uniform illumination with led based lighting cie div1 meeting 14 - 15 june 2008
TRANSCRIPT
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
• 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.
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]
Light & Lighting Lab LED-source
• Technical:
• LEDs: ATLAS Light Engines from Lamina
– NT-43F0-0424 RGB
» Dominant Wavelenghts R: 619 – 629 nmG: 515 – 535 nmB: 460 – 470 nm
• Lenses: ATLAS
– OP-4FM1-0442 (Medium Beam) : » Beam divergence = 33°
(--> luminous intensity = half the peak value)
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
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).
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.
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)
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
Colour coordinate variance
0.3290.33
0.3310.3320.3330.3340.3350.3360.3370.338
0 5 10 15 20 25
radial distance (cm)
xyY
x,y x
y
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)
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.
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