A few night vision glasses are in the Cornucopia at the start of the games. Katniss found a pair in the backpack she grabbed, but she thinks they are regular sunglasses that don't work well until Rue tells her they are night vision glasses and that the Careers also have them. The Careers use the glasses to hunt Katniss and Rue in the night. Katniss uses her glasses to evade the Careers.
The descriptions of the Careers using the glasses portray them as inept, "tromping" (p. 229) with their "heavy, branch-breaking bodies" (p 248). The night vision glasses give them an unfair advantage, perhaps? Whereas Katniss feels "relaxed" and does not use hers to hunt, only to hide and augment her senses after an explosion has made her temporarily deaf.