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Are the light sources of light cabinets temporarily stable?

Workshop on “Colorimetry and Color Imaging”. Almuñécar , September 24th 2008. Are the light sources of light cabinets temporarily stable?. R. Roa, L. Gómez-Robledo, R. Huertas, M. Melgosa. Departamento de Óptica , Facultad de Ciencias , Universidad de Granada.

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Are the light sources of light cabinets temporarily stable?

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  1. Workshop on “Colorimetry and Color Imaging” Almuñécar, September 24th 2008 Are the light sources of light cabinets temporarily stable? R. Roa, L. Gómez-Robledo, R. Huertas, M. Melgosa Departamento de Óptica, Facultad de Ciencias, Universidad de Granada. 18071 Granada (SPAIN). email: rafaroa@ugr.es

  2. Contents Are the light sources of light cabinets…? • Introduction • Method • Results and discussion • Conclusions • References

  3. Introduction Are the light sources of light cabinets…? In a first approach, color specification depends on : CIE 15:2004, 3rd Edition, Colorimetry (Technicalreport), CIE Central Bureau, Vienna (2004). Observer • Light source • Object • Observer Light source Object To standardize color coordinates: • Standards for the observer: Standard Observer 1931 and Standard Observer 1964 • Standards for the light source: illuminants (relative spectral distribution) CIE International Commission on Illumination

  4. Introduction Are the light sources of light cabinets…? On visual experiments: CIE 15:2004, 3rd Edition, Colorimetry (Technicalreport), CIE Central Bureau, Vienna (2004). Cabinets with source simulators which represents properly the considered illuminant. Daylight CIE proposes the following illuminants: Tungsten Fluorescent • A (tungsten), C (average daylight), D50, D55, D65 y D75 (daylight simulators) • Typical fluorescents lamps (FL1 to FL12) • New fluorescents lamps (FL3.1 a FL3.15) and high pressure discharge lamps (HP1 to HP5)

  5. Introduction Are the light sources of light cabinets…? From previous works: • There is an important color variability in commercial light cabinets R. Huertas, M. J. Rivas, M. Melgosa, M. Sánchez-Marañón, S. Bhosle, J. J. Damelincourt. “Uniformirty of lighting in color assessment cabinets”, 19th Congress of the International CommissionforOptics (ICO XIX), SPIE Proceedings, Volume 4829, 879-880, Firenze, Italy (2002). • There are relevant differences between light sources and illuminants R. Roa, R. Huertas, M. A. López-Álvarez, L. Gómez-Robledo, M. Melgosa. “Comparación entre iluminantes y fuentes simuladoras”, Opt. Pura Apl.41 (3) 291-300 (2008). • It is important to know the state of the light we are using (temporarily dependent) R. Roa, L. Gómez-Robledo, R. Huertas, M. Melgosa. “Temporal evolution of light sources available in commercial color cabinets”, 4th Balkan Conference on Lighting - Balkan Light 2008, Ljubljana, Slovenia (2008) (in press) Objective: Study the temporal evolution of some light sources of commercial light cabinets

  6. Method Are the light sources of light cabinets…? • EXPERIMENTAL SETUP GretagMacbethSpectralight III • 2 light cabinets VeriVide CAC 60 D65 (illuminant D65, daylight, 6500K) • 3 sources studied in each light cabinet A or F (illuminant A, tungsten, 2856K) TL84 (illuminant F11, fluorescent, 4000K) • Photo Research SpectraScan PR-704 spectroradiometer • PTFE reference white • We measured the spectral radiance of the reference white at the following hours of use of the light sources: 0, 0.5, 1, 2, 3, 4, 5, 10, 20, 30, 40, 50, 75 and 100 hours Total: 14 hours x 3 light sources x 2 light booths = 84 measurements

  7. Method Are the light sources of light cabinets…? • WAYS OF COMPARISON • Normalizedspectral distributions of the light sources with their respective illuminants. • Metrics for comparison of spectral values: RMSE (Root Mean Square Error) F. H. Imai, M. R. Rosen, R. S. Berns, “Comparative study of metrics for spectral match quality”, Proceedings of the First European Conference on Colour in Graphics, Imaging and Vision, 492-496 (2002). WRMSE (WeightedRoot Mean Square Error) J. A. S. Viggiano, “A perception-referenced method for comparison of radiance ratio spectra and its application as an index of metamerism”, Proceedings of AIC Color 01, 701-704 (2004). SCI (SpectralComparisonIndex) CSCM (Colorimetric and SpectralCombinedMetric) M. A. López-Álvarez, J. Hernández-Andrés, J. Romero, R. L. Lee Jr., “Designing a practical system for spectral imaging of skylight”, Appl. Opt.44, 5688-5695 (2005).

  8. Method Are the light sources of light cabinets…? • Correlated color temperature (CCT) J. Hernández-Andrés, R. L. Lee Jr., J. Romero, “Calculating correlated color temperatures across the entire gamut of daylight and skylight chromaticities”, Appl. Opt.38, 5703-5709 (1999). • CIE Color Rendering Index (CRI) CIE Publication 13.3-1995, Method of Measuring and Specifying Colour Rendering Properties of Light Sources (Technical report), CIE Central Bureau, Vienna (1995). • CIEDE2000 color differences CIE Publication 142-2001, Improvement to Industrial Colour-Difference Evaluation (Technical report), CIE Central Bureau, Vienna (2001). CIEDE2000 color differences between GretagMacbeth ColorChecker chips illuminated by the source simulator or its respective illuminant

  9. Results and discussion Are the light sources of light cabinets…? • SPECTRAL DISTRIBUTIONS Spectralight III TL84 Spectralight III D65 Spectralight III A CAC 60 TL84 CAC 60 F CAC 60 D65 • There are some differences between light sources and their respective illuminants. R. Roa, R. Huertas, M. A. López-Álvarez, L. Gómez-Robledo, M. Melgosa. “Comparación entre iluminantes y fuentes simuladoras”, Opt. Pura Apl.41 (3) 291-300 (2008). • Sources are stabilized after half an hour of use, except sources simulating the A illuminant (approximately stable from the first measurement).

  10. Results and discussion Are the light sources of light cabinets…? • METRICS Values in green are those with the lowest coefficient of variation. The red ones have the largest coefficient of variation.

  11. Results and discussion Are the light sources of light cabinets…? • The results obtained with the 4 metrics are not the same. • Values slightly rise over the time except for the source D65 from Spectralight III cabinet.

  12. Results and discussion Are the light sources of light cabinets…? • CORRELATED COLOR TEMPERATURE • Source A from Spectralight III Lowestvariationover time • Source D65 from Spectralight III Shows anincreaseover time • Source D65 from CAC 60 Largestdecreaseover time

  13. Results and discussion Are the light sources of light cabinets…? • COLOR RENDERING INDEX • Sources A from Spectralight III and TL84 from CAC 60 Lowestvariationover time • Source F from CAC 60 Suffers(insignificant) oscillationswhileevolves • Variation on CRI Almostinsignificant in allthe light sources

  14. Results and discussion Are the light sources of light cabinets…? • CIEDE2000 COLOR DIFFERENCES

  15. Results and discussion Are the light sources of light cabinets…? • Bluish chips 8 and 13 largest standard deviations in color differences • Source A from Spectralight III smallest color-difference variations over time 1 2 3 4 5 6 • Source TL84 from CAC 60 greatest standards deviations 7 8 9 10 11 12 13 14 15 16 17 18 • Achromatic samples (chips 19 to 24) smallest variations when the sources age 19 2021 22 23 24

  16. Conclusions Are the light sources of light cabinets…? • This work is related to current TC1-44 “Practical Sources for Daylight Colorimetry”. • Source A from Spectralight III cabinet is the source which suffers the lowest variations over time, attending to the most of indices. • Source D65 from Spectralight III cabinet gets closer to its illuminant with age. On the other side, source D65 from CAC 60 cabinet gets older faster than others. Final conclusion: • When high precision color measurements (e.g. some threshold visual experiments) are needed, it should be necessary to know the state of the light source we are using, because it is temporally dependent, and it cannot be assumed that it is identical to a CIE illuminant. Future work: • To complete this analysis, it would be interesting to study the evolution of the light sources at more hours of use, increase the number of light sources and use other indices for the comparison.

  17. Contact:rafaroa@ugr.es Thanks for your attention Acknowledgements: Research Project FIS2007-64266, Ministerio de Educación y Ciencia (Spain), with FEDER support. Vicerrectorado de Política Científica e Investigación de la Universidad de Granada, for the research grant to Rafael Roa.

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