The Zapper is an electronic light gun accessory launched within the Nintendo Entertainment System (NES) in North America on October 18, 1985.
source.image: The Slow Mo Guys
The Zapper can be pointed at a television set to interact with on-screen targets in games that support it. Games and game modes that support the Zapper are primarily shooting gallery games, although the sidescrolling platformers Gumshoe and Baby Boomer also support it.
When the Zapper’s trigger is pressed, the game causes the entire screen to become black for one frame. Then, on the next frame, all valid targets that are on screen are drawn all white as the rest of the screen remains black. The Zapper detects this change in light level and determines if any of the targets are in its hit zone.
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If a target is hit, the game determines which one was hit based on the time of the flash, as each target flashes for one video frame, one after another. The Zapper can only be used on CRT displays; it will not work on LCDs, plasma displays or other flat panel displays due to display lag.
