Light and Darkness

GURPS talks about light and darkness penalties in a lot of places, but lacks concrete physical rules for what level of illumination corresponds to a given penalty.

Without guidelines from GURPS, I went to reality. In reality, vision starts having gradual reductions in color resolution below a few thousand lux, but significant problems don't start below a few hundred. I've set the zero penalty at the normal minimum for night or indoor sports, which is in the 500 lux range. The scaling below that level is in part because it does a fair job of matching the few set numbers we have, in part because there are theoretical reasons that spotting distance really does vary with the 1/4 power of ambient light.

Light Sources

GURPS also gives no way of estimating how light level varies with distance from a given illuminated area. The above chart is designed to work with the standard range levels, by a simple formula: determine the brightness of light the source gives at a fixed range, and then add half the difference in range modifiers. Thus, if a given light produces no penalty out to 100 yards (range -10), at 1,000 yards (range -16) it gives a light level of -3. Note that the example above is a very very bright light source.

For standardized light sources, it's useful to give the light source a 'brightness'. This is the range modifier at which it will give a light level of 0. To determine the actual light level, add the brightness and the range modifier, and divide by 2, rounding down (so if b + rm = -1, that's a light level of -1, not 0). Note that published GURPS light sources tend to significantly overestimate how bright they are, there are basically no portable light sources below TL 7 that will reduce darkness penalties to zero at any range (beyond ranges where it will start fires, at least; a candle produces a light level of 0 at about 2 inches).

Firelight is surprisingly dim, and is also quite uneven, making it poor light at best. Typically, a fire has a brightness no higher than its size modifier, and it may be much lower. In addition, anyone seeing by flame, even if they have night vision, should normally take an additional -2 to DX and perception because the light flickers.

Note that many lights only produce light in a relatively narrow cone. Assume that the width of the cone is equal to its length * degrees/60, so a typical flashlight beam is about 1 hex wide per 15 yards distance. There are searchlights that produce 100 million candlepower in a 1 degree beam. There are personal halogen flashlights that produce a million candlepower, again in a very tight beam.

Spotting Light Sources

Light sources are relatively easy to spot in conditions of low light. When spotting a light source, invert the light level modifier (positive light levels do give a penalty), and use brightness +5 in place of size modifier. Thus, under starlight with no urban lights, at a range where spotting a human would be +0 in normal light (about 100 yards, if there's no cover), spotting a human needs a roll vs Vision-7, whereas spotting a candle (brightness -8, effective SM -3) requires a roll vs Vision+4. Spotting a candle at that same distance in full sunlight would require a roll vs Vision-7.

Buying a Light Source as an Advantage

Any moderate light source is basically a perk, as its advantage for vision is negated by its rather large penalty to stealth. Light sources bright enough to be useful as attacks should be bought as Create (Light) or an Affliction.

Vision Between Light Levels

Normally, it's assume that light levels are relatively even. However, sometimes you have situations where you're looking at something that is much brighter (or darker) than its background. In that case, the modifier is (2 * light level of target - 1 * light level of background), so a sunlit satellite (light +3) against a moonless night sky (light -8) gives a modifier of +14. In the particular case where you are carrying a light source, and that source isn't masked in some way that prevents it from blinding you, effective darkness penalty is half the brightness of your light source, plus the range modifier.