halfabsorption

Halfabsorption is a term used in optics and spectroscopy to denote the condition in which a material or medium absorbs one half of the incident radiation at a specified wavelength or energy. In practice this corresponds to transmittance T = I/I0 = 0.5, and an absorbance A = -log10(T) = log10(2) ≈ 0.301.

The concept is linked to the Beer-Lambert law, A = εcl, where ε is the molar absorptivity, c is the concentration, and l is the path length. The thickness or concentration that achieves half absorption, sometimes called the half-absorption thickness l1/2, satisfies εcl1/2 = 0.301, so l1/2 = 0.301/(εc).

Measurement and applications are common in laboratory spectroscopy. Researchers may report the l1/2 or the concentration or path length that yields 50% absorption; however many instruments report transmittance or absorbance directly, and "halfabsorption" is not a universally standardized term.

Limitations exist, as real materials may scatter, reflect, or exhibit non-linear absorption. The Beer-Lambert relation is an approximation that holds under conditions such as monochromatic light and a homogeneous, non-scattering sample. Because of these factors, the exact value of half absorption can vary with wavelength, geometry, and sample state.

See also: Beer-Lambert law, transmittance, absorbance, half-value layer.