There's no difference in the brightness, the effect is caused by the design.
The light source is just like Iron Man, light from the ball joint the head rests on, but with the shape of Batman's head, it sits fairly back from the eyes, and the eyes are just one long hollow tube from teh ball joint to the lenses, so, depending on the angle you look at the figure at, you'll get more or less a direct path to the light source so it can look brighter or darker. I like that style though compared to if they had the light right behind the eyes, that would usually mean a sculpted neck and sacrificed posability, this method gives you posability and the light effect. It's really cool too, if you look closely, the light part of the lens has this faint speckling paint effect to mimic the girtty look of the Sonar eyes from the film, they really did a nice job.
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