I wouldn't use the term INSULATOR in this case, unless it's the jargon of an obscure application that I haven't heard of. Which is possible, I don't actually know everything,(really, it's true, and no snickers from the peanut gallery, please.) On the other hand, you haven't told us what level you're at, or what you class is about. At any rate, I would call something like that a FILTER. Add there are plenty of UV absorbing visible transmitting substances (heard of any?). Such a wide range of wavelengths comprise the IR, it could be possible for some thing to transmit some of them.