Wattage? Initial lumens? Reflector?
A typical T12 bulb puts out about 40 lumens per watt, so assuming we're talking 4' bulbs, you had 40W x 40 lumens/watt x 4 bulbs = 6400 lumens output.
A T5 has more like 90 lumens per watt, and their 4' bulbs are 54W, so you'd be going up to 90 x 54 x 4 = 19440 lumens - 3 times the light intensity.
That does not mean that you simply move them three times as far from the lights, as light intensity drops off at the square of the distance. The square root of 3 is 1.732, so in theory, if you increased the light-to-plant distance by 70-75%, you'd have roughly the same light intensity as you did with the T12's. I say "theoretically", as the inverse-square applies to a point light source, whereas fluorescent tubes are long, and the reflector can play a role in dispersing the light better.
I just built a T5 hood for a terrarium using 24", 24W T5's, and wanted to do some measurements before construction began. I placed a light meter about an inch from the bulb, about where the edges of the reflector ended. I took measurements there, and at 3", 6", 9", and 12" from the middle of the length of the bulb:
Right up close, the length of the bulb and the reflector played only a small role, but their light contribution added up as I moved the meter away.
The red curve applies only to the particular reflector I used (the Hydrofarm "High-Performance" strips have polished aluminum reflectors), so depending upon your upgrade, it will vary some from this.
Incidentally, i took the reflector off and ran the same test, with these results:
Are T5 lights too strong for some orchids?
Linear Mode
