J Schanda University Veszprém, Department of Image Processing and Neurocomputing, Hungary Characterizing illumination systems Colour rendering and beyond

CIE Standards for assessing quality of light sources Introduction Characterisation of the light source Colour rendering Assessing daylight simulators CIE Technical Committees working on the above questions New approaches Summary

CIE Standards and Guides CIE colorimetric illuminants –ISO/CIE 10526, CIE S005 CIE colorimetric observers –ISO/CIE International Lighting Vocabulary –CIE 17.4 Colorimetry: CIE 15.2, new edition coming Colour rendering: CIE 13.3, outdated - experiments needed Quality of daylight simulators: CIE 51, possible improvements

Illuminant, source, simulator CIE Standard Illuminant –Defined as spectral distribution

CIE Standard Illuminants

Illuminant, source, simulator CIE Standard Illuminant –Defined as spectral distribution CIE Standard Source –Source with specified spectral distribution Simulator –Source approximating the illuminant

CIE colorimetric illuminants CIE Standard Illuminant A –New definition, unchanged values

CIE Standard Illuminant A

CIE colorimetric illuminants CIE Standard Illuminant A –New definition, unchanged values CIE Standard Illuminant D65

CIE colorimetric illuminants CIE Standard Illuminant A –New definition, unchanged values CIE Standard Illuminant D65 –Other daylight illuminants: D50 –Evaluation

CIE (Standard) Source(s) CIE Standard Source A –Realization: incandescent lamp D65: no standard source Secondary sources –fluorescent lamps

Source standards and imaging Original –Real life: daylight illumination –paper copy: D50, fluorescent or tungsten illum. –screen representation Reproduction –screen representation –hard-copy

Source standards and imaging Original scene –Daylight: D50, D65 (Jackson - MacDonald - Freeman pictures)

Source standards and imaging Reproduction –screen representation CRT primaries chromatic adaptation colour appearance models –hard-copy CMYK primaries base material (paper reflection properties) gamut mapping

Source standards: original scene Light source colour description –chromaticity of source –colour temperature –correlated colour temperature –metamerism index Colour rendering –special indices: R i –general colour rendering index: Ra

Light source colour description Colour temperature, Tc –The temperature of a Planckian radiator whose radiation has the same chromaticity as that of a given stimulus. Correlated colour temperature Tcp –The temperature of the Planckian radiator whose perceived colour most closely resembles that of a given stimulus at the same brightness and under specified viewing conditions.

Light source colour description

Colour rendering Spectra of two sources of equal chroma ticity

Colour rendering Chromaticity of the two sources and of a sample illuminated by these sources

Colour rendering Effect of an illuminant on the colour appearance of objects by conscious or subconscious comparison with their colour appearance under a reference illuminant

Colour rendering index Measure of the degree to which the phsychophysical colour of an object illuminated by the test illuminant conforms to that of the same object illuminated by the reference illuminant, suitable allowance having been made for the state of chromatic adaptation.

Colour rendering index Comparison of illuminants –test illuminant –reference illuminant: Planckian if CCT < 5000 K Phase of Daylight 5000 K Objects illuminated (test colour samples) Chromatic adaptation Colour difference

Flow-chart of CRI calculation

Test colour samples

CRI calculation - 1 Selecting reference illuminant Selecting test colour samples Chromatic adaptation (von Kries) Colour difference calculation –U*, V*, W* colour space

Chromatic adaptation

CRI calculation - 2 Special colour rendering indices: R k,t = 100 – 4,6 E i,k,r-t. General colour rendering index:

Assessing daylight simulators - 1 For colorimetric practice the evaluation of the UV content also important Assessment based on metamerism indices –5 metameric pairs 3 sets: D55, D65, D75, Set for D50 under development

Assessing daylight simulators - 2 Visible range metamerism index –average colour difference for the five metameric pairs

Assessing daylight simulators - 3 UV range assessment –reflected part of radiation + –emitted part of radiation Final assessment: MI for UV and visible Assessment based on CIELAB or CIELUV colour differences –Five categories established

Assessing daylight simulators - 4

CIE TCs working on colorimetry CIECAM colour appearance models VDU - Reflective media comparison Chromaticity diagram with physiologically significant axes Geometric tolerances in colorimetry Updating the colorimetry and colour rendering documents

Summary Light source colorimetry –lamp light colour –brightness - task-performance relationship –colour rendering Image reproduction colorimetry –colour appearance –colour management