There has been a couple discussion about different T5 HO bulbs. With T5, the bulb choice are still somewhat limited (unlike T8 or T12). There are some specialty bulbs, but they are expensive. This spring I found
a cheap source of plant bulbs (the purplish, gro-lux type). It is from TopDogSellers in ebay (I think the name of the company is GobiiDirect). 2' 24W bulbs are $26 + $12 shipping for 4, and 4' 54W bulbs are $34 + $20 shipping for 4 bulbs. I think the shipping is a bit expensive, but if you order a lot of bulbs in one shot, you might save some. It is still cheaper than some of the plant bulbs you can get from Pet/aquarium Shop.
It is marked as Odyssea Plant bulb.
I wasn't sure about the quality, so I took some simple measurements. I used 2' 24W bulbs. The fixture is Hydrofarm FLT-24, but used only 1 bulb per fixture. Room temp was 66F. Measurement is at the middle, 1 foot from the bulb.
Brand new bulbs:
Code:
bulb PPFD fc.1 fc.2 watt PPFD/fc.2 PPFD/watt
plant 81 230 475 33.3 0.17 2.43
6400K 58 230 472 29.3 0.12 1.98
The 6400K bulb was AgroBrite 6400K.
columns:
PPFD: photosynthetic photon flux density in micromoles/m^2/s, measured by old Li-Cor Quantum sensor (not calibrated recently).
fc.1: foot-candles measured by Gossen Ultra Pro (calibrated, but the spectral response curve deviates from CIE luminosity function).
fc.2: foot-candles measured by a cheap LX-1330B meter (not calibrated, but fairly new, the manual shows a nice spectral response curve, but not sure if it is the true spec.)
watt: this is measured by Kill-a-watt. Since I'm putting only 1 bulb in the 4-bulb fixture, this energy usage is quite a bit of overestimation. Indeed, if you put 4 bulbs, it uses 72.6 watt (not 30wattx4).
PPFD and PPFD/watt columns are the most relevant data for plants. So the plant bulbs give 40% more PPFD per bulb and 23% more PPFD for a given watt. PAR (PPFD) doesn't put weight for red light (which is a bit more efficient for photosynthesis than green light), and it counts the number of photons in 400-700nm range. 6400K contains quite a bit of green (compared to plant bulbs), so the advantage of the plant bulb here is an under estimation.
I have only measured 1 bulb of each type, so I can't say too much, but the cheap plant bulb seems to be pretty good.
Longevity of T5HO.
According to some web site, T5 and T5HO seem to have a bit better longevity (i.e. the output doesn't decline as fast as T8). A couple sites about the lumen maintenance curve can be found:
here 1 and
here 2. However, when I measured my older bulbs, the output decline seems to be pretty significant.
2 year old AgroBrite 6400K had only 57% output in PAR compared to the brand-new bulb (and 61% in fc).
About 5 months old Odyssea Plant had 70% in PAR (79% in fc).
About 3 months old Odyssea Plant had 87% in PAR and in fc.
The decline in output seems to be much more than I expected. I used to think T5HO should be replaced every two years, but now I think I have to replace them much more frequently. I wonder why there is such a big difference in the reported lumen maintenance vs my measurement. It's possible that since I put the fixture directly on top of acrylic enclosure and the bulbs have shorter life due to the heat. Any ideas?
Inverse square law.
The inverse square law states that the light intensity should decrease to 1/4 if you increase the distance twice. But since T5HO is not a point light source, the reduction of light with distance is slightly different.
The fixture with 4 bulbs of T5HO gives following fc at several distances:
1' 970fc
2' 336fc
4' 93fc
So at 4', it gives about 1/10 instead of 1/16 (=1/4 * 1/4).
Ray has a more detailed info.
Cheap T5HO "Plant" bulbs and longevity
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