More than two thousand years ago, the ancient people, living in an area that is now part of Italy, discovered that by throwing salt particles on a fire, a clever Roman citizen could produce yellow flames. Of course, that yellowish color caught the attention of others for only a brief period. The thrown salt did not cause any substance to produce a long-lasting glow.
Many centuries later, following invention of the barometer, observant men and women noted a glowing that lasted a bit longer. It came from the mercury that had been placed inside of a barometer, and it made its appearance once that element had been exposed to a frictional force. Unlike the yellow hue associated with the flames that had intrigued Romans, this new glow displayed a distinctly color.
Fireworks have long offered proof that the lights produced by a particular chemical can have a unique hue. Fireworks experts know that the ignition of certain chemicals can yield a bang, one that can be associated with a special color. For instance, ignition of strontium sends red glares into the sky. Ignition of barium yields production of yellow glares, while doing the same thing to certain boron compounds leads to creation of eerie green lights.
Since the colors observed during a demonstration of fireworks do not last very long, chemists and manufacturers chose to look for an alternate way for making a colored beam. Their efforts led to invention of the gas discharge tube. That tube held a specially-selected vapor. If an electric discharge passed through that vapor, the tube’s contents would light up.
Charged tubes that held the vapor made from strontium salt glowed red. That discovery eventually paved the way for creation of neon signs. Besides being quite eye-catching, such signs demonstrated a fundamental and extremely important fact: Vaporization of certain chemicals can lead to emission of uniquely-colored beams.
Eventually chemists learned that a beam’s hue reflected the wavelength of the emitted light. In the case of a self luminous exit sign, the emitted beams did not represent a response to either an electrical current or an outside source of illumination. Each soft beam produced a natural glow, one such as what had been exhibited more than once in a chemist’s laboratory, by a piece of phosphorous.
Still, glowing phosphorous has not been selected as the chemical of choice, during the manufacturing of present-day self luminous exit signs. That is partially due to the fact that its beams do not have a uniform intensity. Moreover, there have been times when such beams have been known to blaze out, with their extinction preceded by emission of shocking flashes.
Obviously, manufacturers realized that no potential buyer would want an exit sign that might produce shocking flashes. Therefore, the research and development teams in certain plants began to make signage that relied on the presence of zinc sulfide. Unfortunately, only a weak glow came from any signage that contained that compound.
Today most such signs have been based on the chemistry of strontium oxide aluminates. That chemical, while contained in a sign’s tubing, gets exposed to the air. The red glow that it emits demonstrates what is known as light fastness. In addition, it has a bright afterglow. Those two qualities serve to illustrate the highly functional nature of the self luminous exit sign.