Introduction
The electrodeless lamp is a new type of electric light source based on high-frequency electromagnetic induction and electrodeless gas discharge. Because it has no electrode in the traditional sense, it is named as a "new rookie" in the field of green lighting, and is gradually entering the public eye [1, 2]. Since there is no electrode and there is no inevitable component to limit the life, the theoretical life of the electrodeless lamp is infinitely long. According to the "wooden barrel principle", the actual life depends only on the life of the electronic ballast and the natural attenuation of the phosphor. Due to the use of amalgam, the "loss" of liquid mercury is overcome, even if the lamp is broken, it will not cause too much pollution to the environment, and the recoverable rate is over 99%, which can effectively protect the environment. The traditional electric light source emits light at the power frequency, and the operating frequency of the electrodeless lamp is as high as 2.65MHz, which can be regarded as "there is no stroboscopic effect" [3], which will not cause visual fatigue and is more conducive to eye protection.
The illuminating mechanism is shown in Figure 1. After power-on, the electronic ballast outputs a high-frequency current of 2.65MHz, which reaches the power coupler via the high-frequency feeder, and then induces a high-frequency toroidal magnetic field in the bubble through the coupling coil, thereby inducing high A frequency ring electric field, the electric field is perpendicular to the magnetic field. The free electrons make a circular oscillation in the high-frequency electric field, colliding with the gas particles, causing the gas avalanche to ionize and form an inductively coupled plasma (ICP [4]). The ground state mercury atoms are excited to the 6 3 P1 state in the ICP plasma, and then return to the 6 1 S0 ground state, radiating a resonance line of 253.7 nm, which is converted into visible light by the rare earth trichromatic phosphor [5,6]. The spectral distribution in the ultraviolet-visible region when the electrodeless lamp is normally illuminated is shown in the figure.
How to use the least amount of electric energy to emit the most light of the best quality is the constant goal of the field of electric light source. For electrodeless lamps, the key to improving light efficiency is to try to get mercury vapor atoms to radiate more 253.7nm resonance lines. The electrodeless lamp emits mercury vapor from the amalgam placed at the cold end (the amalgam is an alloy of mercury and other metals), and the mercury vapor pressure is determined by the cold junction temperature [7,8], so the radiation efficiency of the 253.7 nm resonance line is The cold junction temperature is closely related.
Experimental part
In order to study the effect of cold junction temperature on the resonance line of 253.7nm, this paper carried out experiments on the 85W induction lamp without phosphor. The cold end was extended, and a UV-transparent quartz window was attached to the outside of the bubble, and a 253.7 nm resonance line was captured there by a Princeton ultraviolet-visible grating spectrometer. The experimental apparatus is shown in FIG. Figure 4 shows the spectral distribution in the ultraviolet-visible region when the electrodeless lamp is stable in the experiment, that is, the atomic emission spectrum of the ICP plasma. It can be seen that the 253.7 nm resonance radiation is the main part of the electrodeless lamp [9,10]. The cold junction temperature is changed by a dry thermostat at different values, and the lamp is illuminated and stabilized by the electrodeless lamp. The atomic emission spectrum at 253.7 nm was extracted at a different cold junction temperature for 60 min, as shown in Fig. 5; the relative intensity of the line as a function of the cold junction temperature is shown in Fig. 6.
2 Results and discussion
2.1 Experimental phenomena
It can be seen from Fig. 5 and Fig. 6 that when the cold junction temperature is lower than 80 °C, the relative intensity of the 253.7 nm resonance line increases with the increase of the cold junction temperature; when the cold junction temperature is 80 °C, the spectral line is the strongest; When the cold junction temperature is higher than 80 ° C, the relative intensity of the 253.7 nm resonance line decreases as the cold junction temperature increases. It can be seen that the radiation efficiency of the 253.7 nm resonant line of the electrodeless lamp is approximately normal with the change of the cold junction temperature.
2.2 Phenomenon analysis
The relative intensity of the atomic emission spectrum depends on the concentration of the excited state atom at the initial energy level, the probability of transition, the self-priming of the line [11]. The cold junction temperature is positively correlated with the mercury vapor pressure. With the increase of the cold junction temperature, the mercury vapor pressure in the electrodeless bulb increases continuously, and more and more mercury vapor atoms participate in the collision. The chance of the mercury atom being excited from the 6 1 S0 ground state to the 6 3 P1 state increases, thereby radiating More 253.7 nm resonance lines are produced. At the same time, a portion of the 253.7 nm resonance line is absorbed by the adjacent 6 1 S0 ground state mercury atoms and transitions to the 6 3 P1 state, which is called resonance absorption [11]. In most cases, the resonance absorption of the mercury atom quickly re-radiates the 253.7 nm resonance line and returns to the 6 1 S0 state. However, if the cold junction temperature is too high and the mercury vapor pressure is too high, the resonance absorption is too heavy. At this time, the 253.7 nm resonance line averages many times of absorption and re-emission processes to reach the inner wall of the bubble, so there is a situation in which the mercury atoms absorbed by the resonance may not be able to jump back to the 6 1 S0 ground state. The first type of inelastic collision is excited to a higher energy level, or the second type of inelastic collision loses energy, resulting in a loss of the 253.7 nm resonance line; and the higher the cold junction temperature, the greater the loss. Therefore, as the cold junction temperature increases, the effective transition probability decreases, and the radiation efficiency of the 253.7 nm resonance line decreases.
3 Conclusion
There is an optimum cold junction temperature for the electrodeless lamp operation, and the 253.7 nm resonance line has the highest radiation efficiency at this cold junction temperature. At this time, the electrodeless lamp consumes the same electric energy, and the emitted light is the strongest. The lamp body structure, especially the heat dissipating part, should be optimized to make the cold junction temperature close to the optimum value when the electrodeless lamp is stably illuminated. On the other hand, different types of electrodeless lamps, due to differences in amalgam type, gas composition, etc., have different optimal cold junction temperatures, and the optimum cold junction temperature should be selected for different working environments. To achieve the most economical lighting effects.
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