There’s film at the end of the rainbow …

On rare occasions, some window films with a standard interior scratch resistant coating will exhibit an effect commonly referred to as a ‘rainbow effect’. This is actually the result of ‘Thin Film Interference’ and will have the appearance of various coloured bands  or blotchiness on the film surface that appear to move over the surface as you change your point of view. Films that are dual-reflective (more reflective on the outside than the inside) tend to exhibit this effect more often.

It is impossible to avoid this effect completely with most films because it is a physical effect, and not completely controllable by the manufacturing process.

Types of interior light sources can also have a dramatic effect on this rainbow appearance. Fluorescent lights, compact fluorescent bulbs, and especially new energy-efficient ‘T8’ bulbs appear to generate the worst effects.  It is also more noticeable when there are low visible light levels outside (eg: at night).

What can be done to reduce the rainbow / oil slick effects on windows with a film applied?
1) The replacement of fluorescent light bulbs near windows with standard incandescent bulbs will very nearly eliminate the rainbow effects.
2) Replacement of the fluorescent bulbs with new LED light bulbs will generally reduce up to about 50% of the rainbow effect.