In today's world, the strongest IR filter you can really use is the Hoya R72. The old-fashioned go-to IR filters like the 87 are no longer useable because the current films' sensitivity stops around 820nm, and the 87 starts transmitting light at 870nm. So you'd essentially end up with a roll full of blank film. The Hoya R72 cuts off at 720nm, so you get useable exposures. The regular red 25 and 29 are useable, but will give you a less pronounced infrared effect.