Rendering just a light pass in blender render

Im compositing a scene where i need 3d lights to seem to interact with scenery from the plate i shot on location.

I created a basic plane and subdivided it for displace map to give a sense of geometry for the 3d lights as they scan the area. and some cylinders for the trunks of the trees etc.

my question is. how to i render out just the light pass without the geometry that is ‘catching’ the light so to speak.

Thanks

uh, not sure what your trying to achieve, but try under the lamp setting uncheck diffuse and leave specular checked. and check the specular pass under render passes that will just illuminate and render the specular highlights from your lamp, you can then play with them in the compositor using the specular renderpass