Sorry for the tardy reply. The x-axis on the graphs would represent distance from the lamp. I did not record that data since this article wasn't meant to compare lamp PPFD at any given distance. Instead I wanted to compare sensor response to PPFD. I've taken a bit of heat for not providing that information and realized after the fact that I should have recorded - and reported - it.
Maybe Richard Harker will supply that data in his follow-up article.
