These bulbs are T5 fitting bulbs, so slightly thinner than the standard T8 bulbs, and provide a much stronger and bright UVB light. The increased UVB output means they last longer, and are ideal for larger enclosures.
The 5.0 T5 is ideal for Red Foot Tortoises, Anoles and Chameleons, but would also suit many snakes in large enclosures that had a good height for the UVB to penetrate through. This means their UVB is optimum and reduces the chance of MBD (Metabolic Bone Disorder).
The 24w bulb is 22" in length, and the 39w measures 34". These should be used with a special T5 controller to match their fitting, and should be replaced every 12 months.