Energy, linear and angular momentum of light: what do we measure?

The most commonly observed quantity related to light is its power or equivalently its energy. It can be either measured with a bolometer, a photodiode or estimated with the naked eye. Alternatively people can measure the light impulse or linear momentum. However, linear momentum is characterized by its transfer to matter, and its precise value is most of the time of little use. Energy and linear momentum are linked and can be deduced from each other, from a theoretical point of view. Because the linear momentum measurement is more difficult, energy is the most often measured quantity. In every physical process, angular momentum, like energy and linear momentum is conserved. However, it is independent and can't be deduced from the energy or the linear momentum. It can only be estimated via its transfer to matter using a torque observation. Nevertheless , experimentally, the torque is found to be proportional to the optical power. This leads to a need of a quantum interpretation of the optical field in terms of photons. Clear experimental evidences and consequences are presented here and debated. light temperature rotation $\theta$ velocity energy flux light angular momentum light radiation pressure