Modifications of the Robertson Method for Calculating Correlated Color Temperature to Improve Accuracy and Speed

Correlated color temperature (CCT) is one of the principal metrics used in the engineering and specification of lighting. One of the most common methods used for calculating CCT was first proposed by Robertson in 1968. It utilizes a 31-row lookup table (LUT) based on isotemperature lines and a formula to interpolate CCT values between the lines. The original Robertson method is known to have modest errors in determining CCT, so many alternative methods demonstrating improved accuracy have been subsequently proposed. Rather than an entirely new method, this analysis proposes three changes to the original Robertson method: recalculation of the LUT with an increase in the resolution to an increment of 1%, a correction to address CCT errors in the region between isotherms with slopes of opposite sign, and the application of a Fibonacci or binary search algorithm to efficiently search the expanded LUT. These changes result in a CCT calculation method, denoted Robertson2022, that produces CCT errors less than 0.1 K throughout the range of 1,500 K to 40,000 K with Duv between −0.05 and 0.05.</p

Supplementary document for Predictive performance of the standard and the modified von Kries chromatic adaptation transforms - 5701581.pdf

The comparison of CMFs and sensor

Road Marking Contrast Threshold Revisited

Sufficient contrast between road surface and road markings is key for a safe and comfortable driving experience. This calls for a comprehensive and well established contrast (threshold) model, which ideally results in a single contrast threshold value independent of object angular size or road luminance. The contrast threshold model introduced by Adrian is still commonly used in road lighting. More recently, new contrast metrics that also predict supra-threshold contrast visibility have been proposed, but the corresponding visibility thresholds are not yet known. In the present study, participants are presented a rendering of a highway, including road marking arrows of various size and luminance and were asked to indicate the direction of the arrow. The luminance of the road surface, acting as background for the markings, was varied too. Due to the very low luminance values and the very small differences in luminance, measurement accuracy and calibration issues require special attention. The results show good agreement with Adrian’s visibility model (R2 = 0.75) in terms of luminance contrast, background luminance and size. In addition, we used our experimental data to define contrast thresholds for several other existing image based contrast models. Unfortunately, it seems to be impossible to state one unique threshold contrast value independent of object angular size and road luminance.</p

Supplementary document for Point-by-point visual enhancement with spatially and spectrally tunable laser illumination - 6142167.pdf

painting design and the detailed scene segmentation result

Road Marking Contrast Threshold for the Elderly and the Impact of Glare

As the global population continues to age, with projections indicating that individuals over 60 will comprise 17% of the world’s population by 2030, it is crucial to address the disparity between current road lighting guidelines, based on data from healthy young adults, and the needs of elderly drivers. This study examines the impact of age and glare on contrast thresholds for individuals above 60 years old in the context of road lighting and its effect on detecting road marking arrows. Results indicate a large negative effect of age and glare, such that the required luminance difference between the road marking arrow and the background should be increased up to a factor of 2 for elderly compared to young drivers. We also found that measured luminance difference thresholds increased for elderly participants compared to the younger ones. In addition, the results show a poor agreement with Adrian’s visibility model which can only be slightly improved by using a different measure for the veiling luminance. Still, given these findings, the current European standard EN13201 should adapt the calculation of the threshold increment to also accommodate the needs for the elderly drivers and warranting safe roadways for the aging population.</p

Improved Method for Evaluating and Specifying the Chromaticity of Light Sources

This article describes a method for calculating and specifying light source chromaticity using the International Commission on Illumination (CIE) 2015 10° color matching functions (CMFs), which, according to analysis of existing psychophysical experiment data, can reduce visual mismatch compared to specifications based on the traditional CIE 1931 2° CMFs in architectural lighting applications. Specifically, this work evaluates, documents, and recommends for adoption by lighting standards organizations a supporting system of measures to be used with the CIE 2015 10° CMFs: a new uniform chromaticity scale (UCS) diagram with coordinates (s, t), a measure of correlated color temperature (CCTst), and a measure of distance from the Planckian locus (Dst). It also presents options for updating nominal classification quadrangles. A complete method of this nature has not yet been standardized, which may be contributing to the slow uptake of the CIE 2015 CMFs. The proposed tools are analogous to u, v, CCT, Duv, and the American National Standards Institute (ANSI) C78.377 chromaticity specifications that are all currently defined in the CIE 1960 UCS diagram using the CIE 1931 2° CMFs. While conceptually equivalent, the differences between the current standard method and the proposed st system are important for reducing unintended visual mismatch in the chromaticity of light. The implications of changing chromaticity specification methods are identified by a comparison over a diverse set of real light source spectral power distributions.</p