TREPR time profiles of cell cultures at 347.9 mT. Black indicates 355 nm, 110 mW; red indicates 532 nm, 110 mW. (Inset) Minimum TREPR signal amplitude (a.u.) versus number of photons per second (1017). Black indicates Sepia–RPE 355 nm, linear fit P < 0.0001; red indicates Sepia–RPE 532 nm, linear fit P < 0.0002; green indicates RPE 355 nm, linear fit P < 0.0001; blue indicates RPE 532 nm, linear fit P = 0.009.