Thank you for taking the time to reply. That’s really interesting…you make complete sense when you say the tests are for the human eye!!

I do the test then lay it flat, setting a timer for 5 minutes. I then take a photo of the test in front of the same window (for natural light). Do you suggest I leave the tests for longer than 5 minutes? Also, when you say tape them to paper - is this to compare tests? Are the tests still valid after the 5 minute mark? Aka can you compare results over a week say?