High Efficiency Uniform LED Surface Light Source

Chih-Chieh Kang, Jeng-Feng Lin, Yu-Chang Wu, Shih-Cheng Yeh, Tai-Rong Chen

Department of Electrooptical Engineering, Southern Taiwan University, Yung-Kang, Tainan, Taiwan, 71005

R.O.C.

Abstract

• A novel LED uniform surface light structure is devised for the use of lighting, especially for a large-size edge-type backlight unit (BLU). Two kinds of embodiments are proposed in this work. Simulation results demonstrate their high efficiency and high uniformity in terms of light extraction and illuminance performance, respectively.

Background

• Going green is the market trend.• Concept of using “empty cavity” as a light-guiding

mechanism was proposed more than a decade ago for small-size LED backlight unit (BLU) .

• As the size and thickness of a BLU are becoming larger and thinner, the optical design of existing structures become difficult and complicated.

• Novel design is needed

Prior Works• Y. Nakamura and E. Furuya, “ Liquid

Crystal Display Device Backlit by LED’s Coupled to Printed Circuit Board,” US Patent No. 5453855, (1995).

• S. Sakai, A. Mori, K. Ishiguchi, K. Kobauashi, T. Kokogawa, T. Sakamoto, and T. Yoneda, “A Thin LED Backlight System with High Efficiency for Backlighting 22-in. TFT-LCDs,” SID Symposium Digest 37, 1218-1221, (2004)

• C. P. Hung, W. S. Chen, J. H. Lin, and W. Y. Li, “Novel Design for LED Lens and Backlight System,” Proceedings of IDMC 2007, Taiwan, 476-479, (2007)

Optical Design Principle

• Bullet-shape LEDs with high-directivity intensity profile.

• One central bump and several pairs of side slant structures on the bottom reflection surface for illuminace uniformity by light modulation.

• Improve illumiance uniformity by a diffusion plate.• Reverse prism layer structure to greatly enhance

luminance performance.

Optical Designs

(b) a diffusion plate with a reverse prism layer structure

(a) only a diffusion plate

High-Directivity LED

• For exampe

Nichia NSPW500B Radiation pattern

Bottom Reflection Surface with Bumpy Structures

• Illuminace uniformity improvement• Enhancement of light extraction efficiency

BLU implemented is of 62.6 × 234× 15.9 mm

Optical Simulation (a)

• only a diffusion plate ontop of the “empty cavity”

Light extraction efficiency = 91%

No diffusion plate presents

Optical simulation (a)

• only a diffusion plate ontop of the “empty cavity”

Light extraction efficiency = 77%

Illuminance uniformity = 72%

Diffusion plate presents

Reverse Prism Layer Structure

• Redirect the light rays toward the direction of normal viewing angle

• Enhancement of light extraction efficiency

Refocusing Effect of Reverse Prism Layer Structure

Light rays emitting from LEDs on the left side modulated by the left side slant structure

Light rays emitting from LEDs on the right side modulated by the central bump

Optical Simulation (b)

• A reverse prism layer structure ontop of the “empty cavity”

Light extraction efficiency = 94%

No diffusion plate presents

Optical simulation (b)

• A diffusion plate with a reverse prism layer structure ontop of the “empty cavity” Light extraction efficiency = 89%

Illuminance uniformity = 60%

Conclusion

• We can highly increase the light extraction efficiency up to be 90% that is comparable to some direct-type LED BLUs .

• Our second devised embodiment can greatly increase the luminance performance by the implementation of the reverse prism layer structure as well as the manipulation the transmittance of a diffusion plate.

• More work is needed on optimizing optical design in order to improve the uniformity on both illuminance and luminance.