Viewing infrared extends one of our existing senses. Kant's point was that could be attributes of the thing that do not, in anyway, relate to our senses.

These attributes can't have any effect on the phenomenal world, or they'd be detectable, and by definition, they're not detectable by us. Unless, of course, our free will is part of the noumenal world, and our actions in the phenomenal world are simply one way of viewing our will.