| abstract |
An optical resonant assembly which comprises a plurality of partially light-transmitting mirrors, a layer of photochromic material disposed between the mirrors, a light source providing a light beam of variable intensity incident on the layer of photochromic material and detecting means for determining transmittance values of said incident beam at different intensity levels, said photochromic material having a low quantum yield for bleaching with light of a wavelength corresponding to said light beam and being selected from pyran compounds of the general formula (I) below: ##STR1## wherein X and Y together represent a spiro-adamantylidene group or a spiro-carbocyclic or heterocyclic group or X and Y independently represent hydrogen, alkyl (preferably lower alkyl having 1 to 5 carbon atoms) or phenyl and R 3 and R 4 each independently represent hydrogen, alkyl, aralkyl, aryl, halogen or a heterocyclic group and R 1 reprsents hydrogen, lower alkyl, aryl, aloxy, alkyl- or dialkyl amino, halogen, or heterocyclic or R 1 represents a fused benzene ring at the 5, 6 or 7, 8 positions. |