Judah ben Barzillai al-Barceloni, Perush Sefer Yesirah, pp. 31-32

Jewish Mysticism
The Creator, blessed be He forever, created a light and a great fire for [His] glory which is called the Holy Spirit as well as Shekhinah… no prophet can look at all upon the beginning of that great light… From the end of the light the Creator shows lights and sparks according to His will to His angels, seraphs, and prophets. At times these sparks and lights emerge from the end of the light whether for the angels or the prophets. There are occasions when they see the light in several images (dimyonot), visions or dreams, or a visible image (demut re’iyah) according to what God wills, as it is written, “and spoke parables through the prophets” (Hosea 12-11)… At times when God shows [the glory] He does so in the image of an anthropos [in the form] of fire or of the great light, and under his feet is a fiery and luminous throne… All the prophets knew that all the forms which they saw were created from light… When God gives strength to the prophet in his eyes and heart on account of his abundant sanctity, the Creator allows him to see something of the end of the light or the fire of the splendor of the Shekhinah, or some of the sparks that come out from the end of the light.

Translated by Elliot Wolfson in Through a Speculum that Shines- Vision and Imagination in Medieval Jewish Mysticism, Princeton- Princeton University Press, 1994.