LEDs an der Schwelle zum Einsatz in Projektionssystemen...

LEDs an der Schwelle zum Einsatz in Projektionssystemen: Herausforderungen, Grenzen und Anwendungen

Dr. Anton MoffatCarl Zeiss Corporate ResearchCarl Zeiss AG, Jena, Germany

[email protected]

Moffat / CZ AG / LEDs an der Schwelle zum Einsatz in Projektionssystemen -2-

Contents

– Introduction– System Design Methodology– Applications– Conclusions


IntroductionMotivation for using LEDs: Colours, Lifetime,

Lifetime• Conventional Lamp:

50% preserve 50% brightness in x h,1/2 can failGuarantee for a few 100 h

• Semiconductor LampMTTF with confidence > 9x%Intensity degradation < 30%in 10.000 h

Guarantee is given for years

Colour Saturation• > 100% NTSC achievable

for saturated colours• Colour space can be made to

match video standards exactly• Selectable white point

Many More:• Cost• More suppliers• Simple electronics• Instant on/off• Low voltage• Low pressure• Colour Break-Up reduction• No Colour Wheel: noise reduction


IntroductionTimeline Starts Now for LED-Based Projection Systems

Critical threshold is screen brightness

LED Roadmap

Customer ThresholdHighly optimized electro-optical system

Standard OpticalSystem with LEDs

2006 2008 2010


IntroductionDMD Microdisplays for High Light Throughput at High Contrast

LCD

LCOS

DMD

• 120 W Lamp : 6000-7000 Lumens– 4000 – 5000 in Aperture– 500-1000 out of projector– ~10% light throughput standard

• Goal: reach same screen brightness (Nits) with only ~1000 Lumens from LEDs

• High light throughput– Large area microdisplays: 0.7", 0.85",

0.9" diagonal– Wide opening angle optics (Low F/#)

• Liquid Crystal Imagers (LCD, LCOS)– Three panels with colour combiner for

polarized light– Required low F/# limits constrast ratio

• Digital Micromirror Device (DMD)– Single panel requires sequential colour


Geometrical Optical RequirementsDMD is the key component

• Etendue is the geometric extent of the optical system– E = π n² A sin²θ– Component with the smallest etendue limits the brightness of the system

(usually the DMD)

LED – small area, large angleDMD – large area, small angle

xx

Geometrical match:etendue, aspect ratio, overfill


Geometrical Optical RequirementsUseable System Etendue Practical Limit on Light Source Area

0 5 10 15 20 25 300

5

10

15

20

25

30

Emitting Area (mm²)

Rel

ativ

e In

tens

ityN max

0

Itotal A( )

Iuseable A( )

N max0 A

+/-60°xHD4F/2.0


Geometrical Optical RequirementsAvailable DMDs Determine Useable System Etendue

DMD Diagonal Area F/2.0 Etendue Max Area (mm²)(mm²) (mm² sr) (Surface Emitter)

xHD5 0.67" 1920 x 1080 14.74 x 8.29 122.2 24.1 7.7HD2+ 0.78" 1280 x 720 17.51 x 9.85 172.5 34.0 10.8xHD4 0.85" 1920 x 1080 18.67 x 10.51 196.2 38.6 12.3sxHD5 0.88" 2560 x 1440 19.58 x 11.02 215.8 42.5 13.5

DMD SizeResolution(pixels) (mm²)

Larger DMD More Light ~ DMD area

But: System is larger and more expensive:Larger optics ~ DMD diagonalLarger LED area ~ DMD area

+ Overfill 10..20%


Geometrical Optical RequirementsLow F-number for Higher Light Throughput

Projection Lens Projection Lens

Illumination

DMD DMDF/2.4 (standard) F/2.0 (wider opening)

larger optics~30% more light throughput


System Design Methodology

• Goals:– Maximum Screen Brightness– Good Image Quality– Competitive Cost

Optimize combination of parameters:– Optical (light throughput, image quality)– Electrical (driving conditions, power consumption)– Thermal (heat dissipation, operating temperature)

• Upstream design:– from the screen– through the lens– to the LED (surface emitter) – instead of to the lamp (volume emitter)


Basic Projection System DesignDMD Lamp-based System

• White light source with rotating colour wheel• DMD – Digital Micromirror Device• F/2.4 – Standard opening optics

OpticalIris

DMD

Field Lens

Mirror

Projection LensF/2.4

Integrating RodUHPLamp UV, IR

Filter

Colour Wheel

Relay Optics

Screen

Video ElectronicsSync

Colour WheelRotation Sensor


Basic Projection System DesignDMD LED-based System

• 3 coloured light sources, electronically controlled• DMD – Digital Micromirror Device• F/2.0 – Wider opening optics

DMD

Field Lens

Mirror

Projection LensF/2.0

MicrolensArray

RedLED

GreenLED

Relay Optics

Screen

CollectionOptics

BlueLED

DichroicMirrors

Video ElectronicsTrigger, Dimming

HeatSink

LED Driver


LED Module Configuration

• Since LEDs are brightness-limited, make full use of available etendue– NB. Additional constraints imposed by power consumption, heat dissipation,

manufacturing tolerances, cost, ...

Pro: Efficient – matched geometry

Con: Costs for yield and custom sizeSpec. uniformity across chip areaThermal stress in pulsed operationLarge drive currents

Monolithic Solution:

Pro: Standard LED chips as building blocksLow Current, Voltage for LED-Strings

Con: Less efficient – gaps, approx. geometrySpec. uniformity across chips on a module2xN arrays (favoured due to bond wires)

Tiled Solution:


Available LED Light Sources

• Osram Ostar: 12 chips (two 2x3 arrays) with primary optics

• Luminus PhlatLight™:PT85 (1-chip), PT180 (4-chips)

• Input power 10 – 60 Watts• Peak output power approx.:

– 200mW/mm² Green– 400mW/mm² Red, Blue

• Here: Experimental results based on Osram Ostar– Method applies to other LED architectures

PhlatLight

Ostar


Physical Optical RequirementsColour primaries and white point

(4000K..15000K)

Green

Red

Blue

• Potential to display oversaturated colours

• Can dynamically adjust illumination source to video standards

• Need an initial setup, specific to each set of LED subassembly


Electro-Optic Transfer Function (EOT)Basic System Performance Data to Optimize Driving Conditions

• Vary driving conditions one at a time• Measure system output• EOT data to optimize driving conditions and establish correlation with testing conditions

Driving Conditions

OpticalSystemLEDs EOTs

RedGreenBlue

Dichroic mirrorsLens coatingsDMD

Integrating sphereSpectrometer

CurrentTemperatureDuty Cycle


Driving ConditionsApproximately Equal Radiant Power for RGB at the White Point

Green determines the Luminous OutputRed and Blue need ~ equal Radiant Flux!

Optimize driving conditions– current density– temperature– duty cycle Watts Lumens


Electro-Optic Transfer Function (EOT)Red LED Most Sensitive to Overdrive Current

Red Green Blue

25°C

45°C

30°C

60°C

30°C

60°C

Nominal750mA

Nominal500mA

Nominal500mA

Temperature Coefficients:

-0.25% / K-0.8% / K -0.14% / K

(thermal rollover)


LED Light Output VariationsAdjust Duty Cycles to Maintain White Point

• LEDs manufactured in brightness bins– Bin width of +/-20% typical for high-brightness LEDs– Full distribution typically 2:1 in luminous flux (4-5 bins) !

~20% Minimum to ensure image bit depth

What happens to projector‘s output?


LED Light Output VariationsProjector Manufacturability with Matched Sets of Three LEDs

• Projector output shows less variability than LEDs– But: total projector output variability should be +/-10% over all components!

• Avoid arbitrary combinations of LEDs– Specify and obtain matched sets of three LEDs

Applications


Mainstream ApplicationRear Projection Television

Samsung

• Recent results from CES in Las Vegas– Samsung (xHD4 DMD) 56”– Akai (xHD4 DMD) 46”, 52”– HP (xHD4 DMD) 52”– Sanyo (3-Chip LCD) 55”– JVC (3-Chip D-ILA LCOS) 46”

(All 1080p HDTV resolution)

Sanyo AkaiLED

PDP


Mainstream ApplicationFront Projection

• Today’s front projectors– 500 - 1000 Lumens– Noisy, heavy and bulky– Brightness versus colour saturation– High lamp replacement cost

New approach: Mobile “Pocket Projector” for controlled ambient lighting conditions


Mobile ApplicationPocket Projector

• Small size paramount: 100x70x40 mm³• Robust and mobile, battery operated• 25 Lumen from 8 W (LEDs)• Illumination path length 30% shorter

– field lens shared in illumination path– use of two LED Modules: RB, G

Core optical moduleassembled withtwo LED modules, heat sink,and DMD on interface board


Mobile ApplicationPocket Projector

Prototype

ProductSamsung


Conclusions

• LEDs provide significant advantages over lamps• System EOTs crucial data for optimizing brightness• Matched set of three colours needed• LEDs have crossed the threshold for use in projection systems

– A highly optimized system is required– LED-based RPTV and the Pocket Projector are ready for the market now


Vielen Dank für Ihre Aufmerksamkeit.

Acknowledgements:• Osram Opto Semiconductors in Regensburg, Germany • Fraunhofer IOF in Jena, Germany• Bundesministerium fuer Bildung und Forschung (BMBF): Grants 01BD150 and 13N8270

LEDs an der Schwelle zum Einsatz in Projektionssystemen...

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