Ansel Adams created the Zone System which was a way to determine the proper exposure and adjust the contrast of black and whites in a photograph. In this system 0 is considered absolute black and 10 is absolute white, 5 being in the middle making it mid gray. Light meters read up to mid gray and then is decided by the photographer where to place the light meter to determine the exposure.
Here are some photos where I worked with a light meter at ISO 400 to determine the exposure which was needed. By placing the light meter by the white light I created a dark silhouette, after I placed the light meter in front of my subject to create a overexposed bright white of the light box.