I've notice glowing objects when the image is dark and the objects ambient color is "higher" than black - even dark red gives you a glow but not quite enough. Check out this image at http://home.satx.rr.com/mtodd/diana/temp/glowing-eyes.jpg for a demo. You can see on the right, the eye is just the ambient color changed on the eyeball. On the left is the spotlight with the bright red color, settings shown and the ambient color on the eye combined.