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Fluorescent Lamp

George Inman lead a group of General Electric scientists researching an improved and practical fluorescent lamp. Under pressure from many competing companies the team designed the first practical and viable fluorescent lamp that was first sold in 1938. According to the GE Fluorescent Lamp Pioneers, On Oct 14, 1941 U.S. Patent No. 2,259,040 was issued to George E. Inman; the filing date was Apr 22, 1936. It has generally been regarded as the foundation patent. However, some companies were working on the lamp at the same time as GE and some individuals had already filed for patents.

The fluorescent lamp is a gas discharge tube whose output of light is so increased by special tools that it can be used for lighting purposes. The inner surface of the wall of the tube is coated with a light-emitting substance-usually fluorescent or phosphorescent metallic salts (calcium tungstate, zinc sulphide, zinc silicate). The tube is filled with mercury vapour at extremely low pressure. The electrons ejected from the incandescent electrodes collide with the mercury atoms and cause these to emit radiation which consists for the most part of ultraviolet rays, which are invisible.

The visible portion of the mercury vapour rays is located in the green and blue range of the spectrum and gives a pale light. The ultra violet light strikes the fluorescent substance with which the wall of the tube is coated and causes this substance to emit radiation with a longer wavelength in the visible range of the spectrum i.e., the coating transforms the invisible rays into visible light. By suitable choice of the fluorescent substance, this light can be given any desired colour.

The lamp has to be operated with a choke which prevents a harmful rise in voltage and serves to ignite the lamp. For this purpose a small auxiliary glow lamp provided with a thermal contact is connected in parallel with the main lamp. When the current is switched on, the glow lamp first lights up i.e. the bimetallic thermal contact is then open.

This causes the bimetallic strip to warm up and close the contact, with the result that the glow lamp is short-circuited and the cathodes of the main lamp receive the full current that makes them incandescent. The bimetallic strip cools and breaks the contact. Through the agency of the choke this interruption of the circuit produces a voltage surge which is high enough to initiate the discharge in the fluorescent lamp itself.

Because it is bypassed by the main lamp, the small auxiliary lamp then ceases to function. The bimetallic strip keeps the contact open. The cathodes of the main lamp are kept glowing at white heat by the impingement of positive mercury ions, and the lamp, thus continues to function and emit light in the manner described.

The light of a fluorescent lamp is not produced by an incandescent body such as the filament of an ordinary electric lamp, but is emitted as a result of the excitation of atoms namely, those of the mercury vapour and the fluorescent coating and is extremely economical. Because of the large light-emitting area, a fluorescent lamp gives a pleasant light which produces only soft shadows.

There's more than a difference in appearance separating fluorescent and incandescent lamps. An incandescent bulb generates light through heat. When electrical current passes through the tungsten filament, it heats to the point where it glows and gives off a yellow-red light. To keep the filament from burning up immediately, it's housed in a vacuum. Even so, the intense heat of the filament ensures a comparatively short and expensive life span.

A fluorescent lamp has no filament running through it. Instead, cathodes (coiled tungsten filaments coated with an electron-emitting substance) at each end send current through mercury vapors sealed in the tube. Ultraviolet radiation is produced as electrons from the cathodes knock mercury electrons out of their natural orbit. Some of the displaced electrons settle back into orbit, throwing off the excess energy absorbed in the collision. Almost all of this energy is in the form of ultraviolet radiation.

To turn this radiation into visible light, the inside of the tube has a phosphor lining. The phosphors have the unique ability to lengthen UV wavelengths to a visible portion of the spectrum. Put another way, the phosphors are excited to fluorescence by bursts of UV energy. The easiest fluorescent fixture to explain is a design offered by Sylvania in 1938. This early preheat model is no longer made, but millions are still in service, and its principle design features are found in every new fixture.