Originally Posted by
staffcc
I checked the episode again just to be sure, but he did not have the red ring on his eyes. In fact, the scene never showed his face close enough to confirm that he was geassed. The red geass light reflected from the mirrors just flew into his eyes, but it was never shown whether it worked or not. The emperor just shouted in reaction, like he got hit, but all of that may be acting.
In another episode, somewhere in season 1 with C.C. during the Mao arc, it was shown that the light will still fly into the target's eyes even if it is immune, but there will be no effect.