Artificial Light Source Illuminating Through Solid Opaque Objects

I have created several exterior lighting components, but am having trouble with one that is illuminated areas that are blocked from the light source by solid opaque objects. The stair riser and tread light fixture looks as intended from the front, but is also illuminating areas under the the stair. Please see my attached images. As you can see, the rectangular light object does not extend past the cladding of the stair risers and treads. Illumination is only set to 10