On Photisms

As a general rule, the capacity to perceive suprasensory lights is proportionate to the degree of polishing, chiefly the work of the dhikr [devotional meditation in Islam], which brings the heart to the state of perfect mirror. In the beginning these lights are manifested as ephemeral flashes. The more perfect the transparency (the "specularity") of the mirror, the more they grow, the longer they last, the more diverse they become, until they manifest the form of heavenly entities. - Henry Corbin, The Man of Light in Iranian Sufism

A flame appeared to me so often and, indeed, in different sizes with a diversity of color and splendor, that during some months when I was writing a certain work, hardly a day passed in which a flame did not appear, as vividly as the flame of a household hearth. It was then a sign of approval, and this was prior to the time when spirits began to speak with me viva voce. - Emanuel Swedenborg, The Word Explained

The person who meditates intensely, seeking inner guidance, may find a signal system to guide him. When what he is thinking meets inward approval, the person may suddenly see a flash of light. The light may vary from a pinpoint to a large area, but it is characteristically bright. One normal individual of my acquaintance sees a bright pale-blue light signaling a correct thought and a black spot signaling a wrong one. Some saints report similar experiences. - Wilson Van Dusen, The Presence of Other Worlds