I ran across this article on Eye Lighting and thought it to be useful information.
For the original article, which is a much different read, please see the link below.
The Eye Light
What Is Eye Light?
The human eye is a smooth, moist surface that reflects everything that comes its way. When a beam of light hits this shiny, smooth surface, it reflects back as a perfect duplicate, much like a mirror. An eye light creates a small sparkle of reflected light that shines from the surface of the eye to achieve this important effect.
We look to the eye to see emotion and as a clue to understanding what a person is saying.
Without the sparkle in the eye, we lose these emotional clues and the character seems distant and mysterious.
We easily become detached from any character whose eyes seem lifeless and unemotional and we struggle to gauge their expressions.
Cinematographer Gordon Willis used this idea to good effective in the Godfather (1972) where he created a very high-angle lighting design that kept Marlon Brando's eyes shaded with no eye light. This brooding look prevented the audience from connecting with what the Godfather was thinking and the emotional void behind his words was amplified.
Analysing Eye Lights
In magazine photographs and on the silver screen models and actors have a spot of light reflected from in their eyes to give their face a sparkle and add depth of character. Pick up any magazine or high quality image and look at a photograph of the model's eyes magnified. Looking at the sparkle in an eye we can determine the exact type of lighting used to light the surface of the model. A smooth-edged square or rectangle indicates the model's eye was lit by a large rectangular silk or other form of softer light. A tiny sparkle of light indicates the lighting director used an light specifically to create that sparkle.
We can keep the approaches others use in mind when setting up our own schemes of lighting.
But what about Three-Point Lighting?
Basic three-point lighting is a great way to start when planning your lighting scheme but it does not take into consideration the specifics of the scene (in this case the subject's eyes). Often when we use three-point lighting, eyes are shrouded with little or no hint of any spot of light reflected back towards the camera. (Note: We usually get this spot to appear in a rendering by adjusting the color, size and intensity of our model's Specularity in the model's surface properties).
In dramatic lighting setups where we have little to no fill and the key light is set at an angle more to the side there will be no light reflection coming from the eyes. You can still see the subject's eyes, but they will be dull and lifeless without a tiny sparkle reflecting off their surface towards the camera. Simply by placing a small low-powered light next to the camera, a sparkle of light is reflected back to the camera, and the eyes will suddenly come to life. (We can even tell A:M's lights to shine only on specific models via Lightlists)
Placement of the Eye Light
Usually we place the eye light on or slightly above the camera position but we must also consider the scene itself and the angle of the key light.
The eye light does not have to be in line with the key but it should come from the same general direction so it will supplement the key and add to the three-dimensional look of the overall lighting.
For low-key dramatic lighting, set the eye light a little more to the side of the camera and towards the key light. This will extend the key light into the subject's eyes and add a sparkle the actual key light cannot. For light hitting your subject from a high angle, place the eye light higher in the shot to raise the sparkle in the eye above the pupil. Control where the reflection in the eye is seen. Experiment to see where the most realistic and dramatic placement would be in your scene. (For ease of moving Lights in A:M consider moving your lights in an Action so lights can be added easily and their positioning and settings can be easily changed, animated and saved for reuse later.)
Fade Out
The eyes are said to be the window to the soul, and the tiny spots of light we see in someone's eyes give us a sense of life and vitality and adds an emotional dimension to the subject. Look at your subject's eyes and determine where their life sparkle should be, then place your eye light where it will bring life to their eyes. Keep a couple of small lights on hand that you can quickly set on or near your camera. Especially when bringing your characters in for a close up.
Unmercifully edited and extended from the original article about real world eyelights:
Source: http://www.videomaker.com/article/14089/
Author: Robert G. Nulph, Ph.D., Contributing Editor, an independent video/film producer/director who teaches college level video production.