Lustre and Colour Variable Pigments

Several names are applied to pigments that show an angle dependent colonr produced by light interference in thin Ahns. For example nacreous, interference. 

The pearlescent effect in natural pearls arises because of the interaction of incident light with alternating layers of high refractive index (CaCOj) and low refractive index (protein), reflected and diffracted light from which leads to the observed iridescent colours. 

Interference between light waves of the same wavelength can be constructive or destructive, i.e. they can add to or subtract from each other, and this interaction is angle dependent. Therefore the colour observed is also dependent on the angle of illumination and on the angle from which the pigment film is observed. 

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