Planck constant over 2 `pi` * c
The reduced Planck constant is the Planck constant divided by 2 `pi`. Planck described radiation emission as discrete packets, quanta, of energy. His constant relates the wavelength to the frequency of the emission. This constant is expressed in units of Mega electron volt * femtometers
The Planck constant over `2pi` times c is specified with the standard uncertainty: `0.0000044` MeV fm
See Uncertainty of Measurement Results, a discussion provided by NIST of the application of uncertainty to the documented constants.