The Scan Visor is one of the two visors that Samus will normally have equipped by default in the Prime games. It allows for Samus to scan specific points of interest, including switches which are activated by scanning them, such as the scan post the activates the second set of targets in Exterior Docking Hangar. Most rooms in the Prime games will have at least a couple scan points. Pressing A or D-pad Up will change back to the Combat Visor.
While the Scan Visor is normally not considered a collectable item, certain randomizers have the ability to make it into an item. This will make it so that Samus will not have the Scan Visor by default.
In Prime 1, the Scan Visor works by locking on to preset points on the map. Enemies will have these points affixed to them so that Samus can scan them as they are moving. The scans are color coded: red scan points are typically more important for the lore, while orange scan points are not as important. The scan points will become translucent when fully scanned.
Scan Visor Crash
If you bring up the Scan Visor at the exact wrong time while transitioning through rooms, this can crash the game. The reason this happens is because the Scan Visor checks to see which scan points are loaded when it is brought up, and it crashes if it looks for these points before they are properly loaded. Scan points are only fully loaded once the room is transitioned. The video below demonstrates one example of this crash when transitioning into Reflecting Pool, though this can happen in any room.
Metroid Prime 2: Echoes
The way the Scan Visor works was changed in Echoes. Instead of locking on to preset points, every object that is scannable will be highlighted in the Scan Visor. The color coding was also changed from Prime 1, with red objects being important scans and blue scans being not as important. Once something is scanned, it turns green.
Since the Scan Visor was changed, a number of unintentional quirks were added as a result.
Out of Range Scans ("Odd" Scans)
The game calculates if a scan is in range by calculating it based on Samus' line of sight. However, this line of sight doesn't always match with what the Scan Visor is focused on, depending on the type of scan it is. This is because of a change in the way scanning works in Echoes. There are two types of scannable objects: "fixed" scans, which automatically direct Samus' facing angle to a predetermined point; and "odd" scans, which do not, and therefore must calculate how far away the scan is from Samus. For these "odd" scan points specifically, if an obstacle is obstructing Samus' view relative to the exact point being scanned, the game will incorrectly calculate how far away the scan is, allowing for the scan point to be scannable outside of its normal range. The video below demonstrates this oddity in the room Watch Station using the Spider Ball track as an example.
Sudden Angle Change (Dolphin Only)
This is a bug that is only known to happen on Dolphin as a result of emulation inaccuracies. In certain cases, such as if a scan is attempted where the scannable point meets with an obstacle that blocks Samus' scan, Samus' facing angle will harshly change to a completely different direction. Sometimes this results in Samus scanning an entirely different scan point. This effect does not happen if the internal resolution is set to Native and there are no anti-aliasing effects.
Metroid Prime 3: Corruption