Artificial vs. natural illumination for space habitats

Joe Strout wrote:

I was recently
reading about this actually being put to use in real buildings -- light
collectors on the roof, fed to fiber optics, which then light up glow
tubes right next to the fluorescent lights. Light sensors automatically
shut off the fluorescents when the piped light is bright enough.

This is common commercial technology except that
the sunlight is not fed into optical fibers but
into large diameter tubes called light tubes.
More info: http://en.wikipedia.org/wiki/Light_tube

Feeding the sunlight into the fibers does not make
sense because there is no economical way to focus
the sunlight into a spot as small as the end of the
optical fiber. The sun disk seen from the earth
has the diameter of 0.53 angle degree (0.00930475
radian). This angle implies that the maximum
diameter of the lens focusing the sunlight is about
100 times bigger than the fiber diameter. The fibers
have the diameter of about 100 micrometers, so the
biggest lens would have the diameter of about 1 cm.

The light tubes are economical means of delivering
sunlight to a room that is too dark because the
architect who designed the room was incompetent.
Using this technology to deliver sunlight to an
orbital greenhouse does not make sense because
mirrors are orders of magnitude cheaper.

Using artificial light sources when sunlight is
available is total absurd!

PS. When you cross post serious technical debate to
sci.space.policy, space cadets infest sci.space.tech.