In practice, the dome is used to integrate the light illuminating the scene. It is normally held somewhere in the scene and pointed toward the camera. Used in this manner it will integrate the the light coming from the light source (sun, flash, etc.) and the light reflected off of the environment in front of the scene, all which illuminate the scene. The meter reads this integrated light and gives you a camera setting that, based on a given ISO should place an 18% gray card in the scene, or any light level, at its proper place on the curve. This is a general description and actual results may be affected by where exactly the meter is pointed but pointing it at the camera is known to give good results.