Opacity
Opacity turns up in 2 fields: lighting, paper optics. Here is what it means to the people in each one.
Opacity in lighting
The degree to which a material blocks the passage of light.
A quantifiable measure of how impenetrable a substance is to visible light or other electromagnetic radiation. It is the inverse of transmittance. In lighting and optics, specifying opacity is critical for designing diffusers, gels, and gobos, determining exactly how much luminous flux will be absorbed or reflected rather than passing through.
"The client wants total blackout curtains, so we need a fabric with 100 percent opacity."
Opacity in paper optics
A sheet's ability to hide what lies behind it, expressed as a percentage from readings over black and white backings.
Opacity compares how much light a single sheet reflects over a black cavity with how much it reflects over a white background, either a pad of the same paper or an 89 percent reflectance backing. A sheet at 100 percent hides everything behind it. Opacity rises with grammage, with light scattering from fines, filler and fiber surfaces, and with absorption from dyes or dark fiber, and it falls when calendering or wetting collapses the air spaces inside the sheet.
"Dropping to 45 gsm cost us almost three points of opacity, so we're back to adding TiO2."