The Romanian word for light is derived from Latin "lumen", not "lux", thus it is "Lumina" (which is the origin of the female Romanian name "Luminita" -> little light).
Ray of light in Romanian would be "Raza de Lumina", which is veeery far from Raluca. "Raza" indeed comes from Latin "radius", but as you can see, it's quite far.
There is a word in Romanian derived from Latin "lux", that is "lucire", "a luci", etc, which means "to shine".
That's the origin of the name Lucia, which among other languages exists in Romanian too. But more interesting, that's also the origin of the word Latin name Lucifer (the one who shines), which in turn resulted in Romanian in the name "Luceafar" (morning star).
If we go by the hypothesis that Raluca comes from "radius" + "lux", it must be related to the latter word. In which case there is NO way the 'ch' sound would be lost in Romanian. I'm sorry, but it's one of the strongest sounds (definitely the strongest africate) in that particular language, and I am honestly not aware of ONE SINGLE INSTANCE of having it lost in word transformation.
So, if it came from "radius"+"lux", and we are replacing the regular (Latin-based too) word for light with the one derived directly from "lux", we would get "raza de lucire" or "raza lucirii" or "raza lucitoare" or something else which doesn't make much grammatical sense. In any case, there is no way the CH (similar to Italian, the C followed by an E or I is read like a "ch") would have been lost unless some other language intervened on the way.
Thus my conclusion is that it's very, very, very hard to believe it means ray of light and it seems it is just a popular pseudo-etymology to a very common name, in order to have it connected to something as poetic as a "ray of light".
If you find anything about this name that might infirm my conclusion and establish a light between "radius" "lux" and "Raluca", please tell me. I'm very open to changing my mind.