当大部分光经过一个样本时，部分光会被吸收。在光谱学，透光率是透射光和入射光的光强比：

；

其中，是入射光光强，是透射光光强。

吸光度的定义为：

。

对于不同波长（λ）的光，样本的吸光度未必一样。

对于较稀的溶液，吸光度和浓度成正比，两者关系可用比尔-朗伯定律说明：

；

其中，是光在样本中经过的距离，是浓度，是吸收系数，是材质的性质。

实验中，吸光度或透光率可用色度计求得。

"Optical density" redirects here. "Optical density" can also refer to index of refraction.

In chemistry, absorbance or decadic absorbance is the common logarithm of the ratio of incident to transmitted radiant power through a material, and spectral absorbance or spectral decadic absorbance is the common logarithm of the ratio of incident to transmitted spectral radiant power through a material. Absorbance is dimensionless, and in particular is not a length, though it is a monotonically increasing function of path length, and approaches zero as the path length approaches zero. The use of the term "optical density" for absorbance is discouraged. In physics, a closely related quantity called "optical depth" is used instead of absorbance: the natural logarithm of the ratio of incident to transmitted radiant power through a material. The optical depth equals the absorbance times ln(10).