A lamp to cast shadow only and not illuminate

Dear All,
I’m assembling a scene and I wish to have a lamp that does not illuminate any objects, it just makes then cast shadows.
How to pull this off?
I tried searching the net of course, and I came across this:
https://www.blender.org/manual/render/blender_render/lighting/shadows/properties.html
But to be frank I do not know how to reach those attributes.
Thanks in advance for your help.

Those settings are from Blender Internal. In Cycles, it doesn’t make any sense a light that cast shadows without throwing light (it is the absence of light that produces the shadows: while in BI the shadows are build to obscure the scene)

You can switch the engine, or render a new pass for later composition. For the later, you can set all materials to pure white diffuse, place your light and render the scene. Then later you can multiply this pass with the original render in the compositor.

Thanks for your reply.
Then I guess I will render out an AO pass for example with overexaggareted shadows and make the wonder in PS with overlaying and adjusting the levels.