Why 40 cm ... that's not how the equipment is actually used. 40 cm would have been a valid distance in the days of traditional overhead phototherapy lamps.
To satisfy my own curiosity ... what does the Bio-Tek meter give at 100 cm; and (or) at what distance do you get readings in the 35 or 50 range?
Meanwhile, I notice that the Olympic Model 22 Bili-Meter
has a special sensor for testing fibreoptic pads; one that sits directly on the pad. My take on all this is that (unlike the old fluorescent blue lamps) output on these new LED units should not deteriorate over time, and what we are more concerned about is uniformity of output across the mat or pad.
Now I'm wondering what you would get if you covered the pad with a piece of cardboard with just a 1 x 1 cm square cut out of it (then took readings at various positions to test for uniformity by moving the cut-out around the pad). As you have already suggested, a "conversion factor" shall need to be applied when using the test equipment you have to hand. My own suggestion is that all we are looking for here is little more than an "is the equipment working, and giving blue light output in an acceptable range" question. I was thinking about something along the lines of applying the "inverse square law" to arrive at readings of comfort. At 100 cm I would have hoped to see something like 1100 (for the 35 pad) and 1580 (for the 50 pad). I was working towards a "fiddle factor" of 31.6 (being the square root of 1000), but your 40 cm readings seem to have blown that idea out of the water! So now I'm guessing that the Bio-Tek meter is being saturated by blue light from the pad (maybe the active area of the pad needs to be factored in) in a way that light scattered from a fluorescent tube would not. All this could be total BS, of course (see the "Bluffer's Guide to Biomedical Engineering")!