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Context.  The colour-ratio technique has been widely used in mapping planetary surfaces, but its solar phase-angle dependence is not well understood. Understanding the phase-angle dependence of the colour ratio would enhance our abilities in interpreting planetary remote sensing data.   Aims.  We aim to investigate the dependence of the colour ratio indices on mineralogy, phase angle, particle size, and the degree of simulated space weathering.   Methods.  We measured the multi-band (i.e. 458, 633, 750, and 905 nm) and multi-angle reflectance spectra of four typical lunar-type minerals with different particle sizes using a custom multi-angular imaging device.   Results.  The colour ratio does have a phase-angle dependence that is more sensitive to the mineralogy and wavelength and less sensitive to particle size distribution.   Conclusions.  The combined analysis of the colour ratio and its phase dependence can improve efficiency in mapping the lunar surface. With a prior knowledge of the phase behaviours of colour ratios of specific lunar-type minerals, an optimised colour ratio at certain phase angles can be found to efficiently distinguish the composition of a target surface.

Phase-angle dependence of colour ratios and potential implications for lunar remote sensing