Most portraits are painted with light coming from about forty-five degrees in front of the model. The light reaches most of the visible form, leaving only a fraction of the form in shadow. The light is low enough to illuminate both eyes.
The illuminated side of the face is modeled in close values, using variations of reds and greens more than tonal changes. . . . Frontal lighting emphasizes two-dimensional design instead of sculptural form. It's a good lighting to choose if you want to emphasize local color or pattern.
The width of the rim light varies according to the size of the planes that face backward to the light. Edge light is not just a thin white line around the form. In the Abe Lincoln cast, the broadest plane and the widest part of the rim light is on the forehead.
Contre-jour lighting is a type of backlighting where a subject blocks the light, often standing against a bright sky or an illuminated doorway. The field of light takes on an active presence, almost surrounding or infusing the edges of the object.
Strong light doesn't usually come from below, so when you see it, it grabs your attention. We tend to associate underlighting with firelight or theatrical footlights, which can suggest a magical, sinister, or dramatic feeling.
In theatrical illumination, the light is almost never completely uniform. Less important areas of the stage fall into shadow . . . a spotlight effect can be used on a small form, too, such as the face above. "Eyelights" were common in classic cinema to concentrate the viewer's attention on the eyes.
Abraham Lincoln bust
Bust of Socrates originally by Jane Heloise.
Bust of Tissot, Countess Armand, and Baronesse Sipiere model originally by Geoffrey Marchal.
Madame de Wailly model originally by Tom Freudenheim.
All models subsequently decimated in Blender.