Night Vision Goggles

The standard Night Vision Goggles (abbreviated as NVG) found in the Metro enhance the user's vision in darkness, allowing them to see without using their flashlight. The night vision goggles are charged by the universal charger along with the flashlight.

Overview
Night vision goggles are used to sneak through areas or seeing traps when the player cannot risk using their flashlight. When the goggles are switched on, the player is much harder to detect than when they have the flashlight turned on. If the player uses the flashlight and the night vision together, they can extend the range of the NVG and see better. This drains the battery faster by around 40 percent, making the battery only last about 2 minutes from the original amount of about 3 minutes 20 seconds, but this makes it easier to see in very dark areas and is of great use when stealth isn't necessary. Like Artyom's watch, the 2033 version of NVG features a stealth meter at the top; red means the player is in well-lit area that easily detectable, yellow means the enemy will find you if they get close, green means you are hidden, but you will still can be seen if you are in close proximity with the enemy, finally "no light" means you are completely hidden, only noise can give your location away.

One should keep in mind the drawbacks to using the goggles however. When the goggles are on, well-lit areas become difficult to manage through. The goggles obscure Artyom's peripheral vision. If the player is swarmed in an area with nosalises or lurkers, it is a good idea to take them off and use the flashlight instead. Taking them off will allow the player to see any mutants or NPCs trying to flank them.

In Metro: Last Light, the NVG return but with some slight differences; they reduce the wearers maximum visual range to some extent as any target the user may see will appear hazy and unclear. They can no longer be used in combination with a flashlight but appears to be equipped with an infrared spotlight (the infrared equivalent of a flashlight) which has the same effect as using the NVG and the flashlight together, but without the extra battery drain or revealing the user's location. The NVG in Last Light no longer obscure the player's vision, has a larger field of view, and the clearer vision means you can snipe with NVG equipped. Also, the wearing animation is removed, you will automatically equip the NVG right after pressing the N (default) key.

Locations
Metro 2033 Metro: Last Light Metro: Exodus Sam's Story
 * Armory level (bundled with the stealth suit).
 * Behind Artyom after falling off of the rail cart during "Front Line", in a small red box protected by Communist soldiers, along with a few MGRs and a moral point.
 * On the communist side of Front Line. The destroyed tunnel in which the injured Communist soldiers are sleeping has a side door. A gas mask is not required to explore the foggy side hall (as long as the player moves very quickly) but it is recommended. Near the end you will find the goggles and a small collection of ammo.
 * After being rescued by Ulman and Pavel, directly behind Artyom through the door.
 * Can be looted from the Nazis in Black Station, or found near the supplies.
 * At Sparta, near the rifles and the ammo.
 * At the start of "Dungeon" in a small room directly in front of Artyom on a small crate with lots of ammo and health packs.
 * Found on the second floor of the abandoned ranger outpost near start of the level "Nightfall", be very careful for the traps inside that post.
 * If you miss the first pair of night vision goggles on the Nightfall level, you can find a pair of them on the level Contagion. The goggles can be found along with a Tihar on a table in the area where two Red soldiers are executing people.
 * Volga: Found in a locked room of confiscated goods from the Fanatics in the Terminal where players must go to find Krest's railcar. The room can be entered near the start of the level through a wooden door (if a key is obtaining from rescued fanatics at a bandit outpost) or by going through a red locked door before exiting the terminal.
 * Caspian: Found in the conference room in the depths of the Caspian-1 bunker, just after retrieving the maps.
 * Taiga: After encountering the Pirates, new NVGs can be found in the basement of the building next to the road. These replace those lost in the river during the rockslide at the start of the level.
 * Vladivostok: Two sets of NVGs can be found in Vladivostok before meeting with the Captain for the second time. NVGs also serve a secondary purpose as the Night Hunters, a small but well equipped Bandit gang that at one point had most of the ruins under their control, hid supply caches around the city and left clues to them that can only be seen by someone wearing NVGs. In addition to rare modifications and supplies, finding all of the Night Hunters caches awards the Great Owl Achievement.
 * A set can be found soon after leaving the Submarine dock in the upper floor of a flooded building you have to pass under. They are found in an small office and can be acquired after a quick time event. During this QTE, a Humanimal is trying on a helmet they are attached to before it notices Sam and starts trying to kill him with the helmet, after successfully killing it the goggles can be taken from where they fell during the fight.
 * In a sewer pipe with a modified Stallion beneath the building the Bandits are using as a cinema, the entrance to the pipe is in the buildings toilets and through a manhole in a courtyard to the side of the building. The toilet entrance is easily found, as it is a gaping hole in the floor with a bandit guarding it.

Trivia

 * The world model of the NVG and the one Artyom uses don't match.
 * These are most likely third or fourth generation night vision goggles, because the vision is quite clear, they are almost silent, and there isn't a loud whine created by them when turned on or in use. (Night vision goggles present in game show in shape some resemblance to real-world Russian military PNV-57e - 1st Generation night vision goggles.)
 * However, you still can hear the NVG activation sound from NPCs, if the enemy nearby put it on.
 * The night vision goggles in the Metro games are passive NVGs, which amplifies the existing light; player can notice the effect of the NVG will slightly worsen when they go to extremely dark areas. However, the Last Light version has the optional IR projector to amplify the effect, which make it closer to the active NVG.
 * Miller, on several cases, will randomly acquire and lose his night vision throughout the game's story.
 * Ulman in 2033 is easily identified because he is the only Ranger to wear his night vision during all missions.
 * Scopes are very hard to use with night vision, as the dirt on the scopes creates a bright green glare while looking through it. Use the red dot laser sight on the scope to help you.
 * In Metro: Last Light, the combination of a scope and NVG is much easier to use than in Metro 2033.
 * It is possible to loot night vision goggles from dead enemies from the mission "Frontline" and onward.
 * The faint light projected from the Night Vision can be seen on the NVG users in the Polis entrance, but in most other cases, no light will be seen on the NVG.
 * In Metro: Last Light and its Redux version, if combining the NVG with an IR scope, the effect of the IR scope is no longer apparent - although it is still possible to aim.
 * The infrared spotlight can prove to be a problem, since it lights walls up so that they're almost completely white, obscuring your vision. This is especially noticeable in the Redux games, where the brightness of the game has been increased substantially. Both Redux games have few areas dark enough that the NVG are essential.
 * A glitch in Metro 2033 Redux causes Artyom to put on the NVG every time a cutscene happens where you lose control of Artyom, such as being knocked out at Defense or right before entering the trolley on Trolley Combat. The bug can be prevented by opening your journal by holding the lighter button, and then equipping and unequipping the NVG's. This glitch is still present in Last Light Redux, though much less frequent.
 * The way NVG's work has been changed slightly in Metro Exodus. Previously the player would simply activate the goggles and Artyom would pull them down over his eyes to use them, and take them off to deactivate them. In Metro Exodus, you can now equip the goggles and keep them on, while activating and deactivating them with a press of the flashlight key/button. Of course, wearing the goggles, regardless of if they're turned on or off, comes with some disadvantages. Peripheral vision is slightly limited, and the flashlight cannot be used at all while wearing NVG's. This does, however, make NVG's slightly more convenient in areas where the player intends to use them repeatedly, as you don't have to go through the animation of Artyom pulling them down and pushing them up every time you want to quickly turn them on or off.