opacity
A measure of relative impenetrability to electromagnetic radiation such as light.
opacity: how much light a material blocks
Opacity is the degree to which a material resists the transmission of light or other electromagnetic radiation. It is measured on a scale where a value of 0 represents complete transparency and a value of 100 represents total impermeability; practical materials fall somewhere between these extremes. The opacity of a substance depends on its physical composition, thickness, wavelength of incident light, and the presence of absorbing or scattering particles within it.
In imaging and optical systems, opacity is critical because it determines how much useful light reaches a detector or sensor. A sheet of frosted glass has higher opacity than clear glass because its surface roughness scatters light in many directions rather than transmitting it straight through. Similarly, in photography and cinematography, neutral density filters are calibrated by their opacity values to control exposure without changing color balance. The opacity of a camera's sensor cover glass or an optical element affects the overall light transmission efficiency of the entire system.
Opacity versus related terms
Opacity should not be confused with density or refractive index. Density is a measure of mass per unit volume, while opacity describes radiation transmission regardless of density. Refractive index characterizes how much a material bends light as it enters, whereas opacity describes how much light is lost to absorption or scattering. A material can be optically dense and still highly transparent, as with certain types of optical glass.
In industrial quality control, opacity is measured using spectrophotometers or transmission meters. For papers and films, opacity is often reported as a percentage; newsprint typically has an opacity around 65 to 75 percent, while glossy coated papers may reach 85 to 95 percent. These values matter in printing because higher opacity reduces show-through, where ink or images from one side of the sheet are visible from the back.
Opacity in a material can increase over time due to aging, oxidation, or degradation of the material's chemical structure. For optical coatings and protective windows, this loss of transparency is a failure mode that shortens service life. In forensic and security applications, opacity is exploited intentionally through specialized materials that block specific wavelengths while remaining transparent to others, enabling hidden information or infrared suppression.