As you might be aware, ひかり comes from the verb ひかる.
This is called the 連用形 of the verb, and it has some grammatical functions as a verb inflection.
It's also a common way to make nouns from verbs.
Sometimes the noun form made from a verb's 連用形 is identical in appearance. This is the case with something like 続き (from 続く). The noun and the renyoukei are identical and both have okurigana.
In other cases, they are different. With the verb 話す, the renyoukei is written 話し but the noun form is written 話. With the verb 次ぐ, the renyoukei is written 次ぎ and the noun form is written 次.
And this is the case with 光る. The renyoukei is written 光り and the noun is written 光.
I'm not sure why some nouns cling to their okurigana, but maybe there are historical reasons for it.
What about the form "光り"?
What about the form "光り"? Is it wrong okurigana usage or not?