Visual Evaluation Experiment on Chroma Saturation Effects (Preliminary)

Yoshi OhnoNIST, USA

Report for TC1-69, June 1, 2009, Budapest

NIST Spectrally Tunable Lighting Facilityfor color rendering and vision science experiments

Specifications

• 22 color channels using 1800 high power LEDs/unit (to be expanded)• 17 narrow bands + 5 broad bands (phosphor LEDs)• Each color channel has 5 to 10 W optical power.• Full power total optical power ~200 W, luminous flux ~50,000 lm.• 500 to 800 lx illumination for most of simulated white Light spectra.

“Color Quality Scale”

Fix the problems of CRI (based on color fidelity)1) use 15 saturated reference color samples

2) use the latest color space (CIE LAB) 3) use the latest chromatic adaptation formula (CMCCAT2000)4) 0 to 100 scale (CRI can have negative score.)5) CCT factor based on gamut area of ref. Illuminant6) RMS combining of color differences7) Scale normalized for consistency with CRI for

 fluorescent lamps

Saturation factor – Considers the effect of enhanced color contrast.

Proposed by NIST

Davis, W. and Ohno, Y. “Development of a Color Quality Scale (CQS),” Proc., Sixth International Lighting Research Symposium on Light and Color, Orlando, FL, February 2006.

Score is decreased for the full color difference

RefTest

RefTest

Score is not penalized for increase of chroma. (Score is decreased for hue and lightness shifts)

Saturation factor

Experiments

CCT: 4000 K (800 lx) and 3000 K (300 lx), Duv=0.000

4000 K 3000 KReference: broadband CRI Ra=98 Ra=974 peak (RGBA) Desaturated-4 Ra=70 Ra=704 peak (RGBA) Desaturated-3 Ra=77 Ra=764 peak (RGBA) Desaturated-2 Ra=84 Ra=844 peak (RGBA) Desaturated-1 Ra=90 Ra=904 peak (RGBA) Neutral Ra=92 Ra=954 peak (RGBA) Saturated-1 Ra=87 Ra=884 peak (RGBA) Saturated-2 Ra=82 Ra=814 peak (RGBA) Saturated-3 Ra=74 Ra=744 peak (RGBA) Saturated-4 Ra=69 Ra=69

Lights tested

8 Subjects: 5 Caucasians, 3 Asians / 5 females, 3 males

Observation: 1) Look at the objects in the room (do not look at hands/face) 2) Look at your hands and face

4000 K Broadband Reference

4000 K Desaturated-4

4000 K Desaturated-3

4000 K Desaturated-2

4000 K Desaturated-1

4000 K Neutral

4000 K Saturated-1

4000 K Saturated-2

4000 K Saturated-3

4000 K Saturated-4

3000 K Broadband (Reference)

3000 K Desaturated-4

3000 K Desaturated-3

3000 K Desaturated-2

3000 K Desaturated-1

3000 K Neutral

3000 K Saturated-1

3000 K Saturated-2

3000 K Saturated-3

3000 K Saturated-4

Neodymium (for comparison)

Results

Results

Additional indices (CQS 7.4)Qa: General Color Quality Scale

Qf: Qa without saturation factorQp: Qa with positive saturation factor Qd: Relative gamut area of 15 samples

NIST Proposal

Color Quality Scale (CQS), Qa, for one number metric.

Additional metrics1) Color fidelity index (Qf or Ra)

2) Color preference index (Qp)