lmcolor and lighting a surface with a scalar field

[Jean M. Favre]

A few weeks ago, I asked the net how to do the lighting of a surface with a 
scalar field and I set the following parameters

lmcolor(LMC_AD) and draw my surface with  a series of

cpack(r, g, b)
n3f(normal)
v3f(vertex)

It works well and does the shading with the color specified and the lighting
with the normal supplied.

The problem is that BOTH sides of the surface are shaded because the material 
properties are modified.

What I want is the side facing away from the normals to be shaded with a
uniform gray color so that I can easily distinguish the inside and outside
of the surface.

Can anyone give me some help?

Thanks in advance.
--
Jean M. Favre. EE&CS Dept. The George Washington University
		(202) 994 7532
		favre at gwusun.gwu.edu