Abstract

The Murujuga Rock Art Monitoring Program (MRAMP) is examining the impact of industrial air emissions on rock art near Karratha in Western Australia. One component of this program is measuring the spectral colour of 54 rock art panels across Murujuga. To date, nine fieldwork campaigns have been conducted between 2022 and 2024, measuring the spectral colour of the same ten spots on each rock art panel during each campaign. Previous research on spectral colour converts the data into CIELAB colour space index, which indicates perceptual colour in three coordinates (L*, a*, b*) defined by CIE international standards. L* represents lightness, a* the red-green balance, and b* the blue-yellow balance. Unlike RGB, CIELAB is perceptually uniform, meaning a given numerical change in coordinates reflects the same perceptible change in colour. This index converts colour spectrum data for wavelengths 380 to 780 nm (n = 950) into three numbers (L*, a*, b*). Traditionally, the Euclidean distance (ΔE) between L*, a*, b* coordinates was used to detect perceptual colour differences, with ΔE > 2 indicating a difference. One aim of the MRAMP is to use the spectral colour measurements to monitor the long-term trends in colour of the rock art panels. However, the extreme weather near Karratha (and other factors) have caused instrument faults. These faults are visible in the spectrum plots but disappear when converted to CIELAB, appearing as valid measurements.