• In lamp-based LCD, LCoS and 3-chip DLP systems, multiple dichroic (“two color”) filters/mirrors, which reflect one part of the spectrum while transmitting the rest, are used to separate out red, green and blue spectral bands into physically separate beams to impinge on their respective micro-display panels.
• In lamp-based, single-chip DLP systems a motorized filter wheel separates red, green and blue sequentially in time (additional
Dichroic Color Separation (LCD & LCoS)
SIM University 2014
Seven Segment Color Wheel

Light
Production
(e.g., lamp)
UV and IR
Control
Light
Shaping
Color
Separation
Image
Modulation
Screen
Image
Projection
Light
Output
Control
RGB
Image
Combining
Eye/brain
SIM University 2014

• The video processing and illumination paths meet at the light engine.
• Red, green and blue colors from a light source are shone onto respective panels (or a single panel sequentially) where they are
modulated by corresponding image data transmitted to the panel(s).
• • The modulated images are then combined to produce a full-color image that can be magnified and projected onto an external screen.
• Light engines differ greatly depending on the display panel technology:
LCD, LCoS, one-chip DLP or threechip DLP.
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• A thin layer of liquid-crystal material is sandwiched between two glass plates, each coated with a transparent conductor, typically indium-tin oxide (ITO).
• By applying an electric field
(voltage) across the plates the liquid-crystal molecules can be thereby changing the polarization of light passing through.
• By using sheets of polarizing material on either side of the LCD panel a change in polarization is converted to a change in light transmission (gray level).
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• Pixels on the panel are addressed by row and column electrodes that activate poly-silicon thin-film transistors (TFT) printed on quartz glass.
• “Fill factor” is the percentage of the pixel area that is not blocked by electrodes and transistors.
• Dichroic filters separate the white light from the lamp into red, green and blue beams and guide them to their respective panels.
Integrator
• A prism assembly (cube) uses dichroic filters to combine the modulated light from the LCD panels into a full color image.
First-surface
Mirror
Lamp
W
Lens
B
G
G+B
R
Dichroic mirror
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LCD
Projection
Lens
Dichroic prism cube
R,G,B

Least expensive panel technology
There can be image smearing on fast motion if the response time of the liquid-crystal (LC) material is insufficiently fast.
LCD absorbs some light, which can cause photo-degradation of the LC material from ultraviolet (UV)
→ this problem has been significantly reduced with the use of inorganic alignment layers.
Poorer contrast ratio than LCoS or DLP (can’t turn pixels off fully)
(typically 50% to 60%)
LCD is effectively driven by analog voltages, which can lead to grayscale and color non-uniformities across the image.
DLP display after 3,000+ hours
LC based display
(without inorganic alignment layers) after 3,000+ hrs
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• LCoS has the same essential operation as projection LCD except it is reflective, not transmissive.
• Light enters from one side, passes through the liquid crystal, bounces off a reflective, aluminum electrode then
Silicon
Substrate again on its way out in the opposite direction it came in.
• Drive transistors and address lines are “hidden” below the reflective electrode where they do not block light (unlike an
LCD).
Driver
Electronics Reflective Electrode
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Light

Very high fill factor (typically 89% to 93%) since transistors are behind the mirror surface
→ smooth images
Can draw heat from the back of the device, which is much better than drawing only from the edges as with LCD
Uses inorganic-alignment layers and VAN mode LC
Much higher contrast ratios than LCD
Much faster response time than LCD because cell thickness is less (typically
1/2)
→ no perceptible image smearing
Scalable to very high resolutions
→ to 8,192 x 4,320 in the lab up o Softer image than DLP with lower pixel-topixel contrast due to “fringe-field” effect
Brightness- and color- non-uniformities caused by variations in LC layer
SIM University 2014

• The liquid-crystal layer in LCD and
LCoS devices is continuous.
• Pixels are defined by the electric fields under individual electrodes.
• The field for a given pixel’s electrode cannot be shaped to be totally uniform over the area of the electrode nor to stop precisely at
Electric field interaction between pixels
• The result is usually overlap and interference between the electric fields of adjacent pixels.
• This reduces both pixel sharpness and contrast at high spatial frequencies (pixel to pixel).
Alternating on/off columns on LCoS projector
(top) vs. DLP projector (bottom)
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• LC pixels are driven by analog voltages or pulse-width modulated
(PWM) signals that the LC material responds to in an analog manner.
• As a result, variations in the response of the LC material across a panel can cause differences in the brightness of pixels in different
• Non-uniformities are easily visible on full black or white fields.
• Small differences in the black level are very obvious due to the eye’s non-linear response to brightness.
• Some LCoS projectors compensate with electronic correction.
Non-uniformities in a white field
Non-uniformities in a black field
(exaggerated)
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• Digital Light Processing (DLP ® ) was invented by Dr. Larry
Hornbeck at Texas Instruments in 1987.
• DLP uses a chip called a Digital Micromirror Device (DMD), which contains anywhere from about 400,000 to nearly nine million square, aluminum mirrors, one for each pixel.
• In currently available devices the pixel pitch is roughly 1/10 to 1/5 the diameter of a typical human hair.
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• Each mirror is mounted on a hinge hidden located underneath, which allows it to flip to either side of perpendicular.
• DMDs are true digital devices since each mirror only has two possible states while operating: titled 10 or 12 degrees from perpendicular in one direction (On) or titled the same amount in the opposite direction (Off).
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• The mirrors tilt because a voltage applied to the pixel from a memory cell on the chip causes an electrostatic attraction of the landing tips to the substrate on one side of the device or the other.
• Light from the lamp is thus either directed into a projection lens or toward a light absorber (light trap).
DMD Unit
Memory Cell
SRAM
Row En.
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(animation)

• Since DLP is digital how does it render shades of gray between black and white? Answer: Pulse Width Modulation (PWM).
• Each mirror vibrates at a rapid rate, flipping back and forth between two extreme positions thousands of time per second, too fast for us to see as individual pulses of light.
• Instead, we see an average light level determined by the ratio of on time to off time during each video frame (more on = brighter).
• The frame is divided into time slices and the binary word for a given grey level controls which slices are on and which are off.
• Example: 16 gray levels from 0 to 15 (0 = full black,
15 = full white) require four slices with each slice half as long as the previous.
Pulse Train for Gray Level of 11
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• Three-chip color uses a separate DMD for each RGB color component, combining the images from them optically before outputting from the projection lens.
• The DMDs are mounted to attached to a Total Internal
Reflection (TIR) prism.
• Efficient use of lamp light
→ best option for high brightness.
• Choose three-chip color if low cost is not a requirement.
Green DMD
Red DMD
TIR Prism
Blue
DMD
Projection
Lens
Formatter
Boards Xenon lamp source after integrating rod
Typical Three-chip DLP Light Engine
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• Single-chip color is also known as field-sequential color.
• With a lamp-based system, white light passes through a rapidly spinning filter wheel containing red, green and blue sectors (at a minimum).
• Image data for each color is displayed in succession on the DMD in synchronization with the wheel.
• If cycled fast enough (
≥
1 the eye melds the sequential colors into a full-color image.
• Less expensive than threechip color but is less efficient with light and can exhibit unique color-switching
“rainbow” artifacts.
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• At least three LEDs are used:
Red, Green and Blue
• Some projectors use additional
LEDs for yellow, magenta, or infrared (e.g., Christie Matrix
StIM)
• Light from the LEDs are combined using dichroic filters.
• • The LEDs are electronically switched on and off in sequence, hence…
• A color wheel is not required
• Much faster color-cycling speed, typically 8 to 24 cycles per video frame
→ greatly reduces or eliminates color switching artifacts.
LED Light Module in Christie Matrix StIM
LED Light Engine in Samsung DLP RPTV
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• No smearing from fast motion in the device (because
DMD’s are very fast, switching in microseconds)
• High fill factor (85% to 90%)
→ smooth images
• Because DLP is entirely digital, grayscale and color across the image can be highly uniform (need only produce full on and full off).
• Image characteristics are inherently stable with temperature.
• • No liquid-crystal material to heat up and/or degrade with UV.
• High reliability (> 100,000 hours)
• Can be scaled from tiny single-chip projectors (e.g., pico-projectors) to three-chip systems with extremely high light outputs (e.g., 70,000+ lumens).
4K Chip & WXGA Pico Chip
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• Lamp-based, single-chip systems:
•
Color-wheel “rainbow” artifact
• Typically have poorer color rendition or lower light output per watt.
• Three-chip systems are more expensive than LCoS or LCD.
• Harder to scale up resolution than
LCoS
→
4,096 x 2,160 is the largest pixel format for the foreseeable future.
• Artifacts due to PWM integration such as false, transient gray levels in gradients (content dependent).
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Color Wheel “Rainbow” Artifact
Color Wheel “Rainbow” Artifact

Light
Production
(e.g., lamp)
UV and IR
Control
Light
Shaping
Color
Separation
Image
Modulation
Screen
Image
Projection
Light
Output
Control
RGB
Image
Combining
Eye/brain
SIM University 2014

• Typical projection lenses use many elements (glass to prevent thermally induced distortion) and are over-sized to allow horizontal and vertical image shift.
• Control of image shift (horizontal and vertical) is provided by the lens mount, typically using DC or stepper motors.
• • Zoom and focus are typically controlled by motors on the lens itself.
Typical 3-chip DLP
Ten Element Projection Lens Prism Panel
Typical 3-chip DLP Lens Mount
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• The primary performance metric for a projection lens is its optical resolution, which is characterized by the Modulation
Transfer Function (MTF).
• MTF is a curve that essentially describes how contrast varies with spatial frequency.
• Due to various optical aberrations in the
MTF can vary in different areas of the image.
• In addition, the presence and quality of coatings on lens surfaces can impact light throughput and contrast performance.
• Hence, lens performance is usually a prime contributor to overall system resolution, contrast and overall image quality.
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Light
Production
(e.g., lamp)
UV and IR
Control
Light
Shaping
Color
Separation
Image
Modulation
Screen
Image
Projection
Light
Output
Control
RGB
Image
Combining
Eye/brain
SIM University 2014

• A projector requires something to focus its enlarged image on, either a front- or rear- projection screen.
• Screens depend on the effect of optical scattering (diffusion).
• A perfectly lambertian front-projection screen scatters (diffuses) light equally
• A screen with “gain” scatters more light in certain directions than in others, typically favoring the direction normal to the surface.
• Gain produces brighter images for viewers directly in front of the screen but dimmer for viewers to the side.
Gain vs. Viewing Angle
(Blue line is a perfectly lambertian screen)
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• Used when significant immersion is required (e.g., flight simulation).
• Most curved screens, especially domes, typically require multiple projectors, each covering a portion of the screen area and edge-blended to eliminate seams.
• Curved screens require greater depths of focal surfaces that are similarly curved.
• A key problem is cross reflection, which greatly reduces the achievable contrast ratio by scattering light back to the screen
→ partially solved by using a screen surface with a gain less than unity
(e.g., a gray surface).
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JTAC Immersive Dome

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