34W tubes are the worst in light output of the four footers. They were designed to use less energy with special ballasts, which get harder and harder to find. So in a conventional ballast, their lumen rating is decreased substantially. Because of their lower power, they bubble like crazy and they have to be over 60 degrees F to achieve their fuller brightness, the slightest draft even breathing on it will reduce their power.

TRY THIS: Have two fixtures side by side, put the 34W's in one of them and put 40W's in the other. You will find out that there is a difference. And once they warm up after a minute and a half, you will still see a difference in lught output.

-j-

Always use some form of reflector even if it is just white card above the lamps and around the sides. No point fretting over every last lumen and then wasting half of them.

Yes, I'm rigging up lots of reflective things - foil on cardboard, white bristol board.

The colour differences are striking. I'm using some 'plant and aquarium' lights as well, that I bought back when I thought they were essential for growing plants.

Reading through past threads here is...an intense experience as it were. That's why I asked from scratch about best light.

A few indicative numbers for you, Paul. They will vary slightly depending on ballast factor. All numbers for a single ballast driving two tubes.

34W T12, 5500 lumens, 82W, 67 lumens/W.
40W T12, 5735 lumens, 96W, 60 lumens/W.
32W T8, 5100 lumens, 62W, 82 lumens/W.

So the T8 is the dimmest bulb but also the most efficient. The 32W bulbs in particular tend to be driven off a low ballast factor and you can get more light using a different ballast. In general, you will the best efficiency by choosing a modern T8. They are readily available in 40W for about $4 (US) and common ballasts will drive them at pretty much catalog lumens (up to about 3600 lumens per tube).