Fundamentals of Projection Technology - Christie · Fundamentals of Projection Technology 2 Alen...

Fundamentals of Projection Technology

2

Alen Koebel, Senior Design Qualification Specialist,Christie Digital

2SIM 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 This Seminar

33

• We will cover the basics of the three main projection technologies in use today: LCD, LCoS and DLP.

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What’s Inside a Typical Projector?

Low-Voltage Power Supply

(LVPS)

Frame

Lens

Lens Mount

Light Engine w/ Micro-display Panels

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Fans

Light Source

Processing Electronics

Light SourcePower Supply

Input Panel

Skins

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• 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), separated into red, green and blue (possibly more) spectral bands.

Projector Functions

55

separated into red, green and blue (possibly more) spectral bands.

• 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

Critical Specifications

66

• Size, shape and weight

• Appearance (Industrial design)

• User interface

• Projector environment

• Reliability

• Regulatory compliance

• Cost

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• 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)

Image Parameters

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• 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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Image Parameters• 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)

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• 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?)

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• 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 take place on different

Video Processing

Back-end processing

Front-end processing

Video input

signals

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take place on different circuit boards.

• All video input is ultimately converted to red, green and blue image components for display.

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• 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

Additive Color

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• 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).

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• 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 central region, they de-

CIE Chromaticity Diagram

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central region, they de-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.

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• 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 gamuts include SMPTE-C and

Color Gamut

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gamuts include SMPTE-C and 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.

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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 colorsrequires avoiding overlap between the regions, which is done by filtering out more light → a tradeoff.

Light Source Spectra

Typical Lamp-based Spectrum

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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 nanometerswide), which produces near-fully-saturated RGB primaries.

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Typical LED-based Spectrum

Typical Laser-based Spectrum

• The path from video input up to gamma correction is called “front-end” processing.

• Transmitting the processed RGB digital video to the panels is considered “back-end” processing.

Video Processing Path

Digitize De-interlace Resize Convert

Analog Video


Video Decoding

De-serialize

interlace (if needed)

Resize Convert Frame Rate

Switch

Digital Video

Video

Switch

Panel(s)Gamma Correction

(Back-End Processing)(Optional)

Warping & Edge

Blending

• 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).

Resizing (Scaling)

• A practical requirement to perform re-sizing at the display’s frame rate limits the allowable complexity of the algo-rithm for a given computational cost.


rithm for a given computational cost.

• 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

Warping and Edge Blending

Digitize

Video Decoding

De-interlace

(if needed)Resize Convert

Frame Rate

Analog Video

Switch

Warping &


De-serialize

Switch

Digital Video

VideoWarping &

Edge Blending



• 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), which occurs when the projection axis is

Geometric Warping

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which occurs when the projection axis is 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

Edge Blending

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• 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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Edge Blending Example

Original Overlapped Images


Poorly Blended

Well Blended

Back-end Processing

Digitize

Video Decoding

De-interlace

(if needed)Resize Convert

Frame Rate

Analog Video

Switch

Warping &


De-serialize

Switch

Digital Video

VideoWarping &

Edge Blending



Back-end Processing• 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)

LCD


• 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

LCoS

DLP

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• 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 frame to track the object’s

Motion Blur

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frame to track the object’s motion while the frame is being held.

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Impulse Response (CRT) Sample & Hold (LCD, LCoS, DLP) TimeP

ositi

on

Impulse Response Sample & Hold

• 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.

AccuFrame™

• Also introduces flicker in the image because of the lower duty cycle


lower duty cycle and reduces brightness in direct proportion.

• A mechanical shutter achieves the same effect for LCD and LCoS projectors.

Optical Processing Path• 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).

Light


Light Shaping

Color Separation

Image Modulation

Screen

Light Production (e.g., lamp)

UV and IR Control

Image Projection

RGB Image

Combining

Light Output Control

Eye/brain

Light Production: HID Lamps• Most projectors today still use high-

intensity 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

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• Low power: 100 – 450 watts

• Long life: 4,000 – 10,000 hours

• Red deficient and spiky spectrum

• Xenon primarily for larger projectors:

• Higher power: 500 – 7,000 watts

• Shorter life: 500 – 3,000 hours

• Wide, relatively flat spectrum

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Lamp Spectral Comparison

0.2

0.4

0.6

0.8

1.0

400 500 600 700 800

Natural Sunlight

Xenon

UHP

Relative Strength

Wavelength [nm]

• 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

LED Light Sources

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• 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 lamp (and even an LED) is a much greater

Lasers

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lamp (and even an LED) is a much greater 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

Hybrid Illumination

Method A (e.g., Casio)

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• 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 B (e.g., Viewsonic)

Color Separation

Light Shaping

Color Separation

Image Modulation


UV and IR Control


Screen Image Projection

RGB Image

Combining


Eye/brain

Color Separation

• 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 cyan, magenta, yellow or white segments are also possible).

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cyan, magenta, yellow or white segments are also possible).

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Dichroic Color Separation (LCD & LCoS) Seven Segment Color Wheel

Image Modulation

Light Shaping

Color Separation

Image Modulation


UV and IR Control



RGB Image

Combining


Eye/brain

The Light Engine• 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


• 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 three-chip DLP.

• 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 made to change their alignment

Liquid Crystal Display (LCD)

3333

made to change their alignment 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.

Liquid Crystal Display (LCD)

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• Dichroic filters separate the white light from the lamp into red, green and blue beams and guide them to their respective panels.

• A prism assembly (cube) uses dichroic filters to combine the modulated light from the LCD panels into a full color image.

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Dichroic mirror

Lamp LensLCD

Dichroic

IntegratorLens

First-surfaceMirror

prism cube

W

R

G+B

G

B

R,G,B

Projection

� 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)

� Pixels can be obvious: “Screen door” effect from low fill factor

LCD Pros and Cons

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� Pixels can be obvious: “Screen door” effect from low fill factor (typically 50% to 60%)

� LCD is effectively driven by analog voltages, which can lead to grayscale and color non-uniformities across the image.

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LC based display (without inorganic alignment layers) after 3,000+ hrs

DLP display after 3,000+

hours

• 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 passes through the liquid crystal

Liquid Crystal on Silicon (LCoS)

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passes through the liquid crystal 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).

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Reflective ElectrodeDriver Electronics

Silicon Substrate

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

LCoS Pros and Cons

� Much higher contrast ratios than LCD� Much faster response time than LCD

because cell thickness is less (typically


LCDLCD

LCoS & DLPLCoS & DLP

because cell thickness is less (typically 1/2) → no perceptible image smearing

� Scalable to very high resolutions → up to 8,192 x 4,320 in the lab

o Softer image than DLP with lower pixel-to-pixel contrast due to “fringe-field” effect

� Brightness- and color- non-uniformities caused by variations in LC layer

• 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 its edges.

The Fringe-Field Effect

Electric field interaction between pixels

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its edges.• 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).

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Alternating on/off columns on LCoS projector (top) vs. DLP projector (bottom)

• 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 areas.

LC Uniformity Issues

Non-uniformities in a white field

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areas.

• 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.

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Non-uniformities in a black field (exaggerated)

• 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.

Digital Light Processing™

• In currently available devices the pixel pitch is roughly 1/10 to 1/5 the diameter of a typical human hair.


hair.

• A DMD is a prime example of a Micro-Electro-Mechanical System (MEMS), which consists of electrically-actuated mechanical structures built up by a process called surface micromachining.

• 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).

How DLP Works


• 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).

How DLP Works


QQE

ND

SRAMBit

Line

Row En.

DMD UnitMemory Cell

(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).

DLP Grayscale

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• 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.

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• 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

• 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 Formatter/Satellite boards

Three-Chip Color

Red DMDTIR Prism

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Formatter/Satellite boards attached to a Total Internal Reflection (TIR) prism.

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Typical Three-chip DLP Light Engine

Blue DMD

Green DMD

Projection Lens

Xenon lamp source after integrating rod

Formatter Boards

• Efficient use of lamp light →best option for high brightness.

• Choose three-chip color if low cost is not a requirement.

• 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.

Single-Chip Color with Lamps

• If cycled fast enough (≥ 1 RGB cycle per video frame)


RGB cycle per video frame) the eye melds the sequential colors into a full-color image.

• Less expensive than three-chip color but is less efficient with light and can exhibit unique color-switching “rainbow” artifacts.

Single-Chip Color with LEDs• 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

LED Light Module in Christie Matrix StIM


• 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 → greatlyreduces or eliminates color switching artifacts.

LED Light Engine in Samsung DLP RPTV

• No smearing from fast motion in the device (becauseDMD’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.

DLP Advantages

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• No liquid-crystal material to heat up and/or degrade with UV.

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• 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

• 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.

DLP Challenges

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• Harder to scale up resolution than LCoS → 4,096 x 2,160 is the largest pixel format for the fore-seeable 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

Image Projection

Light Shaping

Color Separation

Image Modulation


UV and IR Control



RGB Image

Combining


Eye/brain

Image Projection• 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 Typical 3-chip DLP

Projection Lens

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• Zoom and focus are typically controlled by motors on the lens itself.

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Ten Element Projection Lens PanelPrism

Projection Lens

Typical 3-chip DLP Lens Mount

Projection Lens Performance• 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 lens (e.g., astigmatism, chromatism) the

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lens (e.g., astigmatism, chromatism) 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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Screen

Light Shaping

Color Separation

Image Modulation


UV and IR Control



RGB Image

Combining


Eye/brain

Screens• 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 in all directions.

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in all directions.

• 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.

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Gain vs. Viewing Angle

(Blue line is a perfectly lambertian screen)

Curved Screens & Domes• 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 field from lenses or the use of projection Coverage with multiple projectors

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field from lenses or the use of projection 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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Coverage with multiple projectors

JTAC Immersive Dome

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Thank you.Questions?

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