Spot Light Sources

This shader also uses the illuminate construct, but it supplies not just the spotlight position, but also a cone of illumination:

light spotlight ( 
    float intensity = 1;
    color lightcolor = 1;
    point from = point "shader" (0,0,0);
    point to = point "shader" (0,0,1);
    float coneangle = radians(30);
    float conedeltaangle = radians(5);
    float beamdistribution = 2)
{
    float atten, cosangle;
    uniform vector A = normalize(to-from);

    illuminate (from, A, coneangle) {
        cosangle = (L.A) / length(L);
        atten = pow(cosangle, beamdistribution) / (L.L);
        atten *= smoothstep(cos(coneangle), 
                            cos(coneangle-conedeltaangle),
                            cosangle);
        Cl = atten * intensity * lightcolor ;
    }
}

The latter is specified as a direction, A, and a width, specified in radians by the coneangle instance variable.