Most beverages need special protection from visible light, in order to maintain their sensorial and nutritional characteristics. in food packaging applications, such protection is often obtained by using colours, which are dosed at different addition rates. One of the best ways to ensure that the final container will have the necessary light protection is to measure light transmission directly on the PET preform, before it passes to the blowing stage.
The colour amount level in the preform is tested by spectrophotometer techniques or, very often, just by visual control. The first approach is expensive, requires lab skills and often yields more data than needed. Moreover, it is difficult to apply to opaque preforms.