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Uniform Illumination with LED based lighting CIE Div1 Meeting 14 - 15 june 2008

Uniform Illumination with LED based lighting CIE Div1 Meeting 14 - 15 june 2008. Introduction. Investigation of Colour Rendering of LEDs Psychophysical experiments Colour difference measurements … with colour samples AND real-life samples/scenes

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Uniform Illumination with LED based lighting CIE Div1 Meeting 14 - 15 june 2008

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  1. Uniform Illumination with LED based lightingCIE Div1 Meeting14 - 15 june 2008

  2. Introduction • Investigation of Colour Rendering of LEDs • Psychophysical experiments • Colour difference measurements • … with colour samples AND real-life samples/scenes • Lighting system is essential part of the experimental setup • At the 2007 Bejing conference, our ‘Light & Lighting Lab’ was asked by R.Luo if it would be possible to create a lighting system that produces a uniform illumination of 1000 lx at a surface 40 cm in diameter positioned at 80 cm from the source.

  3. Introduction • Total illuminance delivered should be larger than 1000 lux to avoid effects of rod excitation (tetrachromacy) when viewing angle is > 4° (10°Observer) [1],[7] • Most lighting systems use a diffuser to obtain a spatially uniform illumination: • Problem: the use of a diffuser severely decrease the illumination to levels well below 1000 lx. • Viénot: ~500lx at distance ? [2] • Mahler: ~660lx at 50 cm [3] • Tetri: ~250lx at distance? [4] • Thompson: ~300lx at 60 cm [5] • Schanda: 250 lx at distance ? [6],[7] • Sandor: ~400lx at distance ? [8]

  4. Light & Lighting Lab LED-source • Technical: • LEDs: ATLAS Light Engines from Lamina • NT-43F0-0424 RGB • Dominant Wavelenghts R: 619 – 629 nm G: 515 – 535 nm B: 460 – 470 nm • Lenses: ATLAS • OP-4FM1-0442 (Medium Beam) : • Beam divergence = 33° (--> luminous intensity = half the peak value)

  5. Light & Lighting Lab LED-source • Geometry + operating conditions: 4 LEDs in a 2 by 2 configuration: • LED-centre distance r = 23.3 cm ; • Source-surface distance d0 = 81.3 cm • Operating current: • I(R) = 500 mA • I(G) = 400 mA • I(B) = 150 mA • I(Gcentre) = 80 mA

  6. Measurements: single LED engine • Single LED illumination pattern: • For the 3 different LED dyes: • R and B have nearly identical profiles, while G falls of more rapidly. • The combined RGB profile follows the G one as expected (highest illuminance).

  7. Measurements: 2 by 2 geometry • Multiple LED illumination pattern (1): • The ideal separation distance, r, for the LEDs in a 2 by 2 configuration was calculated from the single LED illumination pattern. • ΔE = 100*(Emin-Emax)/E0 • ΔER = 8% (E0= 360 lx) • ΔEG = 18% (E0= 640 lx) • ΔEB = 12% (E0= 75 lx) • The narrow beam of G results in a drop at the centre, which will actually be advantageous in creating a uniform illumination.

  8. Measurements: 2 by 2 +1 geometry • Multiple LED illumination pattern (2): • In a next stage the centre drop in the illumination pattern for G, was leveled by adding a fifth LED engine positioned in the centre of the 2 by 2 configuration. This LED is driven by a separated power source. • ΔE = 100*(Emin-Emax)/Ec • ΔER = 8% (E0= 360 lx) • ΔEG = 6% (E0= 840 lx) • ΔEB = 12% (E0= 75 lx)

  9. Measurements: 2 by 2 +1 geometry • Chromaticity and Luminance uniformity: • Chromaticity coordinates at the centre as measured on a CERAM ceramic standard: 0.3392, 0.3317 • Chromaticity coordinates at the centre as measured on sheet of white paper: 0.3370, 0.3306

  10. Final Results • Photometric specifications: • Ecentre > 1000 lux (“white” light using RGB LEDs) • Source-surface distance = ~80 cm • Spatial uniform illumination of a 20cm diameter surface: • Ecentre /E20cm≤ 12 % (“white” AND R, G, B)

  11. Future Plans • Photometric and visual experiments for evaluation of CIECAM02 colour differences • Phase 1: colour samples: • Standard source (phase 1A) • Different LED sources (phase 1B) • Phase 2: real-life objects / scenes • Different LED sources • Use colour differences data to evaluate CRI and/or formulate an alternative.

  12. References • [1] Schanda, J : Current CIE work to achieve physiologically correct colour metrics • [2] Viénot, F: Color appearance under LED illumination: the visual judgement of observers, research note • [3] Mahler, E: Testing LED lighting for colour discrimination and colour rendering • [4] Tetri, E: Usability of LEDs for General Lighting • [5] Thompson, M: An investigation into Perception of Color under LED white Composite Spectra with Modulated Color Rendering • [6] Schanda, J: Colour Rendering, Past – Present – Future • [7] Schanda, J: The concept of colour rendering revisited • [8] Sandor, N: Visual observation of colour rendering

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