Night Vision Helmet

The effect turns up the apparent brightness of light that is already on screen. In a corridor with a couple of torches it makes the whole thing readable. In a pitch black cave with nothing burning, it does nothing at all. Keep placing torches; this just makes each one go further.

It has to be worn as your real helmet, not tucked into the social slot, so wearing it means giving up whatever head piece your armor set uses and the set bonus with it. That confines it to a mining or building loadout for most people. It also does not stack with the Night Owl Potion, so drinking one while wearing this is wasted.

Two sources, a granite cave or the Cyborg after Martian Madness, and either way the endpoint is the same: it is the base of the Ultrabright Helmet, which is the version worth keeping around long term.