Their only difference is transparency.

In earlier versions of POV-Ray, it was possible to have a background
pigment showing up in reflections and radiosity but not directly
visible, so the output image would have a transparent background, simply
by using a “sky_sphere” and enabling alpha output. This isn
’t possible
now, because you don’t get a transparent background in the output f
ile
unless the “sky_sphere” is transparent, and this in turn re
moves its
effects altogether, including reflections and radiosity.

This behavior does match that of a very large sphere object, but it
departs from that of the “background” feature: the latter d
oes produce
reflections and radiosity when it’s transparent and alpha output is
enabled. Therefore, now, in order to cast environmental light onto the
subject matter, from a non-uniform pattern, a workaround is needed, such
as using an opaque sphere object with “no_image”—if
 it were transparent,
it’d have no effect, just like the “sky_sphere”.

A “sky_sphere” used to be basically a “background
” with a whole pigment,
or a layered stack of them, rather than a solid color. This version has
introduced a significant difference, as a transparent “background
”
generates reflections and radiosity when alpha output is enabled, while
a transparent “sky_sphere” does not. I think this makes a t
ransparent
“sky_sphere” essentially useless. I don’t know if t
his can be fixed
somehow, but perhaps the simplest and more versatile way would be to add
something like the “no_image” switch to the “sky_sp
here” feature. It’d
eliminate the need for a regular sphere.

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