J Fortuna wrote:

Thank you Rob!

I have a follow-up question:

Most of my orchids are Phals and Paphs, but I do have a couple of
mini-catts. I was thinking of mounting the lights in such a way that they
would be about 20" or so above the plants' leaves to leave room for most of
the spikes (though some of them are taller than that, and those I would need
to place elsewhere). Would a 20" distance from the light tubes be enough for
the mini-catts?

Since the window niche is 14 inches deep, according to your rule, two 4'
tubes would be enough? I saw fixtures that held 4 tubes in a similar
space -- would it be ok to buy 4 tubes, or would that result in too much
light and/or too much heat? Would the Phals and Paphs be ok with a higher
light level if I decide to go with more than 2 tubes? ..................
...............................................
Joanna

Joanna,
I'm sure Rob will be along with his advice. In the mean time I'll offer
my opinion.
I don't think you would ever regret going with 4 tubes. I think 20" is a
reasonable distance considering that you are only supplementing the
light from a big north facing window.
Here is a picture of one small part of my growing area:
http://stevewilson.homestead.com/files/lights.jpg
I took that picture when I had one of the benches turned over to fix
something. I use 6 tubes spaced about as close together as I could get
them. I keep my plants only 6 to 8 inches from the lights but then I'm
growing in the basement and there are no windows near that area.
Mini Catts do very well with that light and it's not too much for Paphs
either.
Enjoy your new place!

Steve