Refractive Index is the speed of light in air divided by the speed of light passing through any material. Said another way, Refractive Index describes the degree to which light rays are bent as they pass from one material to another. As light passes through the interface between two materials of different refractive indices, it is bent.

Refractive Index is wavelength dependent, something which is critically important to plastics. Because a polymer matrix is transparent or translucent, light enters the plastic object, is bent, is reflected and scattered by pigmentation particles and bent again when it re-enters the air. When comparing two samples of differing polymer matrices - for example a Cycolac® acrylonitrile butadiene styrene (ABS) standard to a polypropylene (PP) master target - the two objects will bend the light differently, affecting the observer’s perception of the objects’ color and appearance. This must be understood since it has an effect on all color matching and appearance issues. Refraction is a vital part of almost any color match, making perfect matches between different polymers difficult if not impossible to achieve in actual practice.

Refractive Index plays an important role when assessing visual and instrumental evaluations. If Refractive Indices between two samples are not the same, wavelength by wavelength, two scenarios are likely to exist. First, if instrumental values between the samples agree, a visual difference may be noted. Second, if the samples agree visually, most likely there will be a significant numerical difference. The root cause is usually Refractive Index differences between the two samples. Usually, the only times when sample and target can agree visually and instrumentally is if they are made of the same material with the same surface texture.