This detailed paper https://www.aoa.org/Documents/CRG/Blue%20Light%20and%20Eye%20Damage.pdf from the American Optometric Association details the various types of short term (i.e. snow blindness) and long term eye damage resulting from exposure to sunlight, mentioning specifically various portions of the spectrum (UV-A, UV-B, visible blue light). “the most offending portions of the EM spectrum are the UV-A (315 nm to 400 nm), UV-B (280 nm to 315 nm), and “blue-light” portion of the visible spectrum (380 nm to 500 nm)”
Various websites such as https://healthcare.utah.edu/healthfeed/postings/2018/07/sunglasses.protect.php suggest that quality of UV protection in the marketplace varies widely and is not well connected to perceived darkness or cost. From https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5820813/#!po=0.757576: “Shade, sunglasses, and prescription glasses provide some defense against direct solar exposure of the eyes. However, they may not protect the eyes from diffuse, ambient, scattered, and surface-reflected UVR and may cause a decrease in normal defense reactions such as squinting and pupillary constriction in the absence of direct solar light.”
Because “snow blindness” can occur in a single day (https://www.aao.org/eye-health/diseases/photokeratitis-snow-blindness) It is occasionally the case that a pair of questionable (low-cost, borrowed, secondhand) sunglasses are the best available option.
What are good ways to tell if a pair of sunglasses are protecting your eyes, either in absolute terms or just in comparison to other available options?