Fluorescent light

A fluorescent lamp is a type of electric lamp that excites argon and mercury vapor to create luminescence. Fluorescent lights are more efficient than conventional incandescent lamps because less of the energy is converted to heat. Instead, most is converted to usable light.

History

The earliest ancestor of the fluorescent lamp is probably the device by Heinrich Geissler who obtained in 1856 a bluish glow from a gas sealed in a tube, excited with an induction coil. Though to be remembered as a physicist, it is interesting to note that Geissler was educated as a glassblower, which was certainly of some value for this earliest realization.

In 1857, French physicist Henri Becquerel had the idea of a tube encapsulating fluorescent gas while leading investigations on fluorescence, phosphorescence and radioactivity.

In 1894, D. McFarlane Moore created the Moore lamp, a commercial gas discharge lamp meant to compete with the incandescent light bulb of his former boss Thomas Edison. The gas used were Nitrogen and Carbon dioxide emitting respectively pink and white light, and had moderate success.

In 1901, Peter Cooper Hewitt demonstrated the mercury-vapor lamp, which was emitting in the blue-green spectrum and thus was unfit for most practical purposes. It was, however, very close to the modern design, and had some applications in photography where color was not yet an issue, thanks to his much higher efficiency than incandescent lamps.

It remained to Edmund Germer and coworkers to propose in 1926 to coat the tube with fluorescent powder which converts ultraviolet light emitted by a rare gas into better spectrally distributed light (also bringing high pressure of the gas at the same time). Germer is today recognized as the inventor of fluorescent lamp.

General Electrics later bought Germer's patent and under the impulsion of George Inman brought the fluorescent lamp to wide commercial use in 1938.

Principle

A fluorescent light bulb is filled with a gas containing argon and mercury vapor, sometimes referred to as plasma when electrified. The inner surface of the bulb is coated with a fluorescent paint made of varying blends of metallic and rare-earth phosphor salts. The bulb's cathode bombards the vapor with electrons causing it to emit ultraviolet (UV) light at a wavelength of 254nm. The UV light is absorbed by the bulb's fluorescent coating, which re-radiates the energy at lower frequencies (longer wavelengths) to emit visible light. The blend of phosphors controls the color of the light, and along with the bulb's glass prevents the harmful UV light from escaping.

Usage

Fluorescent light bulbs come in many shapes and sizes. An increasingly popular one is the compact fluorescent light bulb (CF) which screws into a regular light bulb socket.

Unfortunately, many people find the color spectrum produced by some fluorescent lighting to be harsh and displeasing. It is common for a healthy person to appear with a sickly bluish skin tone under fluorescent lighting, and many pigments have a slightly different color when viewed under fluorescent light versus incandescent. This is mainly the case with fluorescent lamps containing the older halophosphate type phosphors (chemical formula Ca₅(PO₄)₃(F,Cl):Sb³⁺,Mn²⁺), usually labeled as 'cool white'. The bad color reproduction is due to the fact that this phoshpor mainly emits yellow and blue light, and relatively little green and red. To the eye, this mixture looks white, but light reflected from surfaces has a distorted color. More expensive fluorescent lamps use a triphosphor mixture, based on europium and terbium ions, that have emission bands that are more evenly distributed over the spectrum of visible light and hence lead to more natural color reproduction.

Residential use of fluorescent lighting remains low (generally limited to kitchens, basements, hallways and other areas), but schools and businesses find the cost savings of fluorescents to be significant and only rarely use incandescent lights.

Because they contain mercury, a toxic material, in quantities of a few milligrams per unit, fluorescent bulbs must be properly disposed of in many areas throughout the world. This generally applies only to large commercial buildings which produce many waste bulbs, though restrictions vary widely.

Bulbs are typically identified by a code such as F##T##, where F is for fluorescent, the first number indicates the power in watts (or strangely, length in inches in very long bulbs), the T is for tube, and the last number is diameter in eighths of an inch. Typical diameters are T12 (1½" or 38mm) for residential bulbs with old magnetic ballasts, T8 (1" or 25mm) for commercial energy-saving bulbs with electronic ballasts, and T5 (⁵/8" or 16mm) for very small bulbs which may even operate from a battery-powered device. High-output bulbs are brighter and draw more electrical current, have different ends on the pins so they cannot be used in the wrong