Design of a Head Mounted Display for Image-Guided Surgery

1. Design of a Head Mounted Display for Image-Guided Surgery Bo Bai Stephanie Dalquist Megan Galbraith 16 May 2002

2. Introduction • Develop HMD for surgeons • Projects diagnostic images on patient • Compare digital X-Rays, NMR • Overlay text plans • High resolution and accuracy • Design micro-display for image source

3. Technology goals • Full color, 4 bits/color grey scale • Minimum frame rate of 100 Hz • High resolution – 1280 x 1024 • Pixel pitch at most 24 μm • High contrast ratio, at least 100:1 • Design for hospital environment, 100 Cd•m-2 • Minimum brightness 60,000 Cd•m-2

4. Display technology • Active Matrix Organic LED • Fast response time of 3 μs • Pixel pitch down to around 12 μm • Can reach brightness levels up to 106 Cd•m-2 • Reliable to 106 hours • Preferred over other technologies • AMEPD: color difficulties • AMLCD: less bright • AMELD: not very bright (23 Cd/m2 at 60 V)

5. Grey scale method • Sub-framing • Allows for 16 shades from black to white • Minimizes flicker at high ν (1600 Hz) • Other options • Patterning: Too much flicker, too few greys • Voltage modulation: • Requires high sensitivity to ΔV • More complicated drivers

6. Pixel structure • Stacked OLED • True color • High contrast • Bright, emissive • Flexible • Makes pitch spec

7. Pixel structure • Stacked OLED preferred • Other options: • Side-by-side monochrome • Large pitch or lower resolution • Many have decreased viewing angles • Color filters • Absorption loss • Need efficient blue dyes

8. Pixel circuit • Diode equivalent for each color • Single line addressing • Voltage across rows • Data down columns • High frequency • Simpler, cheaper drivers

9. OLED requirements • 9V to reach 60,000 Cd•m-2 • Current Density of 50 mA/cm2 • Area = Apixel - Atransistors = 570 μm2 • Current =2.87 x 10-4 mA

10. CMOS specification • Choose n-doped CMOS: Kn > Kp • Lowers power consumption • VGS = 0.6V • ID = 0.5Kn(VGS-VTn)2 • VG1 = VGS+Vdiode = 0.6+9 = 9.6V • VG2 = VG1+VTn = 9.6+0.5 = 10.1V • Column voltage = VG1 = 9.6V • Row voltage = VG2 = 10.1V

11. Driver specifications • Generic driver circuit • Can be used with OLED, ELD, LCD, etc. • Row driver draws 10.1 V • Column draws 9.6 V

12. I/O data rate for drivers • Row data rate • Selects a row to update • Consider 16 sub-frames for grey scaling • 1.6 MHz or 205 Kbps • Column data rate • Sends color information • 6.29 GHz or 786 Mbps • Much more data, much faster data rate needed

13. Power consumption • Total power consumption of 4 W • 2 x 10-6 W per pixel • Ppixel = VI = 9V x 2.87x10-4 mA • Display requires 4 W • Ptotal = PpixelNrowsNcolumns • Conservative estimate assuming all pixels on

14. Overall efficiency • Display efficiency ηdisplay= 27% • 4 lumens/Watt power efficiency at 9 V • Input Power Pin = 4 W • Output Power Pout = 60,000 Cd/m2 • Area/(4 lm/W • 4π Cd/lm) = 0.9 W • ηdipslay = Pout/Pin • Optical subsystem ηoptics = 15% • Overall ηdisplay •ηoptics= 4%

15. Conclusions • Meet target specifications at low cost with current technology • Not power hungry • Safe (low voltage, amperage, etc.) • Take it to the lab for prototyping • Revolutionize health-care by 2010

16. questions?