Brightness of a 5 ° spot was varied with neutral density filters (minimum steps of 0.1 log) until a response was obtained that was double the background activity, yielding the threshold level for that point on the visual field. (journals.plos.org)
To adjust the sensor for various skin tones and sunscreen use, the group added UV neutral density filters that can speed up or slow down the discoloration time of the sensor. (sciencedaily.com)
The first way is by measuring the densities of electrons and O2 + ions, as well as neutral densities, and then calculating the oxygen photochemical loss rate. (sciencetrends.com)