[Enhanced Senses] Darkness, Night Vision, and flickering
 

I’ve been trying to figure out how light and darkness work in GURPS for a long time. I did my due diligence, and I am content with the illumination levels table from page 13 of GURPS Enhanced Senses (version 1.1, not 1.0). However, there are some parts of the rules that are little bit unclear to me, and I’d greatly appreciate if somebody would shine some light on them. I feel that light and darkness are very important rules, and since they come up very often, it’s nice to have consistent and clear rules.

1. GURPS Powers: Enhanced Senses says that torch is equivalent to an illumination level of -1. In Basic Set, it is -3, in Template Toolkit 2: Races it is -2, in DF it is 0. Enhanced Senses, however, mentions that flickering lights add another -1 to -3. Is this flickering penalty taken into account in the table? I mean, does -1 mean (0 illumination with -1 for flickering), or should this penalty be applied to this -1? That would bring the torch down to -2, -3, or -4 (two of which correspond to torchlight intensity from other sources).
2. “A light source in a dark environment eliminates or reduces Vision penalties within the illuminated area but *prevents* seeing outside it. Treat the light as “in plain sight” to distant observers who have a clear view of it: +10 to Vision rolls to notice the lighted area (p. B358). However, they’re at ‑7 to see anything else.” This rule also exists in GURPS Underground Adventures, but is phrased slightly differently – “However, they’re at -7 to see anything outside its radius.”
This causes many questions, especially when multiple light sources are visible, and Night Vision is present. I drew a diagram that demonstrates the situation. I don’t know if it is correct. The ambient darkness penalty is -5, and there are two groups with torches, and some without torches.

Diagram link

3. “Prolonged exposure to intense light can cause lasting or permanent vision loss.” How long is prolonged?

Re: [Enhanced Senses] Darkness, Night Vision, and flickering
 

I have GURPS Enhanced Senses version 1.0, so I don't know if that is one of the things that are different, but it shows torchlight having the penalty of -4.

My copy of Basic Set say it reduces it from -10 to -3.

My copy of Template Toolkit 2: Races also has it at -2.

In DF, Torches ELIMINATES light penalties out to 2 yards, at which point:

DF Exploits page 19:
So, in DF, torches reduce the penalty to 0 out to 2 yards, then -3 out to 6 yards, pass that its whatever the original penalty is.

I cannot find anywhere in my copy of Enhanced Senses any mention of flickering lights. Can you provide a page reference?


GURPS Underground Adventures Page 10, above where you found the quote:

The reason it used the word "radius" was to tie it back to that sentence. "within the illuminated area" is the same as "radius"

Anyone looking towards the light source gets +10 to their vision rolls to see whats within the illuminated area/radius, but have a -7 to their vision rolls to notice things outside that area/radius.

This quote does not appear in any of the books, but in Enhanced Senses it has:

And the rest of that sentence:

Re: [Enhanced Senses] Darkness, Night Vision, and flickering
 

If I recall correctly, on the release of Enhanced Senses, there was a forum brouhaha over the fact that its rules for darkness penalties were inconsistent with previous GURPS (and with basic common sense- darkness penalties for fighting in a suburban living room, forsooth!), so later editions had the table modified. Since it's a strictly electronic document, you should be able to re-download it at no extra cost and see what it's like now.

Re: [Enhanced Senses] Darkness, Night Vision, and flickering
 

Enhanced Senses p 13, in the Illumination Levels box. Inline in the second half of first paragraph.

The text seems clear enough (emphasis mine).

So, yes, add a flickering penalty to a torch, should that torch be flickering. (As a properly atmospheric low-tech dungeon fantasy torch ought! :) But not all flames flicker.)

One reason there are new rules for illumination in Enhanced Senses is that the existing examples in early 4e works are simply not consistent. Those early sources weren't examples derived from this set of rules, unpublished but known to SJG authors; this set of rules was worked out afterward, matching the existing rules as nearly as possible, and just ignoring the outliers where necessary. It's nice that this example works out, but don't expect every existing reference in other rules to line up exactly. (I tried, once, while looking at Anthony's house rules for illumination, which were similar to the ones that appeared in Enhanced Senses.)

This one I also find confusing as written. I have some thoughts, but I'll need to think a bit and put them into shape.

However, from your diagram, I'd say that the +10 to spot a light source is for just the light source itself, not everything within its illuminated radius. All those objects already have a light level that gives the penalty to spot them. It's easy to spot the torch, even a long way away. That doesn't make it equally easy to see everything around the torch; that's what illumination levels are for.

The -7 for outside observers looking in is the confusing part to me.

The penalties really ought to be a sliding scale based on the illumination level differences between observer and subject. They seem to be assuming everything not illuminated is at -10 (unlike the ambient illumination in your diagram), and that the observer is at that illumination.

Depends on the intensity of the light and its wavelengths, as well as repetition. Lasers can be fractions of a second. Direct sunlight -- staring at the sun -- can be seconds to minutes. Reflected sunlight from waves or UV exposure can be accumulated over days, months, or years -- the sort of thing that might be better reflected as a kind of Cyclic damage that accumulates vision penalties rather than HP damage, or an additional effect added to aging rolls. Focus -- the amount of area the light affects -- also matters; those lasers can easily burn spots on your retina, even when they seem like they're "low" powered just from the electrical consumption. And a human eye is much, much smaller than the square meter used in the definition of lux and lumen. That 500 milliwatt Class 4 laser spot is a tiny fraction of a square meter, all on the retina, whereas the 100 W lightbulb is throwing out energy in all directions, most of which isn't even visible light. This one's probably up to GM call.

Here's more than anyone wants to know on the topic. Turning that into gameable rules might be difficult.

Re: [Enhanced Senses] Darkness, Night Vision, and flickering
 

For those who are curious about the forum brouhaha, that's https://forums.sjgames.com/showthread.php?t=132662

Re: [Enhanced Senses] Darkness, Night Vision, and flickering
 

The -7 is for the case where you've looked at a flashlight, or a campfire, or a torch, and your eyes have adapted to that light level, and now you turn and look into the darkness and you can't see anything, because your eyes haven't adjusted.

Re: [Enhanced Senses] Darkness, Night Vision, and flickering
 

Also, the listed brightness for a torch is at 1 yard; subtract 1 for 2 yards (actual effective distance will rarely be less than 2 yards, because the light source is usually held more than a yard above the ground). I originally gave brightness at 2 yards because of that. Penalty will routinely be -3 or worse in real situations because of distance.

Re: [Enhanced Senses] Darkness, Night Vision, and flickering
 

Makes sense. Much like my guess about spotting something else in the dark while looking at the light from a distance. (Hm, I deleted that, didn't I? At least that served the purpose of not further muddying the waters.)

How about a time to readjust? A few seconds, just in my personal experience. Or maybe the penalty fades one point per second, if someone's will to track that. Matters in combat, but negligible when you've got time.

Re: [Enhanced Senses] Darkness, Night Vision, and flickering
 

My experience is that it takes minutes, not seconds.

Re: [Enhanced Senses] Darkness, Night Vision, and flickering
 

It says in the box '2 minutes per -1'. Fast adaptation to changing illumination is mostly between levels that would both be considered 'no penalty' in GURPS (e.g. your typical 'come in from bright sun into a house interior' is dropping from something like 100,000 lux to 1,000).