Study of the Advantages of Commercial Phone Camera for Measuring the Salinity of Seawater

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Abstract
The optical refractive index reflects all substances dissolved in seawater. In contrast to conductivity, which can only sense the ionized components of seawater, it is one of the ideal parameters for assessing the absolute salinity of seawater. However, existing methods for measuring seawater absolute salinity based on refractive index still face challenges in balancing the measuring interval ([1.329, 1.348] Refractive Index Unit (RIU)) and resolution (10-7 RIU) to meet the needs of seawater measurement. To address this issue and promote the widespread application of seawater absolute salinity measurement, we propose an innovative system that utilizes the Complementary Metal–Oxide–Semiconductor (CMOS) camera module (CCM) in commercial smartphones as a position sensor. By measuring the position change of light emitted from a V–shaped groove, the system determines the refractive index and salinity of seawater. Experimental results show that the system, combined with an optimized gray centroid algorithm, achieves a resolution of 0.3 pixels, resulting in a refractive index resolution of 3.3×10-7 and a practical salinity resolution of 0.0018, with up to 36,666 measurement point. Compared to traditional Position–Sensitive Devices (PSDs), the system’s resolution is improved by 1.45 times, and the number of measurement points is increased by 9.16 times. Further comparative experiments demonstrate that the position measurement standard deviation of the CCM is unaffected by the light spot position, while the standard deviation the PSD measurements depends on the distance between the spot and the center position, which is unfavorable for subsequent data calibration. Moreover, repeatability and stability tests and uncertainty analysis show that using the CCM as a position sensor for measuring seawater refractive index and salinity exhibits excellent stability and repeatability. The smartphone CCM offers diverse options, compact size, low cost and fast development advantages, providing new possibilities for achieving high–resolution and wide–range seawater refractive index and salinity measurement.
Funding Information
  • National Key Research and Development Program of China (2022YFB3205300)
  • Southern Marine Science and Engineering Guangdong Laboratory Guangzhou (GML2021GD0808)
  • Modern Oceanic Service Project at Oceanic Administration of Shandong Province (37000025050809990004M)
  • Marine Metrology and Testing Technology Innovation Center of Ministry of Natural Resources (2025mmit01)

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