The Hololens uses an IR projection and camera in addition to 4 "environmental cameras", which is why it doesn't work well in very low or very bright lighting.

The Hololens uses an IR projection and camera in addition to 4 "environmental cameras", which is why it doesn't work well in very low or very bright lighting. It also uses the inertial sensors to help correct the map. There are some things about SLAM that are best done optically.