Prana is the sun; rayi is the moon.
Matter is solid, matter is subtle;
Rayi is therefore present everywhere.

The sun gives light and life to all who live,
East and west, north and south, above, below;
It is the prana of the universe.

The wise see the Lord of Love in the sun,
Rising in all its golden radiance
To give its warmth and light and life to all.

Prana is energy, particularly life energy, so the sun is the source of life energy.  No one can doubt that it is the energy of the physical sun that feeds life on the physical Earth, so isn't it logical that the primary source on Earth of spiritual Light is the spiritual sun?  While spirit can exist without matter, it is usually concentrated around it's material counterpart.  So when the Upanishads says "the Lord of Love is in the sun", it is referring to the spiritual sun.