1. Promoptica “Nouvelles Techniques d’Eclairage” Inorganic LEDs: working principles and prospects for general lighting applications Laboratory for Light and Lighting KaHo St.-Lieven University College Gent (B) P. Hanselaer Liège Novembre 8, 2007

2. Laboratorium voor Lichttechnologie 1. Main categories of light sources

3. Laboratorium voor Lichttechnologie Light sources DischargeIncandescent Mercury Sodium Low pressure High pressure FL, CFL Metalhalide Outdoor illumination Outdoor, Shops Solid State LED Low pressure High pressure

4. Laboratorium voor Lichttechnologie 2. Elementary Solid State Physics

5. Laboratorium voor Lichttechnologie Intrinsic semiconductor “Free” electrons and holes Bandgap E g E Conduction band Valence band

6. Laboratorium voor Lichttechnologie Recombination between an electron and a hole: energy can be released by the creation of a photon Band gap E g E At room temperature: only a small amount of free electrons and holes: limited number of photons! Photon Light!

7. Laboratorium voor Lichttechnologie Extrinsic semiconductor (n) Donor-atoms (P); n-type E

8. Laboratorium voor Lichttechnologie Acceptor-atoms (B); p-type Extrinsic semiconductor (p) E

9. Laboratorium voor Lichttechnologie +- Injection of free electrons in p-type and free holes in n-type p-n junction! pn

10. Laboratorium voor Lichttechnologie 3. Electrical characteristics

11. Laboratorium voor Lichttechnologie Diode characteristic Low, dc voltage Forward voltage dependent on bandgap of the semiconductor: AlGaInP: 2.95 V typ. InGaN: 3.42 V typ. Electrical characteristics

12. Laboratorium voor Lichttechnologie Electrical characteristics

13. Laboratorium voor Lichttechnologie 4. Optical characteristics

14. Laboratorium voor Lichttechnologie Photon energy (and colour) is determined by bandgap E g Rather monochromatic radiation Optical characteristics: spectrum

15. Laboratorium voor Lichttechnologie Peak wavelength λ p : from UV to IR Full Width at Half Maximum: from 20 to 50 nm Optical characteristics: spectrum

16. Laboratorium voor Lichttechnologie Optical characteristics: chromaticity CIE chromaticity Purity Dominant wavelength

17. Laboratorium voor Lichttechnologie Optical characteristics: chromaticity CIE chromaticity Additive mixing with wide colour gamut

18. Laboratorium voor Lichttechnologie Bandgap engineering to obtain an extensive range of wavelengths and colours: use of compound semiconductors Optical characteristics: colour

19. Laboratorium voor Lichttechnologie Compound Semiconductors

20. Laboratorium voor Lichttechnologie AlGaInP InGaN

21. Laboratorium voor Lichttechnologie Optical characteristics: white LEDs

22. Laboratorium voor Lichttechnologie Three or more LEDs of different Colors + The more colours one has to mix, the more control one has in producing white light with a high color rendering index. + Photons from each LED contribute directly to the light intensity, i.e. no conversion efficiencies have to be considered. + Extensive range of hue’s can be obtained - Optical control, coloured shadows

23. Laboratorium voor Lichttechnologie Osram 6 lead multiLED

24. Laboratorium voor Lichttechnologie Optical characteristics: Radiant/luminous Flux Φ (e) forward electrical current # recombinations # photons luminous flux

25. Laboratorium voor Lichttechnologie LED’s: current driven

26. Laboratorium voor Lichttechnologie Efficacy red: 55 lm/W (room temperature) Energy-efficiency: 24 % Losses Non-radiative recombination (heat) Internal reflections Optical characteristics: efficacy

27. Laboratorium voor Lichttechnologie Non-radiative recombination Temperature of the semiconductor junction increases!

28. Laboratorium voor Lichttechnologie Total Internal Reflection Substrate Active material absorption Partially reflected Totally reflected

29. Laboratorium voor Lichttechnologie Internal reflections 1. semiconductor- encapsulant 2. encapsulant-air

30. Laboratorium voor Lichttechnologie Optical characteristics: spatial Dependent on position of die and reflector shape of the external dome www.nichia.com

31. Laboratorium voor Lichttechnologie Secundary optics Optical characteristics: spatial

32. Laboratorium voor Lichttechnologie 5. Effect of Temperature

33. Laboratorium voor Lichttechnologie Effect of Temperature: luminous flux Increase of non-radiative recombination!

34. Laboratorium voor Lichttechnologie Effect of Temperature: peak wavelength and light flux Decrease of the bandgap, increase of wavelength!

35. Laboratorium voor Lichttechnologie Effect of temperature: chromaticity Chromaticity versus warm-up time

36. Laboratorium voor Lichttechnologie Effect of temperature: lumen maintenance http://www.lrc.rpi.edu/progra ms/solidstate/ongoingProject s.asp?ID=57

37. Laboratorium voor Lichttechnologie Thermal management

38. Laboratorium voor Lichttechnologie Thermal management 10 °C/W

39. Laboratorium voor Lichttechnologie Determination of junction temperature

40. Laboratorium voor Lichttechnologie 6. Photometry of LEDs Photometer/colorimeter or spectroradiometer

41. Laboratorium voor Lichttechnologie Photometer

42. Laboratorium voor Lichttechnologie Important errors in tails of eye sensitivity curve

43. Laboratorium voor Lichttechnologie Spectroradiometer Bandwidth: 5 nm

44. Laboratorium voor Lichttechnologie Photometry of LEDs: intensity Some LEDs have a very narrow radiation pattern (FWHM 2°) Large distance to detector and small detector aperture required. CIE 127 standardisation: “averaged LED intensity” at 316 mm (A) or 100 mm (B) distance and 1 cm 2 detector area.

45. Laboratorium voor Lichttechnologie Photometry of LEDs: luminous flux Reference light source required

46. Laboratorium voor Lichttechnologie Fast measurements: partial flux

47. Laboratorium voor Lichttechnologie 7. LED penetration into general lighting

48. Laboratorium voor Lichttechnologie LED penetration into General Lighting: main obstacles Luminous flux Efficacy Colour and flux maintenance Thermal management Reproducibility Price

49. Laboratorium voor Lichttechnologie LED penetration into General Lighting: obstacles : luminous flux (white) P(W)I(mA)Φ(lm) 0.070201.5

50. Laboratorium voor Lichttechnologie LED penetration into General Lighting: obstacles : luminous flux P(W)I(mA)Φ(lm) 1.235060 3.61000100 5.0700120 Luxeon

51. Laboratorium voor Lichttechnologie LED penetration into General Lighting: obstacles : luminous flux P(W)I(mA)Φ(lm) 4.7420108 262300567 86013300 “Chip on board” technology Lamina Ceramics, Osram Multiple LED package

52. Laboratorium voor Lichttechnologie LED penetration into General Lighting: obstacles : efficacy/ CRI

53. Laboratorium voor Lichttechnologie CRI (R a ) Test source Colour samples(8 of 14) Standard illuminant Colour coordinates

54. Laboratorium voor Lichttechnologie Low CRI and yet high colour preference? CRI and LED’s: subject of international research CIE TC 1-69 Colour Rendition by White Light Sources

55. Laboratorium voor Lichttechnologie Obstacles: efficacy BUT Lighting Systems Higher Light Output Ratio possible due to a higher directionality of the “naked” light source

56. Laboratorium voor Lichttechnologie Obstacles: efficacy BUT coloured applications LED /Halogen green traffic signal: efficiency (cd/W): 8 / 1

57. Laboratorium voor Lichttechnologie

58. LED penetration into General Lighting: obstacles : lumen maintenance LED lifetime is sometimes specified in MTBF (mean time between failure). Various LED manufacturers predict LED source life up to 100K hours “Lumen Maintenance” is even more important. End-of-Life specification: light output has dropped to 70% compared to the original light output: 50.000 hrs !

59. Laboratorium voor Lichttechnologie LED penetration into General Lighting: obstacles : thermal management - chip

60. Laboratorium voor Lichttechnologie LED penetration into General Lighting: obstacles : thermal management - luminaire Project 2.2 Californian Energy Commission

61. Laboratorium voor Lichttechnologie LED penetration into General Lighting: obstacles : reproducibility “binning”

62. Laboratorium voor Lichttechnologie LED penetration into General Lighting: obstacles : price

63. Laboratorium voor Lichttechnologie Pro and contra: pro Saturated colours, dynamic colour effects with a large colour gamut High efficiency for applications with coloured light (e.g. traffic lights) Liftetime up to 50 000 hours (70% definition) Vibration-proof Low voltage No mercury No UV and IR radiation Instantaneous switch-on Easy dimmable

64. Laboratorium voor Lichttechnologie Pro and contra: contra Reproducibility is difficult (semiconductor processing); binning (sorting by intensity, colour, forward voltage) is required Colour and intensity shift with temperature, driving current and life time Low output/device Low efficacy for white (but is improving) Price

65. Laboratorium voor Lichttechnologie www.lichttechnologie.be Instituut voor de Aanmoediging van Innovatie door Wetenschap en Technologie in Vlaanderen KaHo St.-Lieven Gebr. Desmetstraat 1 B-9000 GENT Tel: + 32 9 265 86 10 Peter.Hanselaer@kahosl.be

66. Laboratorium voor Lichttechnologie Laboratory for Light&Lighting Founded in 1997 with the support of IWT Vlaanderen (Flemish institute for the promotion of innovation in science and technology). Main activities: –Education –Scientific research –Supporting industrial developments

67. Laboratorium voor Lichttechnologie Topics Photovoltaics Lighting Optical design Appearance Measurement Facilities

68. Laboratorium voor Lichttechnologie Lighting Research: –Criteria efficient lighting –LED’s (PhD) Supporting industry –Groen Licht Vlaanderen: promotion of energy efficient lighting (Greenlight) –Shoplighting

69. Laboratorium voor Lichttechnologie Optical design Research: –Luminaire design with ray-tracing (PhD) Supporting industry –Secundary optics for LED clusters –Surface with uniform luminance

70. Laboratorium voor Lichttechnologie Appearance Research: –Gloss (PhD) –Colour rendering with LED’s Supporting industry –Automotive –Wood –Retro-reflection

71. Laboratorium voor Lichttechnologie Photovoltaics Research: –Spectral response –Light trapping in cells and modules Supporting industry –Stand-alone systems –Signalization

72. Laboratorium voor Lichttechnologie Measurement facilities 8/d spectral reflectance and transmittance Goniometer Spectrometers: VIS, UV, near IR Electrical characterization Bidirectional Scattering distribution Photometric/colorimetric camera LED integrating sphere