Discussion on the heat effect of new energy TV general lighting materials on the quality of sent light

1 principle and purpose

When the LED luminaire is just starting up, the temperature is not high, but as the usage time increases, the temperature of the luminaire will gradually increase. At a certain ambient temperature, the temperature of the luminaire will stabilize below a certain temperature. If the ambient temperature rises, the temperature of the luminaire will also increase further. As the temperature of the luminaire increases, the junction temperature of the LED also increases. An increase in junction temperature can have a series of adverse effects.

Studies have shown that when the ambient temperature rises and the junction temperature rises, not only does the forward voltage Vf of the LED change, but its light output also decreases, as shown; the position of the peak wavelength shifts toward the long-wave direction, ie, red shift As shown. Thus, when the LED lamp is severely heated, the operating characteristics of the lamp are deteriorated - the color of the light emitted by the lamp changes, and the light becomes dark. These changes will affect the quality of the TV picture.

In order to ensure the quality of TV studio production, especially the high-definition television (HDTV) picture quality, it is necessary to minimize the impact of the LED light output due to thermal effects. This can only be done after fully controlling the laws of change. To this end, the research on the influence of the thermal effect of LED general lighting fixtures on the light output quality was carried out. The study uses an incubator that can actively adjust its temperature. The various types of LED lamps in the experiment are placed in this incubator to light up. By manually changing the temperature of the cabinet, the actual performance of the lamps (such as color temperature or related) is actually measured. Color temperature CCT, color rendering index, phase contrast, etc.), and summarize the changes. Therefore, it provides a scientific basis for the enterprise to improve the heat dissipation function of the lamp and obtain an effective and stable light output, and provides technical support for the TV station to adopt the LED lighting with excellent performance. The time of this experiment is from April 7 to 11, 2010.

2 experiment

2.1 Experimental conditions

The experiment used SDH4020 low temperature constant temperature and humidity chamber. The incubator has a light-emitting window with a temperature controllable range of -40 ° C to 100 ° C. However, this incubator was found to be not ideal during the experiment.

In order to fully test the chromaticity parameters of the luminaire (ie, the correlated color temperature and color rendering index), the spectral distribution of the luminaire must be measured with a spectrum analyzer, and the chromaticity coordinates (u, v) are obtained on the chromaticity diagram by computer analysis. ), compared with the black body trajectory, find the difference between the color temperature of the lamp and the standard black body, and accurately measure the small change of the color temperature, as shown. The PMS-50 spectral analysis system is well suited for use in such experiments, but the instrument is relatively large and cannot be moved to the incubator. A more feasible method is to use the STC4000 multi-channel spectrometer (common CCD spectrometer), which is compact, small in size and fast in testing. However, due to its lack of stray light, high dark noise, high temperature dependence and narrow dynamic range, such spectrometers are suitable for applications where accuracy is not critical. After research, the experimenter believes that in this study, the change in color rendering is not too large, so it is not necessary to measure the color rendering index, as long as the relevant color temperature changes with temperature. So I decided to use the Minolta CL-200 illuminometer to measure the correlated color temperature and color coordinates.

In addition, the measuring equipment also includes: 1 set of temperature acquisition and recording system, 1 digital thermometer, multi-function AC regulated power supply APC AFC? 500W1 station, etc.

2.2 Experimental lamps

There are two types of lamps for the experiment: three kinds of LED flat light lamps and four kinds of LED spotlights. The total number of experimental lamps is 7, and the number of lamps is shown.

2.3 Experimental content

For different brands of lamps and different lamps of the same brand, when the temperature of the incubator is raised, the relative color temperature, color coordinates and relative contrast of the lamp are measured. Select a luminaire (No. 7) for destructive testing, ie, raise the temperature of the light box as much as possible to see if the luminaire is damaged, and measure the relevant parameters at different temperatures.

2.4 Experimental methods and procedures

The lamp to be tested is aged for 100 h, and the experiment is performed after the parameters of the lamp are stabilized. During the experiment, the lamp to be tested is placed in the incubator, and the optical axis of the lamp is perpendicular to the light exit plane of the incubator. After the lamp is working normally, turn on the heating power of the incubator and raise it to 25 °C. It was found in the experiment that the temperature would exceed the predetermined temperature during the temperature rise. In order to maintain the temperature of the oven at the required temperature, the cooling function of the oven must be enabled. Under the combined effect of the cooling function and the heat generated by the lamp itself, the temperature in the last box reaches the required value of 25 °C. At this time, the illuminance meter was used to measure the maximum illuminance value (relative value), the correlated color temperature, and the color coordinates at 5.9 m from the light exit port.

Then gradually increase the temperature of the incubator to several predetermined values, and measure the maximum illuminance value, correlated color temperature and color coordinate at each temperature.

When measuring the spotlight, it was found that the temperature of the lamp body was not high when the lamp was taken out of the incubator after turning off the lamp; however, when the lamp was measured outside the incubator, the lamp housing was very hot after the lamp was turned off. It is thus conceivable that the method used to control the ambient temperature may be problematic. After analysis, it is found that the temperature rise or decrease in the box is to send the heat source or the cold source to the box through the fan. With this temperature control method, the temperature of the casing of the luminaire is forced to be maintained below the ambient temperature. Under real use conditions, there should be a temperature difference between the lamp housing and the environment, and there is natural convection heat transfer between the housing and the environment. Therefore, the fan must be turned off during the experiment. However, since the heat capacity of the casing is small, the heat of the lamp itself causes the temperature to rise, and the temperature balance cannot be achieved. In order to increase the heat capacity of the box, we decided to put a few bottles of water in the box. In this way, the temperature of the water is raised by heating, and then slowly cooled, so that the tank can be maintained at a certain equilibrium temperature for a long time, which is convenient for measurement.

3 experimental results and analysis

For the sake of convenience, the measurement method before placing the bottled water in the incubator is referred to as the "old method", and the measurement method after the bottled water is placed is referred to as the "new method". The measurement results of the old method are shown in Appendix 1. The measurement results of the new method are shown in Appendix 2.

Taking the lamp 3-1 as an example, the high temperature box experimental record table (see Appendix 1) measured according to the old method is used to draw the relevant color temperature and phase contrast as the ambient temperature changes, as shown. It is the result of measuring the lamp 3-1 with a new method. It can be seen that when the ambient temperature rises, the correlated color temperature and relative light output of the lamp change significantly. When the ambient temperature is raised from 18.9 ° C to 45.5 ° C (here, the ambient temperature is averaged from the upper and lower temperatures), the correlated color temperature is increased by about 200 K, and the relative light output is reduced by about 20%. It is much smaller, because the temperature of the lamp body is artificially reduced. It is the result of measuring the lamp 4-1 and the lamp 6-1. According to the data analysis, when the temperature rises by about 25 °C, the contrast of the lamp 1-1 and the lamp 4-1 is reduced by about 5%, the color temperature is increased by less than 80 K; the color temperature of the lamp 3-1 is increased by about 200. K, up to 7%, the contrast decreased by 20%, the change was obvious; the color temperature of the 6-1 lamp of the lamp did not change much, only decreased by 30 K, about 1%, and the contrast decreased by about 11%.

When the ambient temperature rises, it is easy to understand that the light output of the lamp is reduced. An increase in the ambient temperature will increase the junction temperature of the LED. The decrease in light output is caused by an increase in the visible junction temperature. When the visible ambient temperature rises, the correlated color temperature of the lamp rises; and the opposite is true. When the ambient temperature rises, the correlated color temperature of the lamp decreases. What is the reason for this?

Experiments have shown that CCT will increase or decrease as the junction temperature increases. Whether CCT increases or decreases the grading of the blue light of the excited phosphor. If the blue light used to excite the phosphor is in the rising region of the absorption spectrum of the phosphor, and the absorption spectrum is assumed to be a Gaussian distribution, the blue light is to the left of the center apex of the curve. When the blue light is absorbed much more by the phosphor, the yellow light is increased to form white light. When the temperature rises, the peak wavelength of the blue LED shifts to the direction of the long wave (ie, red). At this time, the blue LED is more effective in exciting the phosphor, and has more yellow light, and the emitted blue light is reduced. Thus, as the junction temperature increases, the color temperature of white light decreases, which is the case.

If the blue light of the excited phosphor is on the right side of the apex of the absorption spectrum of the phosphor, the blue LED pair phosphor is moved as the junction temperature increases and the peak wavelength of the blue LED shifts toward the long wave (ie, red) direction. The excitation efficiency is reduced, the yellow light produced is reduced, and the blue light that escapes is increased. Thus, as the junction temperature increases, the color temperature of white light increases. This is the case.

In order to study the adverse effects of high ambient temperatures on the lamps, destructive experiments were performed on the lamps 7 specifically. The ambient temperature was gradually increased in the experiment, and the maximum illuminance value, correlated color temperature, and chromaticity coordinates at each temperature were recorded. The temperature of the incubator was raised to its maximum allowable value of 100 ° C, at which time the ambient temperature in the tank had reached about 130 ° C and the temperature of the LED substrate was as high as 150 ° C. At this point, the color temperature of the lamp rises by about 300 K, which is about 7%; the contrast decreases by about 300 lx. Although the light output of the lamp is reduced by 1/4, the lamp still works normally and there is no failure. sign.

In addition, it has been said that the instrument for measuring the color rendering index is relatively large, and it is not possible to measure the color rendering performance of the lamp at a high temperature state before moving to the incubator. In order to study the effect of high temperature on the color rendering of the lamp, we measured the luminaires that passed the high temperature box and then measured them on the spectrometer to compare the color change before and after the high temperature measurement. The change of color rendering performance of the lamp 3-1 before and after high temperature measurement is given. It can be seen from the data in the table that after the high temperature measurement, R1-R8 used to calculate the general color rendering index Ra is almost the same as before the high temperature measurement, so Ra is 77 before and after the high temperature measurement; and other special color rendering index R9 ~R15 is basically unchanged after high temperature measurement.

4 conclusions and recommendations

4.1 Conclusion

(1) When the ambient temperature rises, the light output decreases, and the color temperature increases or decreases.

(2) From the results of the high temperature experiment, the performance of the LED lamp with reasonable heat dissipation design is not changed at an ambient temperature of 40 ° C to 50 ° C, which can meet the requirements for use.

(3) The high temperature destructive experiment shows that as long as the design is reasonable, the lamp will not be invalid due to the high temperature of the studio.

(4) Before and after the high temperature experiment, the color rendering performance of the lamp is basically unchanged.

4.2 Suggestions

(1) Tests on imported lamps and domestic lamps show that the performance of domestic lamps can meet the requirements of studio lighting, and it is recommended to use domestic brands first.

(2) The thermal stability of lamps of different brands in China may vary greatly, and users should be cautious when purchasing.

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