I sometimes use a sunlamp or flashlight with my artwork after I feel the painting is finished, especially when I use watercolor media where I can lift the paint. What I attempt to accomplish is to see what the difference the light will make, especially if it is directional. I try to position the light at the angle the sun (or other source of light) would be hitting the subject of the painting. It has the same effect as if the sun were just coming out and it is possible to see the very great difference light (in your painting) would make. It tells you what to do and gives you a preview of how it will look after you do it.

In the painting, Winter Kindling, I did a lot of lifting of the paint on the brightest areas as well as (more importantly) the darker shaded areas. The images in the negative (shaded) areas were very important to give depth to the painting. I never picked up on the necessity until I played around with different angles of light shining on the painting after I thought it was finished.

Tip: When you lift paint from a given area to go lighter, be sure to darken the tones around it to create a value contrast to emphasize the light area.