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A light gun is a pointing device for computers and a control device for arcade and video games. The first light guns appeared in the 1930s, following the development of light-sensing vacuum tubes. It wasn't long before the technology began appearing in arcade shooting games, beginning with the Seeburg Ray-O-Lite in 1936. These early light gun games used small targets (usually moving) onto which a light-sensing tube was mounted; the player used a gun (usually a rifle) that emitted a beam of light when the trigger was pulled. If the beam struck the target, a "hit" was scored. Modern screen-based light guns work on the opposite principle -- the sensor is built into the gun itself, and the on-screen target(s) emit light rather than the gun. The first light gun of this type was used on the MIT Whirlwind computer.
The light gun, and its descendant, the light pen, are now rarely used as computer pointing devices, because of the popularity of the mouse and changes in monitor display technology -- light guns can only work with standard CRT monitors.
Light guns in video games
The video game light gun is typically modeled on a ballistic weapon (usually a pistol) and is used for targeting objects on a video screen. With force feedback, the light gun can also simulate the recoil of the weapon.
Light guns are very popular in arcade games, but have never caught on in the home video game console market after the NES, Master System, Genesis/MegaDrive, and SNES systems. This may be because people are reluctant to buy more than one extra controller for their system, let alone a special-purpose and often expensive peripheral, or because light guns are less satisfactory to use with the small television screens in peoples homes than on the large screens in arcade game cabinets.
The most popular example of the light gun is Nintendo's Zapper Gun for the Nintendo Entertainment System, though there are also light guns for Sega Master System, Sega Saturn, Sony PlayStation, Sega Dreamcast, Microsoft Xbox, Magnavox Odyssey and several other console and arcade systems. Recent light gun video games include Time Crisis 3, Virtua Cop 3, and The House of the Dead III.
How light guns work
The "light gun" is so named because it uses light as its method of detecting where on screen you are targeting. The name leads one to believe that the gun itself emits a beam of light, but in fact most light guns actually receive light through a photoreceptor diode in the gun barrel. *.
There are two versions of this technique that are commonly used, but the concept is the same: when the trigger of the gun is pulled, the screen is blanked out to black, and the diode begins reception. All or part of the screen is painted white in a way that allows the computer to judge where the gun is pointing, based on when the diode detects light. The user of the light gun notices little or nothing, because the period in which the screen is blank is very short.
Method one
The first detection method, used by the Zapper, involves drawing each target sequentially in white light after the screen blacks out. The computer knows that if the diode detects light as it is drawing a square (or after the screen refreshes), that is the target the gun is pointed at. Essentially, the diode tells the computer whether or not you hit something, and for n objects, the sequence of the drawing of the targets tell the computer which target you hit after 1 + ceil(log2(n)) refreshes (one refresh to determine if any target at all was hit and ceil(log2(n)) to do a binary search for the object that was hit).An interesting side effect of this is that on poorly designed games, often a player can point the gun at a light bulb, pull the trigger and hit the first target every time. Better games account for this by not using the first target for anything.
Method two
The second method, used by the Super Nintendo Entertainment System's Super Scope and computer light pens is more elaborate but more accurate.The trick to this method lies in the nature of the cathode ray tube inside the video monitor (it does not work with LCD screens, projectors, etc.). The screen is drawn by a scanning electron beam that travels across the screen starting at the top until it hits the end, and then moves down to update the next line. This is done repeatedly until the entire screen is drawn, and appears instantaneous to the human eye as it is done very quickly.
When the player pulls the trigger, the game brightens the entire screen for a split second, and the computer (often assisted by the display circuitry) times how long it takes the electron beam to excite the phosphor at the location the gun is pointed at. It then calculates the targeted position based on the monitor's horizontal refresh rate (the fixed amount of time it takes the beam to get from the left to right side of the screen).
Method three
With the advent of LCD screens where the second method is useless due to the synchronous refresh of all pixels, a new method was developed. It relies on one or several Infrared emitters placed near the screen, and one IR sensor on the muzzle of the gun. When the trigger is pressed, the gun sends the intensity of the IR beam it detects. Since this intensity depends upon both distance and relative angle to the screen, angle sensors are located in the gun. This way a trigonometric equation system is solved, and muzzle's 3D position relative to the screen is calculated. Then, by projecting the muzzle on the screen with the measured angles the impact point is determined.A simpler variant is commonly used in arcades, where there are no angle detectors but 4 IR sensors. However, this can prove inaccurate when shooting from certain distances and angles, since angles and 3D position are infered, not calculated.
Other variants include 3 or more emitters with different wavelength and the same number of sensors. With this method and proper calibration three or more relative angles are obtained, thus not needing angle detectors to positionate the gun.
Sometimes, sensors are placed around the screen and the emitter on the gun, but calculations are similar.
This family of methods are used on Wii and modern arcade systems.
Combining the methods
Some light guns designed for method one are not timed precisely enough for method two to work exactly as described, but they can use a combination of the two methods. First the screen is brightened and the response time is measured as in method two, but the computer measures only which scanline was hit and not which horizontal pixel was hit. This does not need nearly as fast a timer that pure method 2 uses. Then using method one, the game cycles among those targets on the line.Detection
Once the computer knows where the gun is pointed, it can tell if it coincides with the target or not. However, many guns of this type (including the Super Scope) ignore red light, as red phosphors have a much slower rate of decay than green or blue phosphors.A game that uses more than one gun reads both triggers continuously and then, when one player pulls a gun's trigger, the game polls that gun's diode until it knows which object was hit.
Positional guns
Positional guns are fairly common in arcades. A positional gun is a gun mounted to the cabinet on a swivel that allows the player to aim the gun. These are often confused with light guns but work quite differently. These guns may not be removed from the cabinet like the optical counterparts, which are tethered and stored in a mounted holster. They are typically more expensive initially but easier to maintain and repair. Games that use positional guns include the arcade version of Resident Evil: Survivor, Space Gun and Judgment Day (arcade game). The console ports used light guns.
A positional gun is effectively an analog stick that records the position of the gun to determine where the player is aiming. The gun must be calibrated, which usually happens after powering up. Some games have mounted optical guns, such as Exidy's Crossbow.
Notable home console games with light gun support
See List of light gun games
Game controllers | Computing input devices | Pointing devices | Light guns
"Light gun"