Optical Thickness
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Definition: a measure either for the absorption, or the effective optical path length of a sample
Related: optical densityrefractive indexinterferometers
Units: m or dimensionless number
Page views in 12 months: 803
DOI: 10.61835/wb8 Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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What is an Optical Thickness?
The term “optical thickness” is ambiguous, as two totally different definitions occur in the literature:
- The optical thickness of a light-absorbing medium is its geometrical thickness times its intensity attenuation coefficient. For example, an optical thickness of 1 implies that the transmitted power is reduced to ($1/e$) (≈ 37%) of its original value. This definition is often used e.g. in atmospheric optics. It is larger than the optical density by the factor ln 10 ≈ 2.303.
- The optical thickness of a transparent medium is sometimes understood to be the same as the optical path length, which is its geometric thickness multiplied by the refractive index. This is the quantity which determines the phase delay for light passing through the medium and is therefore relevant e.g. in interferometers.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What is optical thickness?
The term optical thickness is ambiguous. It can either mean a measure of light attenuation, calculated as geometrical thickness times the attenuation coefficient, or the optical path length (geometrical thickness times the refractive index), which determines the phase delay.
How are optical thickness and optical density related?
When referring to light attenuation, the optical thickness is larger than the optical density by a factor of ln 10, which is approximately 2.303.
What is the optical path length?
The optical path length is a transparent medium's geometric thickness multiplied by its refractive index. This quantity, sometimes also called optical thickness, determines the phase delay for light passing through it and is relevant for interferometers.