Hey Eugene- The HO lamps take T-5 bulbs, but I believe the T-# refers to the bulb diameter although certain diameter bulbs are associated with certain types of flourescent bulbs. There standard output T-5 bulbs for instance. I have not tried them, but there are VHO (very high output) fixtures as well which are T-12 size. The HO lamps (and VHO lamps) are higher wattage lamps. For a comparison a standard output flourescent 48" bulb is a 40 watt bulb while a 48" HO bulb is 54 watts and a 48" VHO bulb is 110 watts. The HO bulbs are quite a bit brighter than the standard bulbs. -- Alani Davis