Coloured paints based on coated flakes of metal as the pigment, for enhanced solar reflectance and cooler interiors: description and theory

Flakes of metal coated with thin dielectric films, when used as various coloured paint pigments, can reflect much more of the near infra red component of solar radiation than standard paints of equivalent colour, which makes them attractive for reducing cooling loads in cars and under metal roofs, while maintaining a wide range of decorative options. Spectral responses, solar absorptance and CIE colour co-ordinates are explored theoretically for a single layer, and two layers of different refractive index, on smooth aluminium flakes. Results in air, and in a clear matrix of refractive index close to that of the paint binder, are presented to highlight the colour shifts that are observed in a matrix when colour is due to interference. Thin film models show that the single layers have weak angle of incidence dependence while double layers have attractive variations in reflected colour while retaining their high NIR reflectance. This adds to decorative appeal. The origins and impact of diffuseness are briefly discussed.