The position of the light source is controlled at the Direction panel. Increasing the radius lowers the height of the light source.

You can use the Source Texture and Mask to supply the height data for the function. Increase the Pre-Smoothing value to smooth the heightmap.

The heightmap's values are multiplied with the Amplify value.

The color of the light source is given by the Primary Color and the ambient light defined by the Secondary Color.