Click and drag in a viewport to create the V-Ray
rectangular light. Make the size the same as our box, and position it in the same way. After that, hide the
box. Set the V-Ray light settings as in the image on
the right (click to enlarge). The multiplier should be equal to the multiplier of the V-Ray light material you used on the light box.
We now created a light source exactly the same as
the light box, but it will cast raytraced area shadows