A fiber-optic measurement system (100, 200, 300) for continuous detection of a temperature curve and/or of a temperature of a water-based system in a temperature range from 5 to 95°C, comprising: - a temperature-sensitive element (101, 201, 301) comprising a substance composition, the substance composition having a first thermoresponsive polymer, at least one second thermoresponsive polymer, and a solvent and/or a swelling agent, the first thermoresponsive polymer and the at least second thermoresponsive polymer having different lower critical solution temperatures or different upper critical solution temperatures, the substance composition being adapted so that at a temperature T above an initial temperature T A and below a final temperature T E its optical transparency has a monotonous course as a function of the temperature T, so that measurement of the optical transparency of the substance composition allows optical determination of the temperature T, when T E > T > T A, the solvent and/or swelling agent being water, the thermoresponsive polymer being selected from among poly(dialkylacrylamide)s, poly(diethylacrylamide-co-dimethylacrylamide)s [poly(DEAM-co-DMAM], or poly((methoxyethoxy)ethyl methacrylate-co-oligo(ethylene glycol) methacrylate)s [poly(MEO 2 MA-co-OEGMA], - at least one light-conducting optical fiber (102, 103, 202, 205, 206, 208, 302, 305, 306, 308, 310); - a light source (104, 203, 303); and - at least one optical detector (105, 207, 209, 309); the temperature-sensitive element (101, 201, 301) being connected with at least one light-conducting optical fiber (102, 103, 202, 205, 206, 208, 302, 305, 306, 308, 310), so that light of the light source (104, 203, 303) can be guided to the temperature-sensitive element (101, 201, 301) and from there to the optical detector (105, 207, 209, 309), so that a transparency of the temperature-sensitive element (101, 201, 301), which can change due to the substance composition, can be detected by the optical detector (105, 207, 209, 309) and associated with a temperature on the temperature-sensitive element (101, 201, 301). - a temperature-sensitive element (101, 201, 301) comprising a substance composition, the substance composition having a first thermoresponsive polymer, at least one second thermoresponsive polymer, and a solvent and/or a swelling agent, the first thermoresponsive polymer and the at least second thermoresponsive polymer having different lower critical solution temperatures or different upper critical solution temperatures, the substance composition being adapted so that at a temperature T above an initial temperature T A and below a final temperature T E its optical transparency has a monotonous course as a function of the temperature T, so that measurement of the optical transparency of the substance composition allows optical determination of the temperature T, when T E > T > T A, the solvent and/or swelling agent being water, the thermoresponsive polymer being selected from among poly(dialkylacrylamide)s, poly(diethylacrylamide-co-dimethylacrylamide)s [poly(DEAM-co-DMAM], or poly((methoxyethoxy)ethyl methacrylate-co-oligo(ethylene glycol) methacrylate)s [poly(MEO 2 MA-co-OEGMA], the substance composition having a first thermoresponsive polymer, at least one second thermoresponsive polymer, and a solvent and/or a swelling agent, the first thermoresponsive polymer and the at least second thermoresponsive polymer having different lower critical solution temperatures or different upper critical solution temperatures, the substance composition being adapted so that at a temperature T above an initial temperature T A and below a final temperature T E its optical transparency has a monotonous course as a function of the temperature T, so that measurement of the optical transparency of the substance composition allows optical determination of the temperature T, when T E > T > T A, the solvent and/or swelling agent being water, the thermoresponsive polymer being selected from among poly(dialkylacrylamide)s, poly(diethylacrylamide-co-dimethylacrylamide)s [poly(DEAM-co-DMAM], or poly((methoxyethoxy)ethyl methacrylate-co-oligo(ethylene glycol) methacrylate)s [poly(MEO 2 MA-co-OEGMA], - at least one light-conducting optical fiber (102, 103, 202, 205, 206, 208, 302, 305, 306, 308, 310); - a light source (104, 203, 303); and - at least one optical detector (105, 207, 209, 309);