Anyone who is illuminated, is visible like in broad daylight. There are 15 levels of lighting; each level below 15 darkens the hues by 1 level as well. Anything affected by light level 6 or more counts as illuminated; everything else is safely hidden in the sacred darkness. The illumination generated by light sources drops by 1 per 1 tile. Lights don't add up; only the strongest one counts. Beware of ambient lighting when you're trying to remain unseen; you might not spot it when using the Night Vision mode.