The Type BT tube also known as a blown tube light bulb is a T light bulb that has had the glass blown in the middle so that it appears to have a bubble in the middle of the tube. BT halogen light bulbs can be used to replace incandescent light bulbs. High-intensity discharge light bulbs come in BT28, BT37, and BT56. These are used in sports arenas, car dealerships, canopy lighting, and industrial applications.