1. Uniform Illumination with LED based lighting CIE Div1 Meeting 14 - 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(G centre ) = 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*(E min -E max )/E 0 ΔE R = 8% (E 0 = 360 lx) ΔE G = 18% (E 0 = 640 lx) ΔE B = 12% (E 0 = 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*(E min -E max )/E c ΔE R = 8% (E 0 = 360 lx) ΔE G = 6% (E 0 = 840 lx) ΔE B = 12% (E 0 = 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: E centre > 1000 lux (“white” light using RGB LEDs) Source-surface distance = ~80 cm Spatial uniform illumination of a 20cm diameter surface: »E centre /E 20cm ≤ 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