Light on both side of a glsl sprite

Hi,

I have sprites (sampler2D) rendered from glsl particle system with lights applied in in the pixel shader. They are created from billboards in the a geometry shader with the vertices coming from the points of a line SOP with coordinates from a texture map.

The sprites are only lit on one side so when the geometry is rotated 180 degrees, they are completely dark.

I’m not sure what do I need to look for. Something to do with “Two sided coloring?” Any good reference or documentation?

Thank you very much

Vincent