läpinäkyvyyskohdan

Läpinäkyvyyskohdan is a Finnish term used in optics and materials science to denote the point or condition at which a material, coating, or interface becomes transparent to a given range of electromagnetic radiation. The term combines läpinäkyvyys (transparency) and kohdan (the genitive form of kohta, meaning point or position). In practice, the transparency point is defined by a transmittance threshold, often expressed as T > 0.9 or a similar criterion, depending on the application. It may depend on wavelength, incident angle, temperature, and structural thickness, and thus can vary across the spectrum or under different conditions.

Applications of the concept occur in thin-film coatings and optical filters, where engineers aim to position the transparency point within a desired spectral region to maximize light transmission and minimize unwanted reflections. In spectroscopy and optical diagnostics, identifying the transparency point helps clarify which wavelengths propagate through a sample with minimal absorption, aiding the interpretation of spectral data. In solar energy devices, transparent contacts and substrates are designed so that the transparency point aligns with the visible range to improve device efficiency.

Measurement and calculation typically involve determining transmittance T, defined as the ratio of transmitted to incident light (I_out/I_in). The transparency point is located by measuring T as a function of wavelength and selecting the wavelength where the threshold criterion is met. For thick or scattering samples, diffuse transmittance and scattering components may need to be separated to obtain a meaningful transparency point.

Limitations include the dependence of the transparency point on wavelength, angle, microstructure, temperature, and measurement setup. As a concept, it serves as a descriptive guideline rather than a universal scalar value, helping compare materials and optimize optical performance. See also transmittance, optical coating, absorbance, refractive index, and optical filter.