Influence of Natural Daylight on Soil Color Description: Assessment Using a Color-Appearance Model

Abstract

Soil color is usually described under natural daylight using Munsell charts. Daylight can vary but its effects on the Munsell notation are hardly known. Today, color-appearance models allow quantitative analyses of the perceived color under different lights. Using the CIECAM02 model, we studied the color changes in 238 Munsell chips and 229 soils under 125 types of daylight as well as their effects when matching soil to chips. The different types of daylight were measured from spectral power distributions and color attributes from reflectance spectra. Relationships (r = -0.94 to 0.95) between master variables taken from categorical principal-component analyses showed that as daylight becomes bluer (3758-34,573 K), samples with a concave spectrum between 500 and 600 nm redden while those of a convex spectrum turn yellow. This hue angle change (0.5-29.5°), especially great in slightly chromatic samples (r 2 = 0.90), was different enough in the soil and chip of most color matches to generate paramerism. The practical implication is that 79% of the soils had more than one Munsell notation because of daylight changes, although this decreased to 19% considering only daylight at a solar elevation of >9°. This finding supports the good practice of pedologists in determining soil color in the central hours of the day; however, 45% of soils with reflectance spectra close to several chips had Munsell colors either redder or yellower under midday light (5933 ± 481 K) than under the C reference illuminant of the Munsell system (6800 K). Unless the reflectance spectrum of the sample is available, it is not possible to know if soil colors have been correctly denoted or not, or how to compensate for the differences.