The advantage of using an external light meter is that you deal with an absolute reading of what light there is, not what the reflection is from some default background. Important: If you look at the white bubble on the lightmeter you can see that it is held in an angle so the light and shadow on each side equals the light on the face (the window highlight to the left and the shadows from the room to the right). It is an artistic decision where to hold the light meter, because if you tilt it a bit to the left in this picture it will pick up more light from the window and give a slightly darker exposure. If there is strong light coming in from the window that would be an idea as you then make sure the bright side of the face is correctly exposed, whereas the shadow side of course till be darker (as the whole exposure gets darker)………
 
Source: overgaard.dk

light meters Thorsten Overgaard