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)
Say hello to the NiSi Prosories P1 Kit: a smartphone filter kit that includes a phone clip, medium graduated neutral density filter, polarizer, pouch, and holder. (dpreview.com)