you will only need an IR Filter in your dichroic head, if you are processing color paper. The cyan forming layers of most color papers is sensitive down to ~750nm. Since almost all enlarger lenses are not corrected for IR, part of the paper "sees" an unsharp image. BTW: the same happens with the UV Portion of the light on all papers (including B&W). Fortunately, most of the UV Light is absobed by the lens (which is not the case with IR Light). Good dochoic heads do have both IR and UV filters.