After long-term use of fluorescent lamps, black spots will appear at both ends of the lamp tube, and the light will dim. Why is it physiological?

Fluorescent lamps are also called "fluorescent lamps" An illuminating lamp using photoluminescence. The lamp tube is a cylindrical glass tube, which is actually a low-voltage discharge tube. Electrodes are installed at both ends, and the inner wall is coated with fluorescent substances such as magnesium tungstate and zinc silicate. During manufacturing, air is removed and a small amount of mercury and argon is filled. After electrifying, the mercury vapor discharge in the tube produces ultraviolet rays, which excite the fluorescent substance and make it emit visible light. Different luminescent substances produce different colors, similar to sunlight (the fluorescent substance is calcium halophosphate). Fluorescent lamp has soft light, higher luminous efficiency than incandescent lamp, its temperature is about 40 ~ 50℃, and its power consumption is only 1/3 ~ 1/5 of incandescent lamp with the same brightness. Widely used as lighting source in life and factories.

Working principle of fluorescent lamp

1. Ignition process of fluorescent lamp

(1) Close the switch, and the voltage is applied between the two poles of the starter. Neon discharge glows, and the heat generated makes the U-shaped moving contact expand and extend, and when it contacts with the static contact, the circuit is connected. Current flows through the filament and ballast.

⑵ After the circuit is connected, the neon gas in the starter stops discharging, the U-shaped piece cools and contracts, the two contact pieces separate, and the circuit is automatically disconnected.

(3) At the moment when the circuit is suddenly disconnected, due to the sharp reduction of ballast current, a high self-induced electromotive force will be generated, and its direction is the same as that of the power supply electromotive force. This self-induced electromotive force is superimposed with the power supply voltage to form an instantaneous high voltage, and the gas added to the lamp tube begins to discharge, so the fluorescent lamp becomes a current path and begins to emit light.

2. Fluorescent lamps usually emit light.

After the fluorescent lamp starts to emit light, the self-induced electromotive force will be generated in the coil because of the alternating current flowing in the ballast coil, which always hinders the current change. At this time, it plays the role of reducing voltage and limiting current together with ballast to ensure the normal light emission of fluorescent lamp.

It is concluded that the ballast produces instantaneous high voltage when it is started, and it plays the role of reducing voltage and limiting current when it works normally. The starter acts as an automatic switch.

Under the current heating, the electrons emitted by the tungsten wire bombard the anode to generate ultraviolet rays. Fluorescent substances emit light under the irradiation of ultraviolet rays, forming the luminescence of fluorescent lamps. But after a long time, solid tungsten will sublimate and condense on the inner wall of the lamp.

After several months or a year's use, both ends of fluorescent tubes often turn black, the illumination drops, and the service life is coming to an end. If the fluorescent tube is removed, it is rotated 180 degrees around its axis (the connecting line between the centers of the two ends of the lamp cap), and then the lamp cap is plugged in, that is, the contact pole is reversed, the life of the fluorescent tube can be doubled, and the illumination can be relatively improved.