定义：
几何厚度乘以折射率。
 

光学厚度是非常含糊的，存在两种完全不同的定义： 

• 光吸收介质的光学厚度是其几何厚度乘以强度衰减系数。例如，光学厚度为1代表透射功率衰减到了其初始值的1/e (≈ 37%)。这一定义常用在大气光学中。它是光学密度的ln 10 ∼ 2.303倍。 
• 透明介质的光学厚度有时认为与光程长度相同，即几何厚度乘以折射率。它决定了光通过介质经历的相位延迟，因此在干涉仪中需要用到。

Definition: a measure either for the absorption or the effective optical path length of a sample

Units: m or dimensionless number

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.