Standardizing photographic measurements of coral albedo with RAW imaging and calibrated reflectance targets

Coral pigment condition is routinely assessed from photographs, yet most commonly used image-based “brightness” metrics are derived from rendered images optimized for visual appearance rather than physical measurement. This limits comparability across cameras, workflows, and studies, and complicates interpretation of photographic signals in terms of intrinsic coral properties. Here we evaluated a simple, reference-based workflow for estimating coral albedo from digital images using unrendered camera measurements scaled to a calibrated reflectance standard. Coral specimens were imaged outdoors under diffuse illumination alongside a Spectralon 99% reflectance target, with paired RAW and JPEG images acquired for each scene. Broadband grayscale albedo estimates derived from these images were copared with independent spectrally derived albedo estimates computed from measured reflectance spectra convolved to the camera response. RAW-derived albedo closely matched the spectrally derived benchmark across coral and a ColorChecker target, whereas JPEG-derived albedo exhibited systematic albedo-dependent positive bias and substantially greater error. A brief sensitivity analysis further shows that the use of uncalibrated “white” reference materials produces predictable inflation in estimated albedo when their true reflectance differs from the assumed value. Additional comparisons across multiple coral species showed that the divergence between rendered and unrendered image-derived albedo estimates was consistent across taxa. Together, these results demonstrate that rendered image products are unsuitable for quantitative albedo estimation and that physically interpretable, repeatable photographic measurements require RAW image acquisition, reference-based normalization, and calibrated reflectance standards.