Schlieren detection technology for the weak refractive index gradient of seawater based on photodetectors

The seawater refractive index gradient is a critical optical parameter for characterizing marine environmental dynamics, with significant implications for marine ecology, ocean dynamics, and ocean optics. This paper presents a high-sensitivity schlieren detection technique, where a photodetector replaces the traditional camera for signal acquisition. A theoretical model for the minimum detectable gradient was established and compared with camera-based models. Results indicate that the photodetector-based system can theoretically achieve a detection limit of 1×10^-7<i>R</i><i>I</i><i>U</i>/<i>m</i>, which is 2 orders of magnitude superior to traditional camera-based methods (1×10^-5<i>R</i><i>I</i><i>U</i>/<i>m</i>). A sensing system was designed and constructed, with the refractive index gradient calibrated via knife-edge displacement-voltage response mapping to establish the relationship between voltage variation and gradient changes. Validation was performed in controlled temperature and salinity stratification fields using temperature sensors and refractometers as benchmarks. Experimental results demonstrate an average relative error of less than 8% compared to standard instruments.