Bump mapping
Allows you to select a bitmap file or procedural map to use as a bump map. You use a bump map to make an object appear to have a bumpy or irregular surface. When you render an object with a bump-mapped material, lighter (whiter) areas of the map appear to be raised and darker (blacker) areas appear to be low.
Like opacity maps and shininess strength maps, bump maps use the intensity of the image to affect the surface of the material. In this case, the intensity affects the apparent smoothness of the surface; white areas protrude and black areas recede.
Use bump maps when you want to take the smoothness off a surface, or to create an embossed look. Keep in mind, however, that the depth effect of a bump map is limited. If you want extreme depth in a surface, you should use modeling techniques instead. For example, the Displace modifier pushes surfaces faces in and out based on the intensity of a bitmap image.
Grayscale images can make effective bump maps. Maps that shade between white and black generally work better than maps with hard edges between the white and black areas.
The bump map Amount adjusts the degree of bumpiness. Higher values render as higher relief, low values render as low relief.
The bumps are a simulation created by perturbing face normals before the object is rendered. Because of this, bumps don’t appear on the silhouette of bump-mapped objects.
Now choose the map as you would choose a diffuse map.
Tip: To avoid aliasing caused by the bump map, in the bump map’s Coordinates rollout, set Blur to be in the range 0.3–0.6 and increase Blur Offset to be greater than 0.0.