János Schanda

CRI2012: A proposal for updating the CIE colour rendering index

The CIE colour rendering index (CRI) has been criticized for its poor correlation with the visual colour rendering of many spiked or narrowband sources, its outdated colour space and chromatic adaptation transform and the use of a small number of non-optimal reflectance samples that have enabled lamp manufacturers to tune the spectrum of a light source to yield, in some cases, inappropriately high general CRI values. The CRI2012 metric proposed in this paper addresses these criticisms by combining the most state of the art colorimetric colour difference model, i.e. CAM02-UCS, with a mathematical reflectance set that exhibits a highly uniform spectral sensitivity. A set of 210 real reflectance samples has also been selected to provide additional information on the expected colour shifts when changing illumination.

Testing a calibration method for colour CRT monitors. A method to characterize the extent of spatial interdependence and channel interdependence

Abstract The performance of a colorimetric calibration method for colour CRT monitors has been investigated experimentally for six monitor types representing different quality levels. The colour of luminous patches in the presence of different border and background colours on the screen was measured. Systematic deviation from the calibration method described in CIE Technical Report, Guide to Characterizing the Colorimetry of Computer-Controlled Displays (1993) was found that can only be explained by the violation of two basic assumptions of the calibration method used: spatial independence and channel independence. Measurements were carried out to see the effect of spatial and channel interdependence separately. We recommend a new method to characterize the extent of channel and spatial interdependence of a given monitor set-up. With the help of this method it is possible to predict the CIE XYZ tristimulus values of displayed colours even in the case of such monitors that show channel- and/or spatial-interdependence. This method is especially important if colours have to be displayed accurately on different screen backgrounds.

Spectral discomfort glare sensitivity investigations.

The main concern of car headlamp manufacturers is to provide better visibility. Unfortunately by increasing the luminous intensity of the headlamp the risk of increasing discomfort glare for the other road users increases. One possibility to increase visibility and decrease glare could be the selection of a spectral power distribution for the headlamp such that it emits in wavelength regions (if such wavelengths exist) where luminous sensitivity is high, and glare sensitivity is low. For this the spectral discomfort glare sensitivity of humans under low photopic conditions has to be determined. Spectral discomfort glare sensitivity was determined using ten young observers, requesting the observers to select medium glare settings of monochromatic radiations based on the de Boer glare rating scale, both at near‐foveal and 10° extra‐foveal directions of the 2° diameter glare source. It was found that the spectral discomfort glare sensitivity function could not be described by a simple addition of the photopic spectral luminous efficiency function and the short wavelength cone fundamental, but that the contribution of the chromatic channels of human vision may also have to be considered.

A New Concept of Color Fidelity for Museum Lighting

Abstract In almost all color rendering calculations, where a reference illuminant is needed, a Planckian or daylight spectrum of the same correlated color temperature as that of the test source is used (a few methods tried to use only D65 or a very restricted number of test illuminants, but these have not received general acceptance). In this article, a new calculation method of color fidelity is introduced that uses modern colorimetric practice in order to reproduce the colors of the painting as seen under the light under which it was painted (probably daylight for a picture prepared before the end of the 19th Century) but keeping the correlated color temperature of the light source warm-white for museum lighting. The principles of corresponding color color rendering are described and an Excel workbook is provided for further testing purposes.

Toward a Replacement of the CIE Color Rendering Index for White Light Sources

ABSTRACT The CIE color rendering (fidelity) index (CRI) has remained unchanged for over four decades. Most, if not all, of its components could be updated to more state-of-the-art methods. One of the most critical components of any color rendering (fidelity) metric is the test sample selection. This article therefore addresses the importance of uniform sampling of wavelength space to avoid selective optimization—that is, taking advantage of the unequal contributions of different wavelength regions to the general color rendering score—of light source spectral power distributions. It summarizes the development of a mathematical sample set with undistorted spectral sensitivity, the HL17 set. The set is used in a recently proposed update, the CRI2012 general color rendering index. To assess the impact of the spectrally uniform sample set on color fidelity scores, the CRI2012 index values for each of a set of 139 lamps were compared with those of the CIE CRI. In addition, the impact of updating the other components was investigated. A mean and maximum absolute difference of respectively 5.9 and 21.8 index units were found between the CRI2012 and CIE CRI, although the largest part—respectively 4.03 and 19.7 index units—was shown to be the result of updating the color difference engine and the switch to the CIE 10° observer. The analysis also indicated possible past spectral selective optimization of some warm-white tri-band fluorescent sources for high luminous efficacy of radiation (LER) and (just) sufficient CIE Ra values by taking advantage of the spectral nonuniformity of the CIE reflectance set. Adopting a spectrally uniform sample set in a color rendering metric therefore has important practical implications when designing light source spectra. Finally, possible updates and further improvements of the CRI2012 are briefly mentioned.

Matrix-based color measurement corrections of tristimulus colorimeters.

For colorimetric imaging the tristimulus technique is still the best practical method to keep the measurement time within reasonable limits. However, the achievable color measurement uncertainties for special sources can be large. It is described how the systematic errors can be significantly reduced by using matrix-based color corrections and how the matrix elements can be optimized to obtain the smallest spectral mismatch errors for different light-source distributions. An approach for decreasing the systematic errors is to increase the number of the colorimeter channels (or filters) used for a measurement. Using five channels in a colorimeter is an optimum choice. Determination of the optimum matrices for the five channels is discussed. The correction matrices are designed such that the spectral mismatch errors of the realized functions are minimized relative to the CIE standard color matching functions for several selected test-source distributions. The optimum matrix depends on the (test) light source to be measured. Adaptive matrix values are determined by using the channel outputs and the spectral power distribution of color LEDs approximated with a simple approximation function. The systematic errors are evaluated for a number of colored and white LEDs. The noise propagation with the applied matrix corrections is investigated.

Spectral discomfort glare sensitivity under low photopic conditions

Car headlamp manufacturers have to optimise the headlamp spectrum to produce maximum visibility and minimal glare. There are very few investigations on the best spectrum for mesopic visibility and even less data on the spectrum of discomfort glare. We have performed pilot measurements to determine the spectral discomfort glare sensitivity under situations similar to night‐driving conditions and conclude that it is not a simple additivity of the V(λ) and the V′ (λ) functions.

Effect of chromatic mechanisms on the detection of mesopic incremental targets at different eccentricities.

Spectral sensitivity functions for the threshold detection of mesopic incremental targets were compared for different target eccentricities (10, 20, and 30°) and for different mesopic backgrounds (0.1, 0.5 and 1.0 cd m−2). Relative responsivities of achromatic mechanisms (L + M and rods) and chromatic mechanisms (S and |L–M|) were estimated for each eccentricity and background. Chromatic mechanisms contribute significantly to detection but their effect is lower at 30°. A new contrast metric (CCHC2) is introduced to account for the selective adaptation of the photoreceptors and the effects of the chromatic mechanisms i.e. broadening of the range of spectral sensitivity with multiple local maxima and yellow sub‐additivity of detection performance. The CCHC2 metric is compared with the achromatic contrast metric of the MOVE model (CMOVE). For the same target, CCHC2 generally predicts a higher visibility level than CMOVE. However, in accordance with visual observations, for grey or yellowish incremental targets appearing at the eccentricities of 20 and 30°, the visibility predicted by CCHC2 is less than the visibility predicted by CMOVE.

Colour fidelity for picture gallery illumination, Part 1: Determining the optimum light-emitting diode spectrum

Colour rendering for picture gallery lighting means colour fidelity, showing the colours of the pictures as seen by the painter in the light he used in creating the pictures. As up to the beginning of the 20th century illuminance high enough for good colour vision was possible only in daylight, daylight would be the optimum illuminant. For art preservation and energy saving reasons this is not feasible. Museums often use light of 3500 K correlated colour temperature (CCT). A method is described that takes chromatic adaptation into consideration to determine the spectral power distribution producing least colour distortion of object colours while changing from a higher adaptation luminance at 6500 K to 3500 K illumination at a lower adaptation luminance. The method can be used for any CCT and adaptation luminance values.

Using Multimedia Interactive E-teaching in Color Science

Abstract Color is becoming a key issue in many e-commerce products. This made it necessary to develop a color course easily accessible from the Internet or distributed on CD-ROM. This article describes the advantages of the electronic approach – compared to class-room demonstrations – to bring the fundamentals of colorimetry to the student with the help of demonstrations.