March 27, 1995
One Minute Papers - Questions and Answers
Why does a fluorescent bulb sometimes appear blue, especially right before it burns out?
I'm not aware of any tendency to change colors as it begins to burn out, but many fluorescent bulbs are relatively blue in color. The phosphor coatings used to convert the mercury vapor's ultraviolet emission into visible light don't create pure white. Instead, they create a mixture of different colors that is a close approximation to white light. There are a number of different phosphor mixtures, each with its own characteristic spectrum of light: cool white, deluxe cool white, warm white, deluxe warm white, and others. The cool white bulbs are most energy efficient but emit relatively bluish light. This light gives the bulbs a cold, medicinal look. The warm white bulbs are less energy efficient, but more pleasant to the eye.
Why do fluorescent emissions of light not produce more heat?
When an atom is excited by a collision and then emits energy as light, it converts most of the collision energy into light. Thus the gas in a fluorescent lamp experiences many collisions but emits most of the collision energy as light. The gas becomes slightly hot, but not nearly as hot as the filament of an incandescent bulb. The electrical energy arrives at the fluorescent bulb as a current of charged particles and most of this energy leaves the bulb as light, without ever becoming heat. However the electrical energy arriving at an incandescent bulb becomes heat first and then becomes light. The conversion of electrical energy to heat dramatically reduces the bulb's ability to emit visible light efficiently.
How does an ultraviolet ("black light") fluorescent tube work?
Some ultraviolet fluorescent tubes are simply the mercury discharge tubes (as in a normal fluorescent tube) but without any phosphor coating on the inside of the tube and with a quartz glass tube that transmits 254 nm light. In such a bulb, the 254 nm light emitted by mercury vapor in a discharge is emitted directly from the tube without being converted into visible light. A filter somewhere in the system absorbs the small amount of visible light emitted by a low pressure mercury discharge. For the longer wavelength black light used in most applications, other gases that emit lots of 300-400 nm light are used. Again, these tubes have no phosphor coatings to convert the ultraviolet light into visible light. One other way to make longer wavelength black light is to use a mercury discharge but to coat the inside of the tube with a phosphor that fluoresces ultraviolet light between 300 and 400 nm.
Why do fluorescent tubes explode if broken (is it the compression of the gas)?
Fluorescent tubes operate at very low pressure; roughly 1/1000th of an atmosphere. They do not explode when broken; they implode. The atmospheric pressure surrounding the tube crushes it as soon as it begins to crack. The tube shape of a typical fluorescent tube is chosen because it can withstand the enormous compressive forces of the atmosphere better than most other shapes.