After the previous introduction to prim lights, let’s look at some technical (but not too technical!) background, and a few things about the lighting system which are not immediately obvious:
The graphics system which Second Life uses (OpenGL if you are interested) only allows 6 lights to be visible at any time. If a new light becomes visible, one of the other lights will be disabled, so you should avoid having too many lights too close together.
(As an aside, this is one of the reasons why some people complain about facelights, and especially about facelights which make use of multiple lights — every one of those lights uses up one of the available lights.)
A light has no visible appearance. When you turn a prim into a light by ticking the ‘Light’ checkbox on the Features tab, the prim itself doesn’t change in any other way, it simply has an invisible light-source attached to it. This light source will move with the prim, but is otherwise completely unaffected by anything that you do to the prim, apart from changes to the actual light parameters.
This means that the size and the shape of the prim have absolutely no effect on the light, because the light source is always a single point at the centre of the prim. This might not be what you expect. For example, if you turn on light for a prim cube, then stretch the prim out into a long rectangle, you won’t get an elongated light-source, you will still get a sphere of light originating at the centre of the prim (well … not exactly a sphere — see this previous post if you want to know more about that).
The light is not blocked by prims. Ok, it isn’t quite as simple as that…
Lights won’t illumine prim faces which are facing away from the light-source. This sometimes makes it look as if the light is blocked by prims. Suppose there is a wall in-between you and the light source. You won’t see any light on your side of the wall, because it is facing away from the light, and it looks as if the wall is blocking the light. However, if the light is low enough, you will see light on the ground outside the wall.
This is easier to show than explain, so here is an example. The small cube on the right-hand side of the wall is the light-source. You can see that it lights up the floor on both sides of the wall, but doesn’t light up the side of the wall which faces away from it (the wall is standing directly on the floor):
Another effect of this is that the prim itself is not illumined, because all the visible sides of the prim are facing away from the light. To make it actually look like a light-source you can turn on full-bright for it, and perhaps apply a small amount of glow (a setting of 0.02 is usually sufficient).
Finally, prim lights don’t cast shadows (except as an experimental feature in some third party viewers). If you want to provide a shadow effect, the usual way to do this is by texturing the surface that you want the shadow to appear on, or by creating a thin ‘shadow’ prim with a semi-transparent shadow texture.