The Coral Vue 39 watt 36" Pure Actinic High Output true actinic fluorescent lamp that spikes at 420 nanometers for ultimate fluorescence of capable coral. T5 fluorescent aquarium bulbs are 5/8" diameter fluorescent tubes rather than the standard 1-1/2" diameter. This slim profile makes the Coral Vue T5 fluorescents more efficient than standard fluorescent tubes. T5 fluorescent bulbs also put out more light than standard fluorescent lamps, power compact and VHOs.