From BlenderWiki

Lamps Textures

When a new lamp is added, it produces light in a uniform, flat colour. While this might be sufficient in simple renderings, more sophisticated effects can be accomplished through the use of textures. Subtle textures can add visual nuance to a lamp, while hard textures can be used to simulate more pronounced effects, such as a disco ball, dappled sunlight breaking through treetops, or even a projector. These textures are assigned to one of ten channels, and behave exactly like material textures, except that they affect a lamp’s colour and intensity, rather than a material’s surface characteristics.

Options

Texture and Input panel.

Map To panel.

The lamp textures settings are regrouped in two panels. Here we will only talk about the few things that differ from usual material textures, see the Materials and Textures chapters for details about the standard options.

The two Texture and Map Input panels you are used to are here regrouped into the Texture and Input one. Except some GUI changes, the Texture part remains the same. However, you’ll note there are much less Map Input options – you can only choose between Global, View or another Object’s texture coordinates (as a lamp has no texture coordinates by itself), and you can scale or offset the texture.

The Map To panel is also a subset of its regular material’s counterpart. You can only map a lamp texture to its regular, basic Color and/or to its Shadow color. As you can only affect colors, and a lamp has no texture coordinates on its own, the Nor/Var/Disp, and Warp options have disappeared.