HydroColor: Water Quality App 2.3

HydroColor is a specialized water quality application that leverages the iPhone camera's capabilities to assess the reflectance of natural water bodies. By employing this data, HydroColor estimates key water quality parameters such as turbidity (0-80 NTU), concentration of suspended particulate matter (SPM) in g/m³, and the backscattering coefficient in the red wavelength (1/m). Notably, the app necessitates the use of an 18% photographer’s gray card as a reference for accurate measurements, which can be conveniently sourced from photography retailers or online platforms.

The application features an intuitive interface that facilitates users in capturing three essential images: a gray card, a sky, and a water image. To optimize data collection, HydroColor utilizes the iPhone's GPS, gyroscope, and compass functionalities. Following image acquisition, users can perform immediate analysis. The application calculates the reflectance of the water body across the camera's RGB color channels and subsequently deduces water turbidity expressed in NTU (nephelometric turbidity units).

Data collected is promptly saved for future access via HydroColor or can be downloaded to a computer directly from the documents folder in iTunes. The accompanying text file contains valuable measurement details such as latitude, longitude, date, time, sun zenith, sun azimuth, device orientation, exposure settings, RGB reflectance values, and turbidity levels.

The operating principle of HydroColor positions the iPhone camera as a basic light sensor (photometer) capable of gauging relative light intensity by normalizing pixel values according to exposure levels. The measurement encompasses three color channels—Red, Green, and Blue—yielding data on light intensities across visible wavelengths.

To refine accuracy, light intensity measured from the water image is adjusted for sky reflection by referencing the sky image. This corrected image reveals the true intensity and color of light reflected from the water. Normalization against ambient illumination is accomplished using the gray card image, resulting in a measure termed remote sensing reflectance that is largely independent of lighting conditions. This method mirrors techniques utilized in oceanography where satellites compute similar metrics from space.

Reflectance directly correlates with the quantity and type of suspended particles within the water column. Increased turbidity—often indicative of suspended sediments—enhances light scattering and raises overall water reflectance. Moreover, particles rich in pigments, like phytoplankton (algae), absorb light at specific wavelengths, allowing for their detection through comparative analysis of reflectance across the RGB channels.

This innovative approach to measuring reflectance is substantiated by research published in the peer-reviewed journal Sensors, which is accessible online: Leeuw, T.; Boss, E. The HydroColor App: Above Water Measurements of Remote Sensing Reflectance and Turbidity Using a Smartphone Camera. Sensors 2018, 18, 256. https://doi.org/10.3390/s18010256. Subsequent updates have enabled HydroColor to utilize RAW data from the camera sensor since this initial publication.