GURPS illumination rules are kind of scattered everywhere, so I put this table together, mostly based off of the box on p.20 of High-Tech: Electricity and Electronics, and the errata'd version of the table on p.13 of Powers: Enhanced Senses (which can be found here). You can view my table on google sheets here.
A couple notes on the examples:
Based on this forum thread, any light source which gives a single penalty (like the -5 for candlelight or the -3 for torchlight) is based on the penalty at 5 yards. So candles and torches give -3 and -1 penalties, respectively, at 1 yard.
p.111 of Dungeon Fantasy: Adventurers contradicts that, saying that a lantern or torch "Eliminates darkness penalties in a two-yard radius", which would put them two steps higher on the table. My best guess on how to square that with prior rules to the contrary is that lighting penalties are getting rounded—the table on p.45 of Dungeon Fantasy: Spells only has examples for penalties of 0, -3, -5, and -7. So the -1 or -2 in that two-yard radius would get treated as no penalty.
Also worth mentioning is that Kromm clarified here that light sources measure distance from the center hex, not radius. So the ranges on my table match up with reach (p. B388), not spell area (p. B239), which is off by one.
