Using Beers Law-Multilayer Composite Coatings

By Don Campbell, Antonette St. Pierre

Measurement of light transmission through automotive topcoats and primers has been practiced for many years. In the 1980s, the integrating sphere spectrophotometer was introduced to accurately measure the total transmittance of systems where the transmission depends on the angle of the incident light. Basecoats (or color coats) that contain reflective aluminum flake pigment are a common example of such a system.

Most measurements now use the integrating sphere geometry. Many manufacturers have some form of UV transmittance specifications, and these can cover individual layers or even the complete system of primer (if used) plus basecoat and clearcoat.

Furthermore, some manufacturers have now extended these specifications to include high energy visible light up to 500 nm since learning that some electrocoats can experience photodegradation even at these lower energies.