I found some probable mistake in POV-Ray reference document at spotlight
section:

1. The sentence which is just two column before the light intensity multiplier
diagram states that
"The values for the radius, and tightness parameters are half the opening angles
                                ^^^^^^^^^
 of the corresponding cones, both angles have to be smaller than 90 degrees."
which tightness should be replaced by falloff. Another flaw is that it say both
angles have to be smaller than 90 degrees but I saw POV-Ray 3.7 can give an
angle more than 90 degrees in that term.

2. The description for the forth diagram in this section "To give the tightness
value full control over the spotlight's appearance use radius 0 falloff 90. As
you can see from the figure below." should be correct. ^^^^^^^^^^^^^^^^^^^
I think that is radius 90 falloff 90 for the diagram.

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