Sri Aurobindo translates Mundaka Upanishad, Chapter 2, Section 2, Verses 10-11: “In a supreme golden sheath the Brahman lies, stainless, without parts. A Splendour is That, It is the Light of Lights, It is That which the self-knowers know. There the sun shines not and the moon has no splendour and the stars are blind; there these lightnings flash not, how then shall burn this earthly fire? All that shines is but the shadow of His shining; all this universe is effulgent with His light.”
These verses delineate an inner spiritual experience that comes to the seeker when there is an opening to the inner sight. The light that is experienced is far more intense and brilliant than the outer lights of the sun, moon, stars, lightning or fire, which are the outer forms of light that we can use for comparison purposes. Even the slightest opening of the spiritual vision can be an overwhelming experience and we see in the words of the Upanishadic sage here, the attempt to provide some form of understanding to the disciple who is following in the path of the Spirit.
We see a similar reference in various religious traditions. We generally try to take these statements metaphorically, but in reality, they represent a real and present experience when the spiritual vision takes over from the outer sight. Sages and religious founders have spoken of a state of “illumination”. They are referring to this flooding of inner light which accompanies the spiritual realisation and which provides them the stamp of its authenticity and its veracity. In the glow of this experience, the world itself is transformed and everything radiates and vibrates with an intensity of light and color that far exceeds the normal perceptions of the mental-nervous mechanisms that feed us our perceptions of the world.
Sri Aurobindo, The Upanishads, Mundaka Upanishad, pp. 193-210