Planck constant reduced (over 2 `pi`)

The reduced Planck constant is when the Planck constant is expressed over 2 `pi`.  Planck described radiation emission as discrete packets, quantas, of energy.  His constant relates the wavelength to the frequency of the emission.  The constant is used to define the quantum of angular momentum in physics.  This is also known as the Dirac constant.  This constant is expressed in units of  electron volt seconds.

Notes

The Planck constant over `2pi` is specified with the standard uncertainty (standard deviation): `0.000 000 15 * 10^(-16)` eV s

See Uncertainty of Measurement Results, a discussion provided by NIST of the application of uncertainty to the documented constants.