a paint to isolate from heat

[9873210]

It's the same spectrum

In all cases its the electromagnetic spectrum that you are dealing. For optical light this is about 400 to 700 nm.

Ultra violet often means from about 100 to 400 nm and infra red is greater than 700 nm.

While they are all electromagnetic waves materials can respond quite differently to different wavelengths. That is why things have colours.

Engineers can and do design paints and other substances to have different absorption at different wavelengths. Coloured paints are a notable example. Another is low-e coatings on glass, they transmit most visible light while reflecting most of the UV and infrared. You can see through the film but much of the incoming energy is reflected.

If this is the same paint I have previously read about and IIUC, it is designed to be highly reflective at shorter wavelengths (including visible and short IR) where most of the incoming solar energy is, but highly emissive (which also means low reflectivity) at longer wavelengths that can radiate into the cold of space. The claim is not simply that it does not absorb energy, but that it can transmit more energy than it absorbs even in direct sunlight, making it better than a perfect reflector.

That does sound fascinating - I'll try and have a read about it.
I thought a perfect reflector did not absorb or transmit anything (but this would be vague flashbacks to optical transmission lectures and stuff about total internal reflection)

Your understanding of what I meant by a perfect reflector is sound. If the roof is a perfect reflector there is no absorption or transmission, so trivially no net energy transfer through radiation. The idea is to tailor the spectrum to get a net loss through radiation. The net loss is better for keeping a house cool.

Note that the paint is tailored to the environment, I believe the design case is a roof in the Arizona desert. It would not help, say, a domestic fridge, where there is no sun and no sky in the field of view. Probably would not work as well, or perhaps at all, under a cloudy sky because the sky would look different at all wavelengths.