Several dependent measures were collected at baseline and then once every 3 months : serum L and Z measured by HPLC chromatography ; MPOD measured using customized heterochromatic flicker photometry ; photostress recovery assessed by measuring the time needed to recover visual acquisition of a grating target after 30 seconds of an intense xenon white flash exposure ; glare disability evaluated as the energy in a surrounding annulus necessary to veil a central grating target ; and chromatic contrast assessed by measuring thresholds for a yellow grating target superposed on a 460-nm background .