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Temperature Effect of Phosphor

Previously, the method of distinguishing phosphorescence from fluorescence was to see whether the light output increased or decreased after being activated based on the luminescence process. If the luminescence process of the phosphor depends only on the electron transition in the activator ion, without the participation of free electrons or "conduction" electrons, the process of its luminescence increase and decrease varies with an exponential curve and is almost independent of temperature. However, if free electrons are involved, the luminescence process is similar to a secondary chemical reaction, which will greatly accelerate the reaction rate under high temperature or strong activation. The oxygen phosphors used in most lamps and cathode ray tubes belong to the first type, and their decay time ranges from one millionth of a second to a few hundredths of a second. Cerium and manganese are typical activators of this type of phosphor. Sulfides are typical of the second type, and the decay time can be as long as several hours or days, especially sulfides activated by copper, which have the longest decay time. However, it is also often found that the same type of phosphor powder has several decay phenomena with different time constants.

For a given excitation energy, most commercially available phosphors have a specific temperature of maximum light output, usually close to room temperature. As the temperature rises, the atomic vibrations in the crystal intensify, absorbing a large amount of input power, thereby reducing the light efficiency to zero. This quenching temperature of various phosphors varies greatly. For example, calcium tungstate completely loses its luminous ability at about 150°C, but some chromium-activated aluminates still increase in brightness until 500°C. The phosphors used in mercury lamps must be resistant to high temperatures because mercury lamps must operate at around 300°C. If the fluorescent material is suddenly heated, a sudden burst of light will be produced, and then it will drop to a lower brightness. At this time, in normal phosphorescence, part of the reserved energy used for luminescence is released in advance due to heating. However, its total light output may be lower than the light output without heating to accelerate its luminescence process. If infrared radiation is used instead of heating, the same phenomenon occurs, which is often due to the "stimulation" effect and sometimes quenching to reduce the total phosphorescence output. When heating produces thermoluminescence, a similar situation can also occur if there is stored energy but no visible phosphorescence. Minerals that have been exposed to long-term radioactive substances, such as the mineral specimens brought back by the Apollo spacecraft, show this effect; some synthetic phosphors also exhibit this phenomenon after being activated by ultraviolet light or ionizing radiation.

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