Absorbance, absorptance and friends

Most photobiologists sooner or later have to measure light absorption, reflection and/or transmission by objects such as plant leaves, optical filters or solutes in a liquid medium. The physical quantities we measure may vary: absorbance, optical density, absorptance, transmittance and reflectance. In addition to these extensive properties of objects we use intensive properties such as the molar extinction coefficient and refractive index. This article disusses the defintions of these quantities and exemplifies some of their uses in research in the field plant photobiology including the study and application of UV radiation. It also touches on some less frequently discussed alspects such as the effect of the angle of incidence on the reflectance at the surface of windows and optical filters.