How are safe exposure levels to solar radiation calculated?
Dr. B. Ralph Chou laid out this method in Sky & Telescope, February 1998, p.36-40, which is recast in the FAQ maintainer’s own words: The damage levels for each wavelength of light are well known. This allows the safe level to be calculated for a filter, using as a starting point the ratio between the maximum solar intensity and the damage threshold. Just to be extra safe the maximum allowable transmittance is then set to between 1% and 0.1% of this ratio. For the bandpass between 3800 and 14,000 angstroms (blue through near infrared), a filter that transmits .0032% is safe. This corresponds to a shade number of 12 (welders glass is rated by shade number). For visual comfort, a darker filter of .0003% transmittance (equivalent to shade 14, or density ~4.5) is recommended.
