Light
This simulates light from a spot light, as a cone of light from the position of the light toward the point it is pointed at. 
The position of the light. 
The direction of the spot light is described in terms of the line between two points, "position" and "look_at". 
The light's up vector. 
If Radius is set to zero, then the cone of light from the spot will emanate from a notional point source. With a nonzero value for radius, then the light source will behave more like a spherical source with the radius specified (although again with the light emitted being constrained to a cone). The larger the radius, the softer the lighting from this light source (zero radius will give hard shadows). See also the Radius parameter in the point light. 
When set to zero, the vertex of the spot light cone coincides with the light's specified position. If nonzero, the cone becomes truncated, and the light is assumed to come from a virtual position that falls behind the light's specified position. This simulates the effect of realworld luminaires that bundle together a light bulb and a parabolic reflector inside a small, closed fixture. For example, a Hollywoodstyle searchlight has a big lens radius and a very small cone angle, producing an almost parallel beam of light whose intensity decreases with distance only very slowly, reaching all the way to the clouds. The effect of this parameter is more clearly seen with volumetrics.

The cone angle in degrees. No light will be emitted outside of this cone. 
This angle, measured in degrees from the outer edge of the cone towards the spotlight axis, specifies an area where the light's intensity smoothly falls off to zero at the cone edge. 
Controls the shape of the intensity distribution. If 0.0, the intensity smoothly decreases from "penumbra_angle" to the half cone angle. If greater than 0.0, the distribution has a cosine lobe shape and larger values sharpen it. 
Set this to 1.0 (range is 0.0 to 1.0) for a cone with a circular crosssection, or to another value for an elliptical crosssection. Some theater lights, such as PAR cans, produce elliptical crosssections. 
A roundness parameter, going from a square shape at 0, to rounded corners, to a disk shape at 1. 