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