The principles of calorimetry have been adapted for measuring laser high power up to 25kW continuously, during the program Eurolaser EU 194, from the EUREKA project between 1988 and 1990. This is one of two major French laboratories participating to this program: Laboratoire National d'Essais (LNE), who has developed two laser calorimeters: one with absorbing cone (1 to 5kW) and other with gradient absorption cylinder (1 to 50kW) successfully tested at the time on the main European sources up to 25kW continuously. Article in PDF attached published in the books of the National Bureau of Metrology describes in detail the principles: optical, thermal, hydraulic and metrology used for these instruments.