If you say that a room is incandescent, or that a color is incandescent, you probably mean that it's bright, beautiful, and glowing. But if you say that a person is incandescent, you probably mean that they're so happy, or so excited, or so angry, that they seem to be lit up, like they have a fire in their heart.
Here's Fry, looking incandescent! He's lit up brightly, and he looks very happy.
When you're so mad that you feel like you're on fire, you're incandescent with rage. And when you're so happy or excited that you feel like you're on fire, you're incandescent with excitement or happiness.