I have t5 and they work great but I never grew orchids try posting this question on growing under lights fourm mayby they can help you

I'm sure T5s would be great, but the expense of them keeps me away. I've always used CFL bulbs and I'm stuck on them. My terr. is a little bit bigger than yours-4'x18x2'-and I light everything with 2 65-85W bulbs.

Yeah t5s are looking hideously expensive, and t8 is the other option. I've now changed my mind to the 4'x2'x2' version, just for the depth.

RP1

I would actually go with VHO T5s for a tank that deep. Yes, the initial cost is a lot more, but the bulbs can burn at full strength until they die (unlike CFLs, which sould be replaced every 6-12 months, and the T5s are more energy-efficient. So in the long run, you'd be saving money to go with T5s. They would both work just fine for growing orchids though, so it's really a matter of how much money you have for the initial set-up.

Well the orchids are going to be around 4"-6" away from the lights needing the highest light levels of say 2000fc, I'm still thinking t5hos, or t8 38w x 4.

Was considering gro lux t8s until I thought about what it would make the tank look like.

Now thinking 4 x t8 ge polyluxs, 2 in warm white, 2 in cold white(unfortunately there's no daylight or interna versions in 38w).

I can't really find any suitable t5hos for the tank though, unless you can name any suitable tubes.

RP1

I really don't think there is much difference between the T-5's and CFL's, as far as what I have read. Some of my CFL ballasts say "T-5" on them, but are rated for 96 watt CFL's, and the lumen output per watt is nearly identical from the scant information I have found from manufacturers.

Further, my understanding of T-5's (which is not written in stone, but is my understanding) is that they are simply CFL's in linear form (all CFL's are "bent back" on themselves). Therefore, the only additional efficiency gained with a T-5 is from the fact that the tube is in a straight line, rather than bent back on itself, so that some of the light is not being shot into the tube right beside it (which happens in the case of CFL's--light shot back into the tubes doesn't reach the terrarium, and is merely converted to heat--inefficient).

With that said, I can also say that the recommendation to replace CFL's every 6 months is hooey. I used mine for over 3 years before replacing them, and they were used when I got them. I have Dendrobiums and Oncidiums growing and blooming regularly in a terrarium (2'x 2' x 2') with those three 40 watt CFL bulbs over it. Upon replacing the bulbs (original bulbs were 3000k bulbs, replacements were two 4100k bulbs and one 6500k bulb), I can honestly see not appreciable difference in light output or growth response from the plants.

Just my thoughts and experiences, for what it is worth. Go with whatever works for your space and is most cost effective for the lumen output you need.

All the best.

Im actually looking into lighting a 3 foot tank too. I think I'm settling for the 2x56w CF kit from from ahsupply.com
And yea, I have orchids in mine too.

I may upgrade to VHO's way down the road, but I think that CF's would be fine, and cheaper initially (long term does, as mentioned depend on how often you change bulbs and incorperate the fact that they are slightly less energy efficiant).

I've never tried Hydroponic Grow lights though, so I cant give any input there.