When light is absorbed in a homogeneous medium with a certain absorption coefficient α, the optical intensity decays exponentially in proportion to exp(−α z), where z is the propagation distance. (It is assumed that the intensity is not affected by divergence or convergence of a beam.) One defines the absorption length as the inverse of the absorption coefficient. After that propagation length, the intensity decays to 1/e (≈37%) of its initial value. After four absorption lengths, only ≈1.8% of the initial intensity is left.
The term penetration depth is often used with the same meaning as absorption length, but it should be considered as a more general term, because limited penetration into a material may not only result from light absorption, but also from reflection. That is a typical situation for metals, for example, where the rapid decay of intensity is mostly due to reflection.
Questions and Comments from Users
Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.
Please do not enter personal data here; we would otherwise delete it soon. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him e.g. via e-mail.
By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.