Above I stated that I prefer reflected light, because its possible to
remove the unwanted spectrum outside the habitat. What is beamed in can
then be used at close to 100% efficiency.

I also said the cost of the power is negligible compared to the cost of
the station.

However, one consideration that favours electrical illumination, is the
fact that colonies may well be grouped in pairs. If one were to have
day, whilst the other has night, the same power station could
illuminate both colonies. (Limiting it to 50% light cycle on average).

This could in theory be done with sunlight, but the mirror mechanism
would be complex. Furthermore, it would take time to move the mirrors
to switch the power, so each colony would get a little less than 50%
average of the sunlight.

Complexity should be avoided, not for reasons of cost (as said, its
peanuts compared to the colony construction), but for reliability. If
natural light is chosen, then each of the two colonies would probably
have its own mirror, which would be blacked out at night.

There may be some facility to transfer some of the light from one
colony to the other to provide back up illumination, though this might
be better provided electrically.

So now I'm undecided.

Mike Combs wrote:
I've lost count of how many times on these newsgroups I've encountered
someone saying that providing space habitats with natural sunlight via
mirrors and windows would just be too complicated, and we'll instead use
artificial illumination. Sometimes the advantages of using red LEDs to
raise crops are touted (less wattage, less need for heat rejection).

I've always argued against this, mostly from instinct, but also from knowing
the actual solution the original space settlement studies did settle on. I
was concerned about conversion efficiencies (why convert light to
electricity and then back to light again when light is what you wanted in
the first place?), and it always struck me that aluminized Mylar and glass
panels by the square mile would be pricey, but PV arrays and artificial
lights by the same magnitude would be more so.

Here's a paper that seems to agree:

"Effect of Environmental Parameters on Habitat Structural Weight and Cost"
http://www.nas.nasa.gov/About/Educat...eres/II-1.html

In the midst of looking into a variety of parameters for habitats for 1,000
(early construction shack), 100,000 (intermediate range earthlike habitat),
and 10,000,000 (long range habitat), and for toroidal, spherical,
cylindrical, and Crystal Palace geometries, the paper compares the costs for
artificial vs. natural illumination.

Table 6 indicates that natural illumination with mirrors should have only
about 20% the cost of the artificial illumination option.

The paper does agree that concentrating solar energy so as to minimize
window area is to be recommended. They say you could get up to a solar
concentration level of 70x before getting into problems with the glass
softening.

--


Regards,
Mike Combs
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By all that you hold dear on this good Earth
I bid you stand, Men of the West!
Aragorn