Everything about vision in GURPS
This is going to be yet another large post to compile everything about vision in GURPS, as some of the things are not very clear to a naked eye. I hope that this will be as helpful to you as it will be to me.
Let's start with the basics. Vision, just like all other senses, is based on your Perception secondary characteristic, which in turn is based on IQ. GUPRS Power-Ups 9: Alternate Attributes has alternative options, such as basing Perception on DX, HT, HT+IQ, or making it independent. There's also an option of decoupling senses from Perception, resulting in Vision being at attribute of its own.
Sense rolls, including Vision roll, are described on page 358 of GURPS Basic Set. A successful Sense roll is needed to notice something, but in some cases a follow-up IQ or skill roll is required to understand what you have sensed. Unmodified Vision rolls are made when you are not consciously watching for anything significant, and may or may not notice it in passing, or when a follow-up IQ or skill roll is required. Vision rolls are modified by certain advantages and disadvantages, distance, and by darkness, receiving up tp -9 in penalties for the latter. Unaided Vision rolls are completely impossible in total darkness. Also, you get a +10 bonus to spot something in plain sight, but not to spot hidden objects, read text, identify faces, etc. According to page 15 of GUPRS Powers: Enhanced Senses, this bonus doubles to +20 if you have already located a target and are focusing on it to see small details (such as reading text or seeing facial expressions). Vision-based skill rolls or Quick Contests do not take the +10 or +20 modifier.
When you try to spot something deliberately hidden, this becomes a Quick Contest of the skill used to conceal something (that could be Camouflage, Holdout, Stealth, etc.) and your Vision or another appropriate skill, such as Observation or Search.
The section also talks about the curvature of a planet blocking vision beyond the horizon. On Earth, the horizon for a person five to six feet in height equals about three miles. The exact formula to calculate the horizon for any person of any height on any planet is:
l = sqrt(2Rh+h^2),
where l is the horizon distance, R - planetary radius, h - height
Since h^2 is insignificant compared to R (at least usually), we can waive it, simplifying to formula to:
l = sqrt(2Rh)
Obviously, if you want to get the horizon distance in miles, you have to put R and h in miles too.
GURPS Powers: Enhanced Senses, which is one of my favorite GURPS books, introduces a detailed sensory classification based on range, arc, activity, and the amount of sensory information granted. The same book provides a way to apply various limitations or enhancment to normal vision, pricing them as 1/5 the percentage value of the modifier.
In terms of range, vision is an intermediate-range sense that takes the standard range penalties defined in the Size and Speed/Range Table (p. B550). Thus, Short-Ranged Vision that takes -1/yard in penalties is a disadvantage that costs -2 points, and Long-Range Vision that takes the Long-Disatance Modifiers (p. B241) is an advantage that costs 10 points. Long-Range Vision 2 that takes no range penalties at all costs 20 points. I would be very careful with these, allowing such abilities only with additional limitations as not to make them invalidate Acute Vision or Bad Sight traits.
Arc of vision is important, as it determines what area you can observe, what hexes you can attack, and how well you can defend from attacks from the side or back. By default, you can only make melee attacks against an enemy in one of your front hexes (unless making a Wild Swing), defenses from attacks from the side hexes are at -2, and attacks from behind cannot be defended against. Ranged attacks can be directed at anything within the arc of vision. Also, by default, your eyes can be attacked only from the front or the sides, but certain traits may change that as well. Normal Vision operates in a 120 degree wedge to the front and allows peripheral vision 30 degrees to each side, for a total of 180 degrees. The same applies to vertical arcs.
Standard Peripheral Vision (p. B74) adds 30 degrees in every direction. This improves your observational arc and arc of possible ranged attacks. You can also make melee attacks into side hexes, but one-handed attacks to the opposite sides are still considered a Wild Swing. You can defend against side attacks at no penalty, and can even defend against attacks from behind at -2. Also, you get +3 to Vision rolls to detect Shadowing attempts and ambushes. One of the possible limitations is Easy to Hit, making your eyes easier to hit, and allowing attacks against them from your arc of vision, not just side and front hexes. Realistic Peripheal Vision usually is achieved by moving the eyes to the side. This should not affect the vertical arc, and it can be represented by the No Increase in Vertical Arc, -10% limitation (p. 16 of GURPS Powers: Enhanced Senses). It should also reduce the central overlap from 120 to 60 degrees. If you defend against attacks from outside the 60 degree wedge at -2, then this is a Nuisance Effect, -5%. If you have two tiers of eyes for a full vertical arc, then you have normal side-to-side arc of vision, but vertical arc is 180 degrees. This is a limitation on Peripheral Vision: Vertical Only, -50%.
Furthermore, GURPS Powers: Enhanced Senses mentions Extreme Peripheral Vision on page 16. This one is represented by putting your eyes on the sides of the head (like a rabbit), and covers 300 degrees, costing 20 points. There is no overlap, functionally also imposing Temporary Disadvantage, No Depth Perception, -15% on the advantage. Additionally, No Increase in Vertical Arc, -10% is necessary as well. If you apply both of these limitations, you arrive at 15 points, just as per normal Peripheral Vision.
360° Vision (p. B34) turns your arc of vision into a sphere. You can defend at no penalty against attacks from the sides or rear. You can attack foes to your sides or rear without making a Wild Swing, but are at -2 to hit due to the clumsy angle. This is actually a bit weird, because by RAW you'd take -2 to attack your righthand-side hex with a weapon in your right hand, but you wouldn't have this penalty with Peripheral Vision. However, it is clarified on page 389 that this -2 applies only to attacks to the rear and one-handed attacks to the opposite side. Also, you get +5 to spot Shadowing attempts. Just like for Peripheral Vision, you can take Easy to Hit to make your eyes more vulnerable to attacks. By default you have additional eyes and/or can turn your head 360 degrees. GURPS Powers introduces two new enhancements. Panoptic I, +20% lets you eliminate the need for extra eyes or turning your head (basically, this is No Signature, +20%). Panoptic II, +60% lets you see even if blinded or blindfolded, but you still need light, unless you have advantages that eliminate the need for light. The book also suggests Switchable, +10% to be able to grow additional eyes. GURPS Powers: Enhanced Senses says that realistic 360° Vision requires at least one eye on the back of your head. In that case, it has limited vertical arc and cannot overlap with the two frontal eyes, giving it the limitations for No Depth Perception when looking backward, a Nuisance Effect, -5%. If you have two eyes on the back of your head, then this problem is avoided.
Even if you have Peripheral Vision or 360° Vision, you get an extra -2 to parry an attack from behind, and you cannot block them at all, unless your weapon or shield arm is Extra-Flexible or you have the Double-Jointed advantage.
Whether the movement point costs for moving sideways or backward are affected by expanded arcs of vision is not stated anywhere.
Addendum: It seems that the penalties to defense from the sides and from the back are based on vision, since Peripheral Vision and 360° Vision lower or eliminate them. Won't that mean that a blind person who is aware of the attacker would get an additional penalty to defend as if struck from the back or side? I don't know.
Your arc of vision can also be restricted by disadvantages. No Peripheral Vision (p. B151) removes 30 degrees from each side, turning your arc of vision into a 120 degree wedge. I assume that it does the same vertically, but it's not said anywhere. You left and right hexes become back hexes.
Tunnel Vision (p. B151) turns your arc of vision into a 60 degree wedge in the front. Now you have only one front hex, two side hexes, and three back hexes.
Here's a chart of arcs of vision for you.
Next classification category is passive/active. Obviosuly, vision is passive as it does not emit any signal, but constantly receives light.
Finally, in the sensory hierarchy (GURPS Powers: Enhanced Senses, pp. 6-7), vision is a precise sense. That means that a successful Vision rolls makes you aware of the presence of the target, direction, distance, and details of submillimeter resolution, and that you can Aim ranged attacks using Vision. You can also apply some new modifiers to vision, such as Profiling, +50% or Targeting, +20%. The first one will create a mental database, making visual recognition easier. And the second one will make your aim better.
Now, let's go over all the disadvantages related to vision (aside from No Peripheral Vision and Tunnel Vision that were covered before).
First, we have Bad Sight (p. B123). In the GURPS Basic Set, there's only two forms - nearsighted and farsighted. Nearsighted makes it impossible to read anything more than 10 yards away, and imposes a -6 penalty to Vision roll to spot items more than a yard away. Melee attacks are at -2, and when making ranged attacks you have to double the distance to the target when determining range penalties. Farsighted makes you able to read text thrice slower and with great difficulty. You are at -6 to Vision to spot objects within one yard, and at -3 to DX to any close manual tasks, including melee combat. These disadvantages are quite crippling, but at TL 5+ you must take them with the Mitigator limitation because you can buy glasses. If cheap and easy sight treatment is widely available, then this disadvantage should be downgraded to a quirk as per page 11 of GURPS Power-Ups 6: Quirks. GURPS Powers: Enhanced Senses allows you to take both forms as an alternative disadvantage to represent your eyesight changing over the years. Also, it introduces two new forms. Low Resolution gives -4 to spot objects at any range, and -8 to resolve fine details. If it represents ostigmatism, you can take Mitigator. No Fovea usually is the result of being struck with a blinding laser. You can navigate using peripheral vision, but you defend at -2, and all your attacks count as Wild Swings (it says all attacks; I assume that it means that your ranged attacks are treated as shooting blindly). You cannot read, and must work by touch when performing fine manipulations. Also, you are at -3 to DX for close combat and larger mechanical tasks. Pizard also introduces a new variant - Motion Sensitive that is intended for realistic animals.
After that we have Blindness (p. B124). What I find interesting here is that it works differently when purchased during character generation and when imposed via an Affliction. In the latter case, you fight as if in total darkness - at -10. In the former case, you are more adjusted to blindness, and you are at -6 to all combat skills and cannot target specific hit locations. When using ranged weapons, you can attack randomly or by using hearing. Obviously, it also forbids you to purchase vision-related advantages and makes you immune to vision-based attacks. GURPS Underground Adventures provides rules for navigating caves blind and the Cane Travel technique on page 10.
Colorblindness (p. B127) represents total colorblindness. Some people consider this to be a disadvantage that doesn't really limit you that much, but I'm of the opinion that it's always possible for the GM to come up with situations that make colorblindness relevant.
Night Blindness (p. B144) doubles your darkness penalties. So, when darkness level is -5, you are completely blind. By default, this is incompatible with Night Vision, but Pizard has a houserule on his website where Night Vision cancels the normal darkness penalties, but not the additional ones.
No Depth Perception (p. B145) is identical to One Eye (p. B147) in mechanical effects. You get -1 to DX for any tasks that require hand-eye coordination, including combat, and unaimed ranged attacks take -3. Also, you get -3 to operate fast-moving vehicles or mounts.
Noisy (p. B146) gives others a bonus to Hearing rolls to detect you, but I see no reason why a Vision-based variant would be inappropriate.
There's also some vision-related quirks from GURPS Power-Ups 6: Quirks: Cosmetic Eyeglasses (p. 13), Obvious (p. 12), Limited Colorblindness (p. 23) (the one is appropriate for realistic animals, according to GURPS Animala), Photosensitivity (p. 24), Poor Night Vision (p. 24).
And now let's go over vision-related advantages, except for the ones that affect arc of vision (they were covered before) and the ones that shift your vision to another spectrum (they deserve a special, more in-depth look later).
First, we have Acute Vision (p. B35) that simply gives you a bonus to Vision rolls.
Then, we have Affliction (p. B36) and Innate Attack (p. B61). Why are they vision-related? Because you can make them be. First, you can have an attack be aimed with Innate Attack (Gaze). That means that you cannot use it when blinded, and you must face your target. But what does facing here mean exactly? Does it mean that the target must be in the front hexes, or just in your arc of vision? I believe that it's the latter, since it seems to work well with the established ranged attack rules. Also, if you have 360° Vision with Panoptic I or Panoptic II, does that mean that you should be able to use Innate Attack (Gaze) even at your back hexes? I don't think that's the case. But without these limitations, you would be able to do that. You can also make a Vision-based attack. For normal Afflictions, this is a +150% enhancements, meaning that it ignored DR if the subject is using the aforementioned sense. Thus, a Vision-Based Affliction that imposes Blindness would blind anyone who is not blind, blindfolded, is not turned away, or has his eyes closed. For Maledictions, this is a -20% limitation, because Maledictions already ignore DR. Maledictions can also take Vision-Based, Reversed, -20% to be able to affect the subject only if the user is using his unaided vision to target. A Malediction can also take both Vision-Based and Vision-Based, Reversed to combine the requirements. Did you know that Malediction and Cone have a special interaction? It is shown off on page 195 of GURPS Banestorm in the medusa's petrifying gaze ability. That one has Vision-Based as well, and has rules for fighting by looking at a reflection of the target. Also, there's a special enhancement Increased Range, LoS on page 15 of GURPS Power-Ups 4: Enhancements to increase your range to the line of sight. Low or No Signature can make attacks less visible or completely invisible. You may also use the more granular variant from GURPS Power-Ups 4: Enhancements.
Chameleon (p. B41) makes the user harder to spot. This one can be extended to other spectra or senses, and has some new modifiers in GURPS Powers.
Clairsentience (p. B42) can displace your ranged senses, including sight, or just the sight with the Clairvoyance, -10% limitation. GURPS Powers provides extended rules, including rules for performing actions when having multiple different viewpoints.
Damage Resistance (p. B46) does not protect the eyes, unless with the Force Field, +20% enhancement. I also like to allow the Includes Eyes, +10% enhancement, but it's not in the books.
Dark Vision (p. B47) allows you to see in total darkness, but with no color. Color Vision, +20% eliminates the latter. GURPS Powers: Enhanced Senses also has a new modifier - Hypersensory that makes it depend on the combination of all other senses, which is pretty cool, as it allows you to see through most kinds of invisibility.
Detect (p. B48) allows you to build a vision-like ability. That would be something similar to Detect Light with Precise and Reflexive.
Elastic Skin (p. B51) improves Disguise, which in turn is opposed by Vision.
Enhanced Time Sense (p. B52) significantly improves the rate at which you can process visual information.
Enhanced Tracking (p. B53) makes your eyes independent, allowing you to aim at multiple targets at once.
Extra Head (p. B54) give you an additional head with additional eyes (if they were on your head originally). If you have heads facing opposite of each other, then you should buy 360° Vision too.
Injury Tolerance (No Eyes) (p. B61) makes you immune to blinding attacks and to eye attacks. You still are able to see somehow.
Invisibility (p. B63) makes you invisible. This is an expensive ability, but also a very powerful one. Unlike many other vision-related abilities, this one covers the entire electromagnetic spectrum by default, not just visible light. We'll come back to this ability at least twice later in this post. I have to mention that the D&D-style "dispels after an attack" limitation is priced as -20%.
Microscopic Vision (p. B68) allows you to discern microscopic details. One level is enough to just barely make out single cells as tiny dots, three levels is enough to see organelles within cells or structural features on inregrated chips made before 2008. Five levels is enough to see viruses. Six levels is enough to study the atomic structure.
Nictitating Membrane (p. B71) provides DR to the eyes and a bonus to HT rolls concerned with eye damage. But did you know that this isn't it yet? According to GURPS Fathom Five from Pyramid #3-26, even clear water imposes a -2 to Vision rolls. GURPS Martial Arts: Yrth Fighting Styles says on page 22 that Nictitating Membrane 1 makes the penalty -1, while Nictitating Membrane 2 eliminates it (but not the additional penalties for murkiness).
Night Vision (p. B71) neutralizes up to -9 in darkness penalties. This can cause some weird interactions. For example, in an environment with a darkness penalty of -5 a character with Per 15 would be able to see as well as a character with Per 10 and Night Vision 5. However, the former would fight at -5, but the latter will not have any darkness penalties in combat. I don't know how to handle this. Night Vision can also be a feature that sets the native illumination level. Thus, a creature with Night Vision 4 [0] will be at -4 in normal illumination and at -1 in darkness that usually imposes a -5 penalty. This was introduced in GURPS Fantasy.
Obscure (p. B72) can be used to obscure vision. It only affects normal vision by default, with the possibilities to extend the spectrum with enchantments. Without Stealthy, +100%, the borders of the effect will be much easier to detect. Also, Pyramid #3-99 has additional modifiers to make Obscure only affect combat rolls, or make them provide a bonus to Vision rolls (I assume that this can be used to negate existing darkness).
Penetrating Vision (p. B74) lets you see through barriers. Neat.
Protected Vision (p. B78) gives you +5 to resist attacks that disable your vision.
See Invisible (p. B83) lets you pierce invisibility. This advantage must be bought separately for each kind of invisibility, be it an ability type (for example, etherealness or hyperspace) or a power source (for example, magic). The True Sight, +50% enhancement introduced in GURPS Powers extends it to other deceptions, such as camouflage, disguise, illusions. Would Obscure 10 count as invisibility or an invisibility-related deception for the purpose of this? I think the former fits better.
Telescopic Vision (p. B92) lets you ignore -1 per level in range penalties to Vision rolls, doubled if you take an Aim maneuver to zoom in. It can also act as a telescopic sight, increasing Accuracy of your aimed ranged attacks. This can be combined with a custom-made scope (double cost), but not a normal scope.
GURPS Powers also has Illusion (p. P94) that can fool visual senses. By default it only fools normal vision, but additional modifiers can extend the spectrum.
There's also some perks that can be found in GURPS Power-Ups 2: Perks: Illumination (p. 10), Limited Camouflage (p. 11), Long Fingers/Thumbs (p. 11), Periscope (p. 11), Robust Vision (p. 14).
Illumination is also described on page 26 of GURPS Powers: Enhanced Senses. The same page adds two more vision-related perks - Polarization Detection and Tetrachromatism.
VISION SPECTRUM
We humans see in the visible light spectrum, while even some animals can see partially in the infrared or ultraviolet parts of the spectrum. In games with supernatural elements or high-or-ultra-tech advancements, the rest of the electromagnetic spectrum becomes accessible. You probably know that in D&D many creatures have darkvision that in its 3e+ form functions like the Dark Vision advantage, but prior to that it was stated to work in the infrared spectrum. A few months ago I found an extremely interesting article in Dragon Magazine #211. The article is called Sight in the Darkness, and it details how infravision works and all the implications of relying on infravision underground or on the surface. I found that article to be immensely useful in GURPS as well.
But... there's three kinds of Infravision in GURPS! And this makes things more complicated.
Infravision (p. B60) can be an advantage, allowing you to switch between normal vision or infravision, or just a feature, in which case you can only see in the infrared spectrum. First of all, this allows you to fight in complete darkness, but only against enemies that emit heat (this includes all living beings and most machines). Undead creatures and creatures with Supernatural Features (No Body Heat) will probably be in thermal equilibrium with the surroundings. According to GURPS Underground Adventures, most underground environments are in thermal equilibrium, so seeing most inanimate objects is at -2 because of low contrast, and I assume that undead beings fit right in - you will be able to fight them at -2. Also, aren't Cold-Blooded being always (more or less, at least) at the ambient temperature? They should not stand out and be at -2 to detect and fight too. Human beings and other heat sources often stand out: +10 to Vision rolls to detect them. In more normal conditions, such as outdoors during the day, this bonus is only +2. Infravision also gives you +3 to Tracking, if the trail is no more than an hour old. Also, infravision has low resolution - roll at -4 to distinguish details, such as faces or text read via reflected light). Sudden bursts of heat, such as explosions or infrared lasers may blind you. And you cannot see color.
Infrared light is reflected by most objects, so it can be used to navigate and detect everything. Realistically, you'd need very large eyes and a way to insulate them from your own body heat to use Infravision, but this is not a concern. Aboveground, object such as large rocks usually are hotter than grass or leaves that lose their heat due to the wind, so rocks will be "brighter" to Infravision and will retain some of the glow at night. As the night falls, a character with Infravision will be able to see normally, as the terrain will radiate heat. As was mentioned, rocks will be brighter than trees and ground. While water is usually colder than the rest of the environment, it also loses heat more slowly, making it "glow" in the dark. Air is assumed to be invisible, unless very hot. Worn and wielded objects in contact with living beings will radiate some heat, and footprints, physical hits, and other friction-related phenomena leave heat marks. This is probably what gives you +3 to Tracking.
Fires emit a lot of heat, making it impossible, for example, to see a living being in front of a bonfire. I can imagine that there would be situations where you would need to know how far this blinding halo extends. I think it would be fair to impose invert the combat and vision penalties for heat-based light sources within their radius of illumination and halve the inverted valued, rounding down, for a radius three times larger than primary illumination radius. For example, a torch illuminates a 2-yard radius, dropping the darkness penalties to -3. For an Infravision-using character, this would be -7 within that 2-yard radius and -3 within 6 yards. I'm not sure how to treat blinding effects of a fire-based attack (or an infrared laser), but I suggest the following - make an HT roll at -1 per full die of burning damage, and at +1 per yard of distance to the attack (any point along its path). If you fail, you get a penalty to Infravision-based tasks, including combat, equal to the margin of failure for margin of failure seconds. If you fail by 5 or more or fail critically, you are blinded for margin of failure of seconds instead.
You should also remember that Infravision can detect the lack of heat. Thus, ice, snow, white dragons, and other cold-producing creatures would look very "dark". Also, apparently glass is impenetrable to infrared light, appearing like a mirror to those with Infravision. That's interesting.
So, when I convert D&D races and creatures, I usually represent darkvision with Infravision and, if the creature has light sensitivity or light blindness, Night Vision 5 (or more) as a feature. I often thought that this light sensitivity does not really impair them enough, that Infravision is a very good advantage to negate the darkness penalties, but after reading that article, I realized that it seems fair. For example, a drow would be able to live in the Underdark, mainly relying on Infravision. However, he would need to take extra time and risk when interacting with others, because it is very difficult to distinguish not only one drow from another, but a drow from, for example, an orc. I can imagine them using normal dim lights in places of social gathering and libraries. What about fighting an invader from the surface who is wielding a torch? They would be able to barely see and fight them off using Infravision, but they would still be penalized if they decide to switch to normal vision. Same thing for aboveground raids - there's some ways to foil infravision that would no doubt be known by the surface dwellers (because Infravision is very common in D&D worlds). And you will not even be able to look up, because the sun is absolutely blinding to Infravision! I guess that's why it is only 10 points compared to Dark Vision that costs 25 points.
Speaking of foiling Infravision. First, you can mask yourself with a heat source - sure you will stand out very obviously, but seeing any details will be almost impossible. Second, characters with Stealth might want to hide or sneak behind/in front of heat sources, such as fires, fresh piles of bodies, hot rocks, and avoid "cool spots". Third, one can just hide under a thick blanket as not to stand out against the background - but this is a temporary measure, as it will get warm in, let's say five minutes. A gelatinous cube might looks like a black wall. Glass spells can create impenetrable barriers, and fire spells cause blindness or mask heat signatures. Covering yourself in mud or clay (that probably takes at least a minute) should also prevent you from standing out to creatures with Infravision. Spells, such as Cool, Warmth, Frost, Snow, can all affect Infravision. Get creative!
If you'd like to get tricky, you may adapt the rules for Light Adaptation (GURPS Tactical Shooting, p. 18) when switching between Infravision and normal vision.
Near Infrared Vision (GURPS Powers: Enhanced Senses, p. 8) works with frequencies close to the visible ones, improving resolution - there's no penalty to see facial features or print. However, living creatures do not radiate in this band and can only be seen by reflected near infrared light from the sun, very hot objects, or technological sources, such as active IR. Fire at 350°F emit a dull glow or a bright one at 1,350°F. When you have such sources, you gain the same benefits as from Ultravision (basically, +2 to Vision). You can also ignore up to two levels of Obscure (Vision) or equivalent penalties from natural conditions, such as murky water. You are blind in total darkness! Some fish have this trait.
Thermal Infrared Vision (GURPS Powers: Enhanced Senses, p. 8) doesn't actually require eyes, as it uses specialized organs. It works with longer wavelengths, including those emitted by living creatures. Resolution is too poor to read by reflected infrared. You detect warm-blooded creatures, or objects hotter or colder than the background, at ‑1 to your Sense (not Vision!) roll per yard of range. You can’t see color and have only a vague sense of shape, but you fight at no penalty even in total darkness against a target you can detect. You gain +2 to Vision to spot any such target, and +3 to Tracking for a trail no more than an hour old, but you are at ‑4 to Vision to distinguish similar objects. This is a trait that can be found naturally in some cold-blooded animals, such as snakes, and some bats.
Ultravision (p. B94) allows you to see ultraviolet light that is emitted by the sun, penetrating even the clouds, and fluorescent lamps. However, it is blocked by any physical barrier and glass. It allows you to see finer details, such as scratches and contaminants, giving you +2 to all Vision rolls when UV-light is present, and a +2 to Forensics, Search, and Observation to spot clues or hidden objects. Also, at night you get some extra UV light from the stars, lowering the darkness penalties by 2. Also, UV light penetrates water better, halving all vision penalties underwater. You still are blind in total darkness. Wait a second, it doesn't work that way in real life! Here's Pizard's justification for discontinuing this advantage. And he knows his stuff when it comes to light.
Just like Infravision, you can pick Ultravision as a 10-point advantage, or as a 0-point feature, in which case you can only see in the UV spectrum, being blind indoors or underground. However, unlike Infravision, you do not have to switch between UV and visible light - you see both at once. If you can only see in UV light, soil, grass, and water will appear dark to you, but dry sand will be bright, and snow might be blindingly bright.
Hyperspectral Vision (p. B60) combines Infravision, Ultravision, and normal vision with some additional synergetic benefits. You suffer no vision or combat penalties if there's any light at all (basically, you have Night Vision 9). In total darkness, Hyperspectral Vision works exactly like Infravision. It also grants +3 to all Vision rolls and rolls to find hidden clues or objects, and to Tracking as well (there's no time limit mentioned, not sure if that's intentional).
The Extended Low-Band, +30% enhancement lets you also see (but not undeerstand) microwave, radar, and radio waves. This gives an increase of 8 points. If you replace the power modifier with Cannot Analyze, -10% for Radio Source Detection [7] (GURPS Powers: Enhanced Senses, p. 7) and Radar Detection [7] (GURPS Powers: Enhanced Senses, p. 7) you get 14 points. A little bit inconsistent. Keep in mind that radio waves can be generated by lightning and astronomical objects, such as stars, nebulae, and galaxies, and can penetrate wood, brick, and concrete easily.
The Extended High-Band, +30% enhancement lets you also see x-rays and gamma rays. X-rays can be reflected by gold and iridium (although only at an angle of 2.4 degrees or less), so you might be able to see something other than the active source of these rays. Gamma rays cannot be reflected at all. Again, in terms of cost it should be roughly equivalent to Radiation Sense [13] (GURPS Powers: Enhanced Senses, p. 16).
Some advantages that obscure vision (Chameleon, Invisibility, Obscure, and Illusion) affect different parts of the spectrum. For example, Chameleon by default conceals the user from normal vision, Obscure affects one kind of vision (normal, Infravision, Ultravision, etc.), Invisibility affects the entire electromagnetic spectrum (including Radar and Dark Vision), and Illusion creates illusions that only fool normal vision. Their capabilities can be extended, but differently. Chameleon and Obscure can take Extended, +20% for each additional related sense - normal vision, Infravision, Ultravision, Dark Vision. Invisibility seems to imply that Radar is related to vision, so it should be included there too, so you will not have to take a separate Obscure Radar ability. Illusion, however, works differently. Its Extended enhancement costs +1% per point of a new vision-related advantage. Thus, Illusion that affect both Infravision and normal vision will have Extended, +10%. Also, it says that Radar is a different, unrelated sense, so it will cost +20%. This seems to contradict what Invisibility said earlier. Then, if we take a look at the Fog Cloud ability on page 140 of GURPS Powers, we will see Extended, Ladar, +20% on Obscure Vision. That seems to imply that Ladar is on the list of vision types too. Finally, where does Hyperspectral Vision fit into all this? Do you have to take it separately for the purpose of Extended, or does Extended, Infravision and Ultravision cover it as well? But if it does not, what about Hyperspectral Vision with Extended Low/High-Band? Do you have to conceal your gamma ray emissions separately? These questions have no official answers, I think. But let's take a look at the Broad Spectrum technique for Hologram on page 36 of GURPS Psionic Powers. This one allows the hologram to affect Ultravision, Infravision, Hyperspectral Vision, and imposes a -6 penalty. Since this ability is based on Illusion, there's four ways that could have gone:
1. Ultravision (+10%), Infravision (+10%), Ladar (+20% regardless of it being an unrelated sense or a related sense, because it costs 20 points), Hyperspectral Vision (+25%), which results in 65% of enhancements. This shouldh've been a -7 technique then.
2. Ultravision (+10%), Infravision (+10%), Ladar (+20% regardless of it being an unrelated sense or a related sense, because it costs 20 points), Hyperspectral Vision (+5%, because the difference between the point costs of Hyperspectral Vision and both Infravision and Ultravision is 5 points), which results in 45% of enhancements. This shouldh've been a -5 technique then.
3. The technique says nothing about extended low-band and extended high-band, but let's assume that they are affected as well. Ultravision (+10%), Infravision (+10%), Ladar (+20% regardless of it being an unrelated sense or a related sense, because it costs 20 points), Hyperspectral Vision with Extended Low-Band and Extended High-Band (+40%), which results in 80% of enhancements. This shouldh've been a -8 technique then.
4. The technique says nothing about extended low-band and extended high-band, but let's assume that they are affected as well. Ultravision (+10%), Infravision (+10%), Ladar (+20% regardless of it being an unrelated sense or a related sense, because it costs 20 points), Hyperspectral Vision with Extended Low-Band and Extended High-Band (+20% because 40 points - 20 points for Ultravision and Infravision equals 20 points), which results in 60% of enhancements. And this results in a -6 technique. This seems consistent, so I assume that you have to treat Hyperspectral Vision as a difference between its cost and the combined Infravision and Ultravision, at least for the Illusion advantage.
But now let's take a look at the Broad Spectrum technique for Photorefraction (same page). Photorefraction by default affects normal vision and Infravision, and is based on Obscure Vision. Broad Spectrum is a -4 technique that adds three senses - Hyperspectral Vision, Ladar, and Ultravision. That should've been a -6 technique then, unless Hyperspectral Vision is ignored for Obscure (and Chameleon?) if both Infravision and Ultravision are affected.
It's still not clear whether Invisibility includes ladar among its senses by default (I think it should), and whether T-Ray Vision is blocked by the same advantages that block normal Radar (including Invisibility).
VISION AND ACTIONS
EYE DAMAGE
Eyes can be targeted with impaling, piercing, and tight-beam burning attacks from the front or side at -9 (unless you're an octopus or a fish, in which case the penalty is -8). An eye hit may result from a critical hit against the face or skull. Eyes have a crippling threshold of 1/10 HP. A crippled eye, if there are two eyes, gives the One Eye disadvantage, and if all eyes are crippled, this becomes Blindness. Having more than two eyes is a 0-point feature. Major wounds to the eye impose a -10 to knockdown rolls. Corrosion attacks that target the face have a chance to blind the subject. Also, did you know that eye crippling works from penetrating damage instead of injury? Sean Punch explains it in one of the forum posts. Now I wonder how it interacts with eyes on stalks that result from 360° Vision or Peripheral Vision takes with the Easy to Hit limitation. Those should not get the x4 injury multiplier and -10 to knockdown rolls for brain damage, right?
GURPS Martial Arts also introduces some additional rules regarding eye hits. Bleeding rolls from eye injury are made every 30 seconds instead of every minute, and major skull wound may result in impaired sight or even blindness. There's also several technique that target the eyes.
ENVIRONMENT
Vision can work a bit differently in different environments. GURPS Fathom Five from Pyramid #3-26 says that even crystal-clear water imposes a -2 penalty to Vision rolls, plus additional penalties for murkiness. You may even stir up the silty bottom to make it -10. Judging the distance imposes an additional -2. GURPS Basic Set also says that Vision is at -1 in a thin atmosphere and -2 in a very thin atmosphere. Eyes can also be damaged due to explosive decompression in vacuum.
DARKNESS
GURPS often mentions darkness penalties that depend on the illumination levels and lists the sample conditions for different illumination levels in many sources with various degrees of consistency. The first one is, of course, GURPS Basic Set on page 394. Quite unintuitively, this is where you also have rules for using torches as weapons. Penalties are also described on page 18 of GURPS Tactical Shooting, and this section also has rules on light adaptation and dazzling others with flashlights in the dark. There is another table on page 13 of GURPS Powers: Enhanced Senses that not only has more examples, but also has illumination values in lux, and additional rules regarding being dazzled by bright light and a light source making you stand out in the dark and preventing you from seeing anything outside of the light radius (these rules are also present in GURPS Dungeon Fantasy 2: Dungeons and GURPS Underground Adventures). Finally, there is a table that is similar, but not identical to the previous one on page 17 of GURPS Template Toolkit 2: Races, plus more granular rules regarding dazzling. So, depending on which book you use, your torch can eliminate darkness penalties completely, drop them to -3, or to -2. Since GURPS Template Toolkit 2: Races is the most recent publication, I assume that it's the version that should be used.
What does darkness affect? It affects all Vision-based tasks, including combat. It does not seem to affect Dodge by rules-as-written, but Sean Punch says in one of his forum posts that since darkness affects all DX-based tasks with a visual component, it should also affect Parry, Block, and Dodge. I actually did not know that before writing this post, and I always used full Dodge even in darkness.
Finally, page 394 of the GURPS Basic Set has a Visibility section that describes various situations that can occure in combat with invisible enemies.
If the attacker cannot see anything at all, he may use Hearing-2 or some other sense to find the enemy. Otherwise, he can attack blindly in a chosen direction.
If the attacker cannot see his foe, but can see his other surroundings, such as when fighting an invisible enemy, the rules are the same, but the penalty is only -6, not -10.
If the attacker cannot see his foe, but knows his location for sure, for example, when the foe is in a single smoke-filled hex, the penalty is decreased to -4, and no Hearing roll is required.
If the defender cannot see his attacker, including his weapon, the he may dodge at -4, if he is aware that he is being attacked. If he succeeds on a Hearing-2 roll, then he may parry or block at -4. If he is unaware of the attack, he gets no defense. If the foe is in the aforementioned smoke-filled hex, but the defender is outside, he can defend normally because he can see the weapon coming.
But what about the situations where the foe is visible, but his weapon is not? There is no clear answer. While there is the Stealthy Attack imbuement skill, it does not really say much aside from giving a penalty to Per to notice the attack. However, the Transparent Blade technique on page 19 of Pyramid #3-69 that makes a sword invisible says that noticing the blade requires Per-6 modified by range. Failure to detect the weapon means that the first attack cannot be defended against, but thereafter it can be defended against at -4.
SKILLS
Now let's go over the skills that are related to vision.
Blind Fighting (p. B180) is a cinematic skill that allows you to fight blindly (or fight invisible foes) more effectively.
Body Language (p. B181) is based on Vision, so it takes all Vision-related penalties.
Camouflage (p. B183) allows you to hide from others and is opposed by Vision.
Disguise (p. B187) is opposed by Perception or by Criminology or Observation, the latter of which is based on Vision.
Innate Attack (Gaze) (p. B201) was discussed in Affliction and Innate Attack above.
Invisibility Art (p. B202) is opposed by Vision, and does not state whether is only fools normal vision or works like Invisibility, fooling the entire electromagnetic spectrum.
Observation (p. B211), obviously, is based on Vision and is used to oppose some other skills.
Search (p. B219) is not based on Vision, but it does get a bonus from Ultravision and Hyperspectral Vision.
Shadowing (p. B219) is opposed by Vision.
Sleight of Hand (p. B221) is opposed by Vision.
Stealth (p. B222) may be opposed by Vision or Observation. Stealth may work a bit off, but there are alternative stealth systems created by fans. For example, I really like this one.
Tracking (p. B226) gets a bonus from Acute Vision, Infravision, and Hyperspectral Vision.
Traps (p. B226) benefits from Acute Vision, but only when used to detect traps.
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