Right near the end of the movie, when Glover is fighting the predator. The predator slashes Glover's stomach and Glover stumbles. We then see the predator looking down at Glover. However, when we see through the predator's vision, its in infa red, which is weird because he is no longer wearing his mask. When the mask is off, he should see all red.
Predators do not see infra-red because of the mask. They naturally see in infra-red. The mask simply allows them to see in frequencies they normally can't. Its the same in the comics. They changed this for the Aliens versus Predator 2 game, but that was only to make the Predator a bit less powerful. In predator, after the predator loses his mask, he still sees in infra-red, but its a bit washed out, like the ambient heat is overloading his senses. The same thing is happening in Predator 2. So the mask makes it easier to see when there is a lot of heat in the environment, like sunglasses help us see when it is bright out. But the predators do not see like us (that is, in the optical spectrum) what we see in the movie is just a representation of what their vision could possibly look like to us.