We want to make a shader for a texture to cut transparency through a semi-transparent black material in such a way to simulate lighting.
We want to create a radial gradient on a texture to map the light source, the texture map should be such that it will cut full transparency through the semi-transparent texture behind it in the center (i.e. cull through the texture) to fully opaque (i.e. we can 100% see the semi-transparent texture behind it) on the outside.
When two of these lightsources overlap we would want the intersection to cut extra transparency through the semi-transparent black layer (that is, appear to be extra "bright" in that it is taking away more shade than when just one light source is over it).
Attached is an example of ALMOST what we want. The problem is that we go from cutting full transparency in the middle to BLACK (this black just happens to match the color of the shade, but because it goes from cutting transparency to the black texture, when two sources of light overlap the blackness adds and the overlap of the two lights appears to be DARKER, rather than lighter.