A Sky Factor is simply the ratio of the amount of light received directly from a sky of uniform luminance, compared to the total amount of light available on a horizontal plane at that same point. In the example below, we show a ball sitting on a plane, being illuminated by a uniform overcast sky. We see that the sky factor on top of the ball is 100%, since at that point light is coming from the entire hemisphere. As we move down the ball, less and less of the hemisphere is visible, until at a position half way down, light is only incident from half the hemisphere, so the sky factor is 50%. This allows one to understand why the maximum value for the Vertical Sky Component (VSC) is 50% (for a uniform sky). For an identical ball being illuminated by a CIE sky, the maximum Sky Component value is 39.6%. Moving further down the ball, the Sky Factor reduces further, then increases again as we move out along the horizontal plane. This is what one would expect, since more of the sky can illuminate the plane as we move further away from the ball.