Quote Originally Posted by OleTj
Can you use the 89c with Konica IR? That film only goes to 720nm, which is about the cutoff for the filter...
Not quite true. Konica 750 has a "bi-modal" sesitivity curve with one "peak" at something like 550um, and the other "IR" peak at 750um. Both peaks are uniform (they look like semi-circles) so there is an amount of sensitivity out to ~ i don't know .... 850um or so.

A "Red 25" filter cuts off (attenuates) light below abut 690um rather effectively, so there is no intense gain from an 87 0r 89 filter - and they both have the disadvantage of removing a great deal of the visible light - therefore they are "opaque" - to normal vision.