Within Poser, you need to do two things - make the surface give the appearance of giving off light; and set an actual iight object by it to light the objects around it.

The simplest way to give something the appearance of giving out light is to turn your diffuse and specular settings down to 0, your ambient settings to 1, and your ambient colour to white. Your surface now glows white. If you want it to give off a colour, or perhaps use a texture map to drive the colouring of the light area, then you need to add a texture map, just as you would with most object in Poser (but add it to the ambient colour channel, not the diffuse colour).

Depending on the kind of light you're looking at, I would recommend either point or spotlights to be used for your actual light source. Spots are good for giving a definite directional light, but can only have a maximum of 180 degree of arc. This is fine if you don't want to illuminate anything behind the light, but doesn't look too realistic in some situations. If you want a more natural look to the settings, then a point light is the better way to go. Either way, set them just in front of the light source surface, so that the surface doesn't interfere with the spread of light.

Hope this helps,

JonTheCelt

P.S. As I wrote this, I found myself wondering something which has cropped up a couple of times in the past for me. Is it possible to use the various math nodes to create a proper inverse square decay on the lights within Poser? Since the parameters for setting the lights angle and distance are in th parameters box, rather than the materials settings, I'm not overly hopeful, but maybe I can be pleasantly surprised. The obvious person to tackle this would be BagginsBill, if he's not too busy. Or can anyone else 'shed light' on this question for me?