Optical system considerations for rear projection display 11

Optical system considerations for rear -projection display 11 March 2003 R. Edward English Jr. , Ph. D. Chief Optical Scientist 3 M Precision Optics, Inc. Cincinnati, Ohio

A brief history of 3 M Precision Optics, Inc. • Founded in 1930 as U. S. Watch Crystal Manufacturing Company - became the largest American maker of watch crystals in the 1930’s and 1940’s – Began making plastic watch lenses in 1950’s • Changed name in 1970’s to U. S. Precision Lens after achieving major success in plastic lenses for LED calculators and optical components – Invented “Delta” lens design in 1978 • Acquired by Corning in 1986 and renamed Corning Precision Lens Incorporated in 2000 – Won Technical Emmy in 1997 for “the processing design and development of high efficiency optics which made possible the growth of the video projector industry” • Acquired by 3 M in 2002 and renamed 3 M Precision Optics, Inc. • Headquartered in Cincinnati, OH with over 550, 000 square feet of operating space employing over 1, 500 employees 3 M Precision Optics © 3 M 2003

Projection television is becoming a commonplace consumer electronic item 3 M Precision Optics © 3 M 2003

Why are projection TVs “grody to the max” ? 3 M Precision Optics © 3 M 2003

Outline of presentation • • Large area consumer display market overview CRT-based projection system design requirements Lens design and performance General lens design requirements for microdisplay – compare and contrast • Summary 3 M Precision Optics © 3 M 2003

The US projection TV market has grown an average of 23% per year since 1999 3 M Precision Optics © 3 M 2003

Many observers predict that the demand for large area displays will continue to grow 3 M Precision Optics © 3 M 2003

CRT-based PTV system components • The standard configuration in today’s CRT-based systems comprises – 3 CRTs (red, green, blue) • commonly called PRT (projection ray tube) – Projection lens (3 per TV) • liquid coupled to PRT – Mirror – Screen COUPLER • Lenticular and Fresnel – Cabinet – Electronics ELECTRONICS 3 M Precision Optics © 3 M 2003

The lens architecture and design are strongly influenced by system considerations • Low cost & high volume • Image contrast • Brightness – f/1 • Optical configuration – compact, wide fov, 3 CRTs, curved faceplate • Resolution (NTSC vs. digital vs. HDTV) • Thermal environment – liquid coupled • Spectral – phosphors, filtering, color-corrected? 3 M Precision Optics © 3 M 2003

The lens design requirements were set to meet consumer digital TV performance • 5” raster diagonal (object size for 7” CRT) – 350 mm faceplate radius • • • f/1. 07 40” to 70” screen diagonal (image size) 77 mm efl Standard RGB phosphors; liquid coupled Four element design (high volume) – non color-corrected • Resolution to meet digital TV performance 3 M Precision Optics © 3 M 2003

Modern CRT projection lenses began with Betensky’s Delta lens patent (USP 4, 300, 817) • Weak aspheric front element • Biconvex second element provides most of optical power • Negative field flattener 3 M Precision Optics © 3 M 2003

The three projected images are angularly converged on the screen • The red and blue CRTs are tilted to satisfy the Scheimpflug condition for uniform focus across the screen – The shape of the raster is adjusted to compensate the keystone • Additionally, there is about 10% distortion in the projected image, which is pre-corrected by the raster 3 M Precision Optics © 3 M 2003

The three CRT phosphors have very different characteristics 3 M Precision Optics © 3 M 2003

Absorption of side-bands reduces need for color correction and improves color purity SMPTE phosphors w/ ECP 3 M Precision Optics © 3 M 2003

A four element design was developed to meet the requirements acrylic aspheres phosphor screen fluid glass (SK 5) 3 M Precision Optics © 3 M 2003 faceplate

3 M Precision Optics © 3 M 2003

Exploded view of PRT light box assembly [courtesy of HED(US)] tube coupler AB lens yoke C lens 3 M Precision Optics © 3 M 2003

The monochromatic MTF meets the performance requirements 3 M Precision Optics © 3 M 2003

A slight adjustment to the C-element flattens the wavelength dependent field shape 100% 85% 70% 35% 0% green red 3 M Precision Optics © 3 M 2003 red

The lens operates in a thermal environment that ranges from 20°C to 65°C • The optical powers in the lens are optimized during design to balance most of thermal drift – fluid index changes from 1. 437 (cold) and 1. 423 (hot) • For higher performing lenses, a thermal bar is used to “adjust” the focus during operation 3 M Precision Optics © 3 M 2003

Microdisplay technology has improved in cost and performance to enter the consumer market • Devices are ~1” on the diagonal – transmissive LCD (high temperature poly-silicon, HTPS) – digital micromirror device (DMD) – reflective liquid crystal on silicon (LCo. S) • Most devices being integrated into RPTV systems are wide XGA (i. e. , 1280 x 720) – high definition wide screen LCo. S available (i. e. , 1920 x 1080) • The optical system must generate light, “homogenize” the light, manage color, illuminate device, imprint on/off + grayscale image scene, and project the image onto the screen 3 M Precision Optics © 3 M 2003

Typical schematic for DMD system 3 M Precision Optics © 3 M 2003

The standard architecture for transmissive LCD uses an X-cube for color management PROJECTION LENS X-CUBE RELAY OPTICS LAMP LCD DEVICES DICHROICS 3 M Precision Optics © 3 M 2003 from USP 5, 959, 778

3 -panel LCo. S can use a similar arrangement LCo. S DEVICE PROJECTION LENS LAMP LCo. S DEVICE RELAY OPTICS (not shown) 3 M Precision Optics © 3 M 2003 from USP 6, 273, 567

Microdisplay system requirements will move lens designs in different directions • Still a projection system – total conjugate distances are comparable – thermal effects are important (but environments are different) • 1 lens (vs. 3 lenses) – full color correction (<1 pixel vs. ~N/A) – telecentric – f/2. 4 -2. 8 (vs. f/1. 1) and magnification of ~50 -80 x (vs. ~8 -13 x) • Pixelated display – Nyquist frequency of ~50 lpm (vs. ~5 lpm) – distortion correction (<1% vs. 10%) • More complex optical system – interaction between illumination system, imager “physics” and lens design is important 3 M Precision Optics © 3 M 2003

USP 5, 870, 228 USP 6, 144, 503 USP 5, 042, 929 USP 5, 760, 965 3 M Precision Optics © 3 M 2003

In the end, the same things will be important • • Low cost & high volume Image contrast Brightness Optical configuration – compact, wide fov • • Resolution (HDTV) Thermal environment Color-corrected Low distortion 3 M Precision Optics © 3 M 2003

3 M Precision Optics © 3 M 2003