Stealth Boy

Overview 

Developed by Robert Mayflower, the RobCo "Stealth Boy" Model 3001 Personal Stealth Device was the first portable hand-held stealth field generator, worn on the wrist. Adapted from captured Chinese Hei Gui Stealth Suits (belonging to the black-ops group Black Ghost) during the lead-up to the  Great War, the Stealth Boy generates a modulating field that transmits the reflected light from one side of an object to the other, making a person much harder to notice (but not completely invisible). The field only lasts for a limited amount of time before the Stealth Boy runs out of power. While the technology is nearly identical to the Chinese Hei Gui, the two pieces of equipment differ in several distinct ways. The Stealth Boy's modulating field is more powerful and therefore better at concealing the user, trading in longevity for power. Conversely, the Hei Gui suits have a much more subtle field generation but can be used practically indefinitely and with relative stability.  
 

 The Nightkin

The Nightkin are a special elite group of Super Mutants, hand-picked by The Master for their skill and are the best equipped of his Super Mutant army. Many of the Nightkin are equipped with Stealth Boys, though in the original Fallout, this is merely a cosmetic effect and has no impact on gameplay. They are nonetheless a terrifying enemy for the player to face, being some of the toughest enemies in the game. They returned as an enemy in Fallout 2, and  though they did not appear Fallout 3, they were referenced in the computer hacking minigame. According to pre-release information from Obsidian concerning Fallout: New Vegas, prolonged usage of the Stealth Boy has adverse effects on the user's psyche, causing paranoid delusions, schizophrenia, and insanity. It is likely (though conjecture at this point) that the Nightkin are themselves insane (more than the usual anyway) because of their Stealth Boy usage. 
 

Gameplay Differences

  The Stealth Boy showed up as a usable item in Fallout and Fallout 3, and it's functionality differs drastically between the two games. In the original Fallout, the devices adds a 20% sneak modifier to the player. Essentially, the device helped a player sneak better, but not render them totally invisible and a character with low sneak skill would not benefit greatly from it. Conversely, the Stealth Boy in Fallout 3 grants the user 100% sneak AND a 75% modifier to a unique stat called "stealth field", ensuring that as long as the Stealth Boy was active, and no action was taken, the player would remain completely invisible to anything. The Fallout 3 interpretation of the Stealth Boy is somewhat contradictory to the original lore established by the first Fallout; the Stealth Boy's field makes the user merely "hard to see" rather than outright invisible. Also, there are only 5 places in Fallout where the player can obtain a Stealth Boy, while players in Fallout 3 can find them lying all over the place. Even though Fallout 2 has Nightkin, they do not drop Stealth Boys.