Then God said, "Let there be light," and there was light. Gen 1:3

The first light of the icon is the created light of God, the light of the cosmos (body).  We are the vessel which receives the presence of God, one step at a time, gradually opening the Divine image.  This first light is the vertical plane (heavenly plane) of the icon.  The light comes from within, not shining from an external source. For the iconographer, care must be taken not to become prideful during this step. The light should speak out, but it needs humility too. Thankfully, humility comes in the next step, a "float" or "veil" of color, the horizontal plane.  May God awaken my heart to receive His goodness and mercy, to shine from within, to diminish my ego, to obey His will.