Hello everyone I just found another way to simulate the microscopic aspect of organic tissus.
-Apply some material, with minimal specular amount.
-Then apply the Velvet material shader, wich can be found in Shader_LibFrabric. Leave the defaults settings.
-Still on the material editor, apply a Static Blur. The values I recommend are 0.5/0.5, just to give a small blurried contour.
-Finally, applying strong bump map is recommended. In my case I used a 3D texture.
The Velvet shader, at its default values, give the same results as an ambiant light of 1.0 combined with a spotlight of -1 (constrained to the camera), like sonne previously suggested.
I suppose that playing with the transparency and maybe with some volume shaders could give good results for radiography simulation too.
Bernard Lebel