Läbilaskvus

Läbilaskvus is the fraction of incident wave energy that passes through a material or coating. It is a dimensionless quantity, often expressed as a percentage, and it generally depends on wavelength, thickness, and the properties of the medium. In optics and spectroscopy, transmittance is used to describe how much light or other electromagnetic radiation is transmitted through a sample.

In a simple, non-scattering, and non-fluorescent system, transmittance T is defined as T = It/I0, where It is the transmitted intensity and I0 is the incident intensity. For many materials, transmittance follows Beer–Lambert behavior: T = e^(-αd), with α as the absorption coefficient and d the thickness. In real samples, scattering, multiple reflections, and interface effects can modify T, and total transmittance may be separated into specular and diffuse components. The sum of transmittance, reflectance, and absorptance equals one: T + R + A = 1, under appropriate conditions.

Transmittance depends on wavelength, angle of incidence, polarization, and surface coatings. Anti-reflective coatings, for example, can significantly increase transmittance by reducing Fresnel reflections. Materials with high transparency in a given spectral range, such as clear glass or water, exhibit high transmittance in that range, while metals or highly absorbing pigments show low transmittance. Applications include optical filters, solar panels, windows, and spectrophotometric analyses where transmittance data underpin calculations of absorbance and material properties.

Terminology varies by field; transmittance is often abbreviated as T and measured with spectrophotometers or integrating spheres to capture both direct and scattered light.