This bulb also contains a partial vacuum; but in addition, mercury and phosphor vapors flow freely in the glass coils. When electricity excites the mercury vapor, it produces ultraviolet light. Then, the phosphorous vapor emits visible light caused by the UV light. These bulbs not only use a fraction of the energy, but can also last up to 10X longer than the previous incandescent bulbs. However, these bright lights can be quite harsh, so it is recommended by our electricians to have them in larger rooms such as kitchens, workshops, and great rooms.