HydroColor is a water quality application that uses the iPhone camera to determine the reflectance of natural water bodies. Using this information, HydroColor can estimate water turbidity (0-80 NTU), concentration of suspended particulate matter (SPM) (g/m^3) and the backscattering coefficient in the red (1/m). IMPORTANT: HydroColor requires the use of an 18% photographers gray card as a reference. Gray cards are widely available at photography shops and online. Visit the supporting website for more information on gray cards.

HydroColor has an easy to use interface that guides users through the collection of three images: a gray card image, a sky image, and a water image. To ensure the best results, HydroColor taps into the iPhone’s GPS, gyroscope, and compass to aid users in the collection of these images. After images are collected they can be analyzed immediately. In the analysis of the images, HydroColor calculates the reflectance of the water body in the RGB color channels of the camera. It then uses the reflectance values to determine the turbidity of the water in NTU (nephelometric turbidity units).

The data is instantly saved and can be accessed again via HydroColor or downloaded onto a computer from the documents folder (accessible in iTunes). The text file contains additional information about the measurement including: latitude, longitude, date, time, sun zenith, sun azimuth, phone heading, phone pitch, exposure values, RGB reflectance, and turbidity.

How it works:

HydroColor uses the iPhone camera as simple light sensor (photometer). Relative light intensity can be measured by normalizing the camera pixel values by the exposure. Therefore, the three color channels of the camera (RGB: Red, Green, Blue) provide a measure of light intensity in three regions of the visible spectrum.

The light intensity measured in the water image is corrected for sky reflection off the surface (using the sky image). The corrected water image provides the intensity and color of light emanating from the water. This is normalized by the ambient illumination using the gray card image. The final product is a nearly illumination independent measure of the water’s reflectance, known as remote sensing reflectance. In oceanography, satellites are used to calculate the same product (remote sensing reflectance) from space.

The reflectance is directly related to the amount and type of suspended particles in the water. An increase in turbidly (i.e. suspended sediments) will cause greater scattering of light and increase the overall reflectance of the water. Particles containing pigments, such as phytoplankton (algae), will absorb light in specific regions of the visible spectrum. Thus, pigment containing particles can be detected by comparing the relative reflectance in the RGB channels.

The method HydroColor uses to measure reflectance has been published in the peer-reviewed journal Sensors and is freely available online (note: HydroColor has been updated to use RAW data from the camera sensor since this publication):

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.