1. The spell says "every object in a 20' cube", so according to the spell everything, including the ground, glows with luminous light.
2. The purpose of the spell is either to "see" invisible creatures/objects, and/or to make it easier to hit someone/something.
Why not just allow the caster to specify what is glowing within the 20' cube when casting the spell?
If the caster want everything to glow, then everything glows. If the caster only want invisible creatures to glow, then only invisible creatures glow.
Problem solved.