Another writer that described this mystical light beautifully was the 16th century Spanish Franciscan friar, Francisco de Osuna, (c 1492-1540). He wrote The Third Spiritual Alphabet which was one of the first books on prayer that Teresa of Avila read and which influenced her life toward prayer. He explained many of the mystical experiences which were very important to the formation of the Golden Age of Spanish mysticism.
He explained the term "light" in this way: In the physical realm, the light of our eyes does not allow us to see in the dark. It must be joined with the outside light for us to discern visible things. It is the same thing with spiritual matters. Our understanding is dependent upon the fusion of natural light which is on our soul with the divine, heavenly light, so that what we did not know previously, we now know. David said, "In your light, Lord, we see light" (Psalm 36:9). "Faith is the light that illumines us, and when we receive it willingly and with pious devotion, it joins with the light of our soul and in this fusion of lights we see through faith the heavenly things to which our soul by nature is inclined". He adds that we then realize how meager our own light is compared to God's immense light and that our own light produces less understanding.
Francisco de Osuna, "Meditation of the Day" 'And he saw clearly', from The Third Spiritual Alphabet, quoted in the Magnificat, Vol. 10, No. 13, February 2009, for Wednesday 18th, p. 267.