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Working principle of high pressure mercury lamp

A considerable part of the cross section of the discharge tube of a high-pressure mercury lamp has not been effectively used for luminescence. In order not to affect the life of the lamp and reduce the energy consumed on the wall of the discharge tube to a certain amount, this non-luminous part is still necessary. However, if another element with a low excitation potential can be introduced without interfering too much with the basic discharge of mercury, this element may be excited in the non-luminous part. In this way, the characteristic spectrum of this element can be added to the mercury spectrum to improve the light color, and its increased light output can also improve the light efficiency.

However, although some elements have appropriate radiation characteristics, their excitation energy levels are different from those of mercury, so the utilization rate of the discharge tube space cannot be well improved. The use of such elements (such as thallium and indium) should be strictly controlled, and it must be ensured that the light color can be improved and the light efficiency can be increased without damaging the basic mercury discharge.

In order to design a lamp with good light color and high light efficiency, one thing must be noted: in the yellow-green spectrum, 1 watt of 555 nanometers of electromagnetic radiation can produce 680 lumens, while 1 watt of 715 nanometers of deep red can only produce 1 lumen of light. Therefore, the radiation ratio that element additives can produce must be properly studied in order to achieve a comprehensive balance between light color and light effect.

The above only considers the effect of excitation on radiation of added elements in high-pressure mercury lamp discharge. However, when studying the practical problem of how to keep other elements in a vapor state in the discharge tube, it is found that although the maximum arc temperature of 6000K can meet this purpose, the lowest temperature of the tube wall (behind the electrode) cannot adapt to all available elements. Alkali metals may be an exception, but alkali metals will discolor and damage the silicon dioxide of the discharge tube due to corrosion in a short time.

It is now recognized that the vapor pressure of many available elements is very low, and the temperature problem caused by this can be solved by citing iodides of various elements. Moreover, the use of iodides of alkali metals can also eliminate chemical corrosion of silicon dioxide. It is now further recognized that many elements in the periodic table can be added to mercury discharge as iodides. In this way, their visible spectrum is naturally included in the light output of the lamp. This appropriate amount of iodine has little or no effect on the discharge characteristics.

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