A luminous intensity sensor element having a flat disk-shaped element (2) made of material with a high thermal expansion coefficient and of low thermal conductivity, and in turn having, on opposite faces (3, 5), an optically absorbent layer (4) and a reflecting layer (6). On receiving excitation radiation (R), the optically absorbent layer (4) is heated and transmits heat to the flat disk-shaped element (2), which in turn is heated internally in spatially nonuniform manner and is deformed together with the superimposed reflecting layer (6). The reflecting layer (6) receives an incident reference layer beam (S) to generate a reflected light beam having optical characteristics ( DELTA phi ) depending on the aforementioned deformation ( DELTA d) and varying in response to the excitation radiation.