product of the Planck constant and frequency
Note 1 to entry: Photon energy can be emitted, transferred or received.
Note 2 to entry: The energy of monochromatic radiation,
Note 3 to entry: The corresponding radiometric quantity is "radiant energy". The corresponding photometric quantity is "luminous energy".
Note 4 to entry: The photon energy is expressed in joule (J = W·s).