infrared - Would this IR-transparent plastic be useful for blocking unwanted control flash?

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?

Answer

Yes! It works very well. I carefully scored the 1"×1" square and broke it in two; the resulting 1"×½" piece fits exactly over my pop-up flash. I hold it in place with transparent double-sided tape, which occasionally needs refreshing. A little bit of visible red light does get through, so I added a couple of layers of Congo Blue filter gel (which is also IR transparent). Perfect.

Unfortunately, it's too thick for my pop-up flash to close with the filter attached. I've considered carefully removing the built-in plastic cover and replacing it with this, but I haven't worked up the nerve to risk not being able to put it all together again.

Still, it works just fine and is easily kept in a little pocket on the bag which I store my external flash in.