Hide the Wall - Paint Opacity

When you paint over a surface, you want a highly opaque paint that will hide the previous color. Paint are tested for opacity by spreading a coat over a black and white test card. A highly opaque paint (high opacity) will hide both black and white surfaces.

In this situation, equal amounts of light will be reflected from the coated black and white areas and consequently an opacity value of 100% will be obtained.

The test is done by doing a draw down of a wet paint over a black and white substrate. After the film has dried, a colorimeter or spectrophotometer is used to measure the amount of reflected light from the coated black and white area of the substrate. This ratio expressed as a percentage of the amount of light reflected from the coated black to white area is opacity.

Opacity = R (black) / R (white)