Surface treatments of aluminium to enhance optical reflectance

Aluminium has good reflection properties. Generally, reflected light has three characteristics: distribution as a function of wavelength (colour), distribution as a unction of the angle (diffuse or specularly reflected light) and the total amount of reflected light (total reflectance). Here we focus on the last two properties. Industrially rolled aluminium shows a rough topography and contains a high amount of oxides in the subsurface layer. The former causes diffuse scattering, the latter decreases the total reflectance. The total reflectance of industrially rolled Al can be increased by etching the top layer of the Al. This causes an increase in roughness and so more light is diffusely scattered. If specularly reflecting samples with high total reflectance are needed, electropolishing is an appropriate treatment. Electropolishing smoothens the surface and removes the top layer, changing the reflection properties. The distribution of the reflected light can be found using Angle Resolved Scattering (ARS), and the results of ARS are related to the visual appearance of an Al sample. Electropolishing also enhances the total reflectance in comparison with rolled Al.