Lustre, a characteristic of the property of a surface that reflects light. Lustre is due to the specular reflection of light from a surface, which for the most part occurs at the same time as scattered (diffuse) reflection. The human eye perceives the specular reflection against the background of the diffuse reflection, and the quantitative measure of lustre is determined by the ratio between the intensities of the specularly and the diffusely reflected light. Lustre is often characterised by qualitative descriptions, for example metallic, adamantine or vitreous lustre.