https://phys.org/news/2018-05-power-meter-door-in-situ-real-time.html

"Measuring laser power by measuring the pressure of a laser beam hitting a mirror is a very unique technique, [and] so far it is the only laser power measurement technique that is truly an in-situ process," said Alexandera B. Artusio-Glimpse, a scientist of NIST in Boulder, Colorado, and the primary author. "Unlike any other optical power measurement techniques, our method allows us to continue using the laser for work while a measurement is being taken."

Artusio-Glimpse explained that traditional high-power meters measure laser power by absorbing all the energy of a laser beam as heat and measuring the temperature change. The calorimetric measurement has to temporarily stop the laser beam from work for around tens of minutes.