>Now, your query: by "essence" of light, do you mean what it is or what it does?

I am familiar with the word Noor (Nuur) which also exists in Urdu. I think the essence consists of both what it is and what it does. To my mind the word Noor would be in the category of radiance, effulgence, brilliance.
However none of these words have the poetic connotations that the word Noor has. Noor is also a woman's name.