The appearance of the invisibility power-up.
The invisibility power-up in Doom 3 is a multiplayer-only item, occurring in most of the multiplayer maps of the original game and those added in the Resurrection of Evil expansion. When picked up, the player who collected it will become translucent for 30 seconds, making them difficult to see. The status of the power-up is reflected through the player's first-person view of their active weapon, which also becomes translucent. The item respawns every 60 seconds.
The invisibility power-up is not coded to have any effect on monsters, and so the power-up is not effective if placed in single-player maps.