Infravision describes a character or monster's ability to see clearly up to 60 ft., whatever the amount of illumination. This is due to the anatomy of the creature's eyes, allowing the creature to discern thermal imaging that is blended and enhanced by the collection of very small quantities of light. This works in tandem with visible light, so that unlike rules that have been written in AD&D, infravision is not spoiled by the addition of other light. This means that pitch blackness does not exist for these characters or monsters. They will always be able to see normally to 60 ft., and even farther if a light source is present. An appearance of red in the seer's eyes is present, but difficult to distinguish under the conditions in which infravision is used.
Rules for how infravision and other light meshes together to determine visibility can be found on the light range adjustment table.