highopacity

High opacity describes a property of a material or medium that effectively blocks or greatly reduces the transmission of light or other radiation. It is used as a contrast to transparency and translucency, and can vary with wavelength and viewing conditions.

Opacity arises from absorption, scattering, and sometimes reflection. The greater the absorption coefficient or scattering coefficient, the less light passes through. In many materials, opacity is wavelength dependent; a substance can be opaque at visible wavelengths but partially transparent at infrared or ultraviolet, although there are exceptions such as metals which are highly reflective across broad ranges.

Quantifying opacity often uses transmittance, T = I/I0, or its logarithmic form absorbance A = -log10(T). The optical depth τ or absorption coefficient α, along with thickness l, gives T ≈ e^{−αl} in simple cases. In practice, opacity is a spectrum, with a material described by an opacity function κ(λ) that varies with wavelength. High-opacity materials have large κ(λ) over the wavelengths of interest.

In optics and coatings, high opacity materials are used as covers, filters, or shielding; in displays and printing, opacity indicates how well an overlay hides the underlying image, controlled by the alpha channel in digital imaging. In astrophysics and atmospheric science, opacity governs radiative transfer: it determines how deeply light from a source can penetrate a medium and affects temperature structure and spectra.

Notes: The term "high opacity" is context dependent and can refer to physical opacity (light attenuation) or perceptual opacity (visual concealment).