Looking at the Flaming Sphere spell (p. 242 PHB), the spell description uses the phrase “sheds bright light in a 20ft radius”. Can it be inferred that this does radiant damage?
The spell specifies the damage in the last sentence of the first paragraph.
The creature takes 2d6 fire damage on a failed save, or half as much damage on a successful one.
So moving the sphere into a creature and forcing a saving throw against the damage is only causing fire damage
If you ram the sphere into a creature, that creature must make the saving throw against the sphere’s damage, and the sphere stops moving this turn.
It only explicitly states that it gives off light or ignites (starts on fire) objects;
The sphere ignites flammable objects not being worn or carried, and it sheds bright light in a 20-foot radius and dim light for an additional 20 feet.
If it was going to do radiant damage, it would explicitly state so.