Baal HaSulam, “Introduction to The Book of Zohar,” Item 46: But the Kelim, called KHBTM in each of the lower worlds—Beria, Yetzira, Assiya—are not considered Godliness, but are rather covers that conceal the Light of Infinity within them and ration a certain amount of Light to the receivers. Each of them will receive only according to the level of its purity.
It is as if I am next to some pipe that passes and receives the Light through it only according to the level of my purity, according to my equivalence of form. Suppose I am in the world of Assiya. On the whole, the whole ladder of the worlds is filled with the Light of Infinity, but every time the system stabilizes itself with regard to the one who receives and gives him Light according to the attribute of bestowal.
Thus, I receive my private Light that matches my desire and according to the level of my correction, my Masach (screen). I receive according to my equivalence of form with the Light.
From the 4th part of the Daily Kabbalah Lesson 2/12/14, Writings of Baal HaSulam