WHEN LIGHT ENTERS A PAINT FILM, SOME OR ALL of it is absorbed or reflected by the film before reaching the substrate, thereby hiding the substrate to a lesser or greater degree. The light that reaches the substrate is partly absorbed by it and partly reflected back in conformance with the substrate's visual pattern. Reflection from the substrate eventually emerges from the film carrying the substrate reflectivity information perceived as visibility or lack thereof, and referred to as hiding. Opacity may be qualitatively defined as the property of a paint film that enables it to prevent the passage of light and thereby to hide the substrate on which it has been applied. Note that opacity is a film property, whereas hiding power is a property of the whole paint. Hiding is a more general term used frequently to refer to either opacity or hiding power (HP).