In Generation I, Light Screen/Reflect double the user's Special/Defense respectively when the user is hit by a special/physical move.
In Generation II, their effects aren't exclusive to the user and will double the Special Defense/Defense of all Pokémon on the user's side. If multiple Pokémon benefit, damage is reduced by one-third instead.
In Generations III and IV, instead of boosting stats, these moves now halve damage done by special/physical moves (resulting in a near-identical interaction).
In later generations, Double Battles, Triple Battles, and Battle Royales use a damage multiplier of 2703/4096 (Gen V) or 2732/4096 (Gen VI+), equivalent to reducing damage taken by roughly one-third. This is the case regardless of whether multiple Pokémon are benefiting from the effect.
Light Screen (Bulbapedia)
Reflect (Bulbapedia)
