# Extinction coefficient & quantum yield units? **URL:** \ **Category:** Theory\ **Tags:** fluorescent-proteins\ **Created:** [May 26, 2020, 3:31pm UTC](https://forum.microlist.org/t/extinction-coefficient-quantum-yield-units/1007 "2020-05-26T15:31:18Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![LPUoO](https://avatars.discourse-cdn.com/v4/letter/l/9fc348/32.png) [@LPUoO](https://forum.microlist.org/u/LPUoO)\ **Post date:** [May 26, 2020, 3:31pm UTC](https://forum.microlist.org/t/extinction-coefficient-quantum-yield-units/1007/1 "2020-05-26T15:31:19Z")
Hi, If the extinction coefficient is the amount of light absorbed at a given wavelength, at a given excitation intensity why isn’t it a percentage? Idem for the quantum yield? What are the standard units? Thanks ---
**Author:** ![jennifer](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.microlist.org/jennifer/32/634_2.png) [@jennifer](https://forum.microlist.org/u/jennifer)\ **Post date:** [May 26, 2020, 3:45pm UTC](https://forum.microlist.org/t/extinction-coefficient-quantum-yield-units/1007/2 "2020-05-26T15:45:14Z")
Tagging @joachimgoedhart & @talley ---
**Author:** ![joachimgoedhart](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.microlist.org/joachimgoedhart/32/26_2.png) [@joachimgoedhart](https://forum.microlist.org/u/joachimgoedhart)\ **Post date:** [May 26, 2020, 5:44pm UTC](https://forum.microlist.org/t/extinction-coefficient-quantum-yield-units/1007/3 "2020-05-26T17:44:19Z")
The extinction coefficient is a parameter that reflects how efficiently light is absorbed (or goes ‘extinct’) at the maximum wavelength. This will also depend on concentration and path length. Hence, the units are ‘per molar’ and ‘per cm’ or: M-1 cm-1 You could express a relative extinction coefficient, which would be the absorbance at a certain wavelength relative to the peak wavelength, but this is uncommon (but perhaps useful). The quantum yield does not have units, it is a relative measure. Usually it is expressed as a number between 0 and 1, but it can equally well be reported as a percentage. Hopefully this answers (part of) your questions… ---
**Author:** ![bbeliveau](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.microlist.org/bbeliveau/32/32_2.png) [@bbeliveau](https://forum.microlist.org/u/bbeliveau)\ **Post date:** [May 26, 2020, 6:53pm UTC](https://forum.microlist.org/t/extinction-coefficient-quantum-yield-units/1007/4 "2020-05-26T18:53:50Z")
@LPUoO If you are interested in the math behind the extinction coefficient, you can check it out here: [https://en.wikipedia.org/wiki/Beer–Lambert\_law](https://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law) ---
**Author:** ![kdean](https://avatars.discourse-cdn.com/v4/letter/k/f475e1/32.png) [@kdean](https://forum.microlist.org/u/kdean)\ **Post date:** [May 26, 2020, 9:43pm UTC](https://forum.microlist.org/t/extinction-coefficient-quantum-yield-units/1007/5 "2020-05-26T21:43:25Z")
If you want to have a little extra fun, you can convert the extinction coefficient into the absorption cross-section of a molecule which is usually around 10^-16 cm^2.