I know this picture won't be of much value since I can't compare the lighting with that of T-12, T-8, or T-5, but it will perhaps give you some idea of the brightness that T-5HO fixtures produce. You are seeing the light transmitted not only through polycarbonate but also 16 mil solar blanket material. Also, in the greenhouse on the left, the right side is illuminated by T-5 fixtures and the left side is illuminated by T-5HO fixtures, two fixtures on each side. My more light-sensitive plants are positioned under those standard T-5 fixtures.

Click on the picture to see both greenhouses.

I just replaced an old 48 Watt, 48", two-tube T12 fixture in my kitchen with a new 32-Watt, 48", two-tube T8 fixture. They weren't even 42 Watt T8s.

T8 instead of T12, but using 33% less power.

That new light is BRIGHT!! I mean, the cat scrunches up his eyelids and then glares at me as he stalks out of the kitchen when I turn it on.

I mean, I almost think about wearing sunglasses, and sometimes I flinch.

Maybe choosing "cool blue" tubes added to the shock. The old tubes were "warm" color and many years old, so the tubes and ballast were on their last legs. Whatever it was, it feels like 2-3 times as much light.

Rick, I think you will find that the same length T-5 fixtures will be even more economical and brighter. The T-5HO WILL require sunglasses.

blink-blink-blink

I wish I had had T5 bulbs years ago when I grew some plants indoors!

I've been reworking the lighting in my GH since my seedlings are too scrawny and feedback (on this site as well as from Rare Seeds/Baker Creek indicates I likely don't have enough light). So I purchased some T5 tube fixtures and bulbs. I still have 6 clip on lights with CFL . However, there is such conflicting information on the web about whether lumens is important. One very detailed site talked about the need for 3,000 lumens per sq. ft - that was an article on growing marijuana (which is not in my GH), but it's a green plant. Other sites say lumens are not important - only K rating. My new T8 tube fixtures will each have two 6500 K bulbs at 2900 lumens each - My intent was to put that over a 18" x 3' shelf - and I had hoped that would be enough light - but maybe it won't. Maybe I'll have to put both fixtures over one shelf and buy two more for the next shelf over ?

Both lumens and Kelvin are important but for different reasons. The amount of brightness (lumens) is going to be more important though. Without adequate brightness, it really doesn't matter what the Kelvin (range of light, from yellow/amber to blue/purple). I published two articles on Light in January/February, 2014, and these articles may be of some help. The 6500K fluorescent tubes will produce more lumens than the 4500K tubes.

I am a little bit confused. You say you just purchased T5 fixtures but then say that you are using T8 fixtures. I prefer the T5 fixtures because you'll get more lumens from those. The Kelvin will be the same from either T8 or T5. 6500K is great for plant growth/greening. The 4500K stimulates flowering. If you are using the T8 fixtures, they should be adequate supplemental light for a single shelf. Just remember that you'll lose lumens very rapidly as the plants are positioned further away from the fixtures. Thus, you'll want the tubes as close as reasonably possible to your plants. I haven't used T8 fixtures in quite a few years, but I had my orchids within 6" of those tubes, probably more like 3" away. If you are using T5 fixtures, you'll be able to get more lumens at the plant-level than with the T8. I now use T5 HO fixtures and I have to position my orchids at least 24" from those fixtures. These fixtures produce huge amounts of lumens (30,000, 1" from the tubes!) and get hot as well. These fixtures that I now use are 4'x6tubes.

@drdawg very helpful. Thank you. I meant to say I purchased T8 - not T5. Great - I'm hanging the lights this afternoon (well, "Mr. Ed" actually is .. haha - he's building a support around both of the shelving units and hanging the T8 lights from them) . I'll watch closely and make sure the plants don't cook. I think I've had inadequate light both last year and this year - so hopefully this will make a big improvement in my yield. I am growing only greens (no flowers) - lettuce, kale, pak choy, etc. We'll see how it goes. Thanks for the input and I'll look up both of your articles.

Carol, if I remember correctly, T8 lights produce very minimal heat. That should not be a problem at all. The 6500K should be right for what you are growing.