The visual appearance of an object is determined by three factors:

• the illumination source;
• the angles of illumination and observation;
• the 'optical response' of the object.

This section deals with the optical response of aluminium surfaces, and explains how it is related to compositional and morphological surface features.

The spectral reflectance of aluminium is determined by the free electron metallic behaviour of the bulk, but as we will see, also to a great extent by the optical properties of the top surface layers that play a major role in the appearance of aluminium.

Learning Outcomes for this Section

After completing this section, you should be able to:

• list the four key parameters which can influence the visual appearance of industrial aluminium substrates
• relate the optical response of the substrate to the microstructure and other morphological features of the aluminium surface
• explain the terms specular reflectance, diffuse reflectance, interference colouring
• calculate at which oxide layer thickness interference colours can appear
• select an appropriate process to enhance reflectivity
• select an appropriate process to increase the scattering
• select an appropriate process to introduce a colour on the surface
• illustrate with examples why studying the optical properties of aluminium is important