Note
            
               - Avoid
                  using Environment textures for displacement
                  mapping. The basic characteristics of environment mapping preclude
                  accurate calculations when used for displacement mapping.
               
- During
                  bump or displacement mapping, if an image file contains a mask channel,
                  the mask channel is used for displacement and bump mapping. If the
                  mask channel is absent, the luminance of the RGB is used to displace and,
                  or bump map.
               
If you prefer to use the luminance information
            as the alpha, turn on the 
Alpha Is Luminance attribute
            (in the 
Color Balance section of the
            file texture’s 
Attribute Editor).