Pentax's wireless P-TTL system is optical, and requires a control flash. I'm using the built-in flash on my camera, but one can also use a hotshoe-mounted flash with the right feature set. If you have this flash in "master" mode, it contributes to the exposure (in addition to the light from any slave flashes). In "controller" mode, it's not supposed to.
Unfortunately, with the Pentax K-7 (unlike the previous models) even in this mode the control flash is still bright enough to make obvious reflections. On the one hand, this is good because it gives greater range — a common complaint, and surely why they made the change. On the other hand, well: unwanted reflections.
Nikon sells a product to solve this same problem: the SG-31R. It works because the black plastic blocks visible light, but lets IR through, and the slave flash sensors are sensitive to that. And it should fit any standard hotshoe (possibly with the removal of the locking pin — no big deal), but it looks a little dorky in action.
I'm thinking of fashioning something of my own, made to be a little less obtrusive, and for that purpose I was thinking of using this bit of IR-transparent plastic — Optical Cast Infrared (IR) Longpass Filters from Edmund Optics. But, I'm not sure if it goes far enough into the deep reds: will my subjects have red reflections in their eyes? There's a handy graph on the product web site, but I don't have enough expertise in this area to understand how that'll practically look. Many sources say that visible red goes to 700nm, and this lets plenty of that through.
Any guesses (or charts!) as to the transmission profile of the Nikon product? Should I just break down and buy it?