An icon is not meant to be a likeness, not meant to be a physical portrait. It is, rather, a spiritual portrait. It shows a glorified soul, transfigured by grace, filled with the light and radiance of God’s love. This light is interior, glowing from within, where the Spirit dwells.
There is no external light source: no sunshine, no candle, no flame nor lightbulb–not even a heavenly beam directed onto the figure in the icon. No shadow is cast by any object or person in the icon.
One who has lived a holy life has a soul filled with light and overflowing with grace, and it is this radiance that illuminates an icon.
In many classical styles of iconography, particularly the Russian techniques which I have studied, shadows of any kind are rarely painted in at all.
One begins with flat, dark shapes or areas of color. A few lines are put in to clarify detail, serve as a guide to placement of the highlights, and to provide a bit of contrast.
Then layers of light are built up, little by little, with yellow ochre pigments at first, gradually adding white.
The figure is modeled and defined entirely through the painting of the light.