Underwater imaging without color distortions requires RAW capture

1. Consumer cameras are not designed to be scientific instruments, yet they are increasingly used in science not just to see, but to measure. This is possible because camera sensors respond approximately linearly to the light they receive, a property that can, in principle, support quantitative measurements of color. 2. Whether that quantitative measurement potential is preserved depends on what happens to the image after capture. Minimally processed sensor images, commonly known as RAW, retain the approximately linear response needed. Instead of RAW, however, the default output of most cameras is a heavily processed image optimized for visual appearance, compact storage, and immediate use, typically saved as a JPEG. 3. The convenience offered by an in-camera processed image comes at a scientific cost. RAW capture requires more storage and processing effort but leaves calibration choices open. Processed images reduce those demands at acquisition and in post-processing but transfer irreversible decisions about color manipulation from the researcher to the camera. 4. The preservation of RAW sensor data is necessary but not sufficient for quantitative color measurement. Pixel values must also be calibrated to account for the light conditions under which the image was acquired. Underwater, this calibration is more challenging than in clear air, because light is absorbed and scattered as it travels through water, resulting in effects that depend on both wavelength and distance. Consequently, color distortions vary across the scene and generally cannot be corrected with a single global color transformation like white balancing; instead, calibration requires information about the imaging distance. We explain the consequences for quantitative imaging and introduce the 3P protocol for distortion-free color imaging underwater.