LED light emitting diode characteristic test (II)

3 Measurement results and analysis

　　3.1 LED volt-ampere characteristics

　　The volt-ampere characteristic curves of five color LEDs at room temperature were measured by Figure 1 and are shown in Figure 6. The voltage at the critical conduction state of the LED is called the threshold voltage. According to the fitting formula in Figure 7, the intersection of the fitting straight line and the horizontal axis is calculated to obtain the normal voltage of the five LEDs, preparing for the normal light-emitting conditions of the subsequent test. According to the formula λ=[（hc）/e]Ud and the above data, the calculated emission wavelength of the light-emitting diode is consistent with the theoretical main wavelength. As shown in Figure 6, at the beginning, the LED current changes almost unchanged with the voltage. After it is greater than the threshold voltage, the current increases linearly with the voltage at a rate of 104~236mA/V, among which the red square LED increases the fastest, while the white round LED increases the slowest. The normal working voltage, threshold voltage and emission wavelength of the five LEDs are shown in Table 1. Table 1 shows that except for the red square LED, the normal voltage and threshold voltage of the other four LEDs are approximately 3V and 2V respectively; the red square LED has the longest emission wavelength, and the emission wavelengths of the other four LEDs are all around 500nm.

　　3.2 LED light intensity distribution characteristics

　　The test results of the light intensity distribution of LEDs are shown in Figure 8. The light intensity distribution curve can properly reflect the spatial distribution of the light source energy. The luminous range of the five LEDs can be determined from the spatial distribution of light intensity. The sensor area of ​​the illuminance meter used in the experiment is 9mm2, and the test radius is 8cm. The data obtained by calculation is consistent with the data of the light intensity conditions for measuring LEDs specified in the international standard, that is, the measured light source can be approximated as a point light source. From the measurement results, it can be found that: square LEDs are more directional; the light intensity of all LEDs is the strongest at their central normal. The half-value angle is often used to describe the LED luminous distribution characteristics. The smaller the half-value angle θ, the stronger the corresponding directivity (see Table 2), which can provide a reference for users to select diodes according to usage conditions.

　　3.3 Spectral characteristics of LEDs

　　The TCS230 color sensor is used to measure the spectrum in a dark box. The color sensor used in this article integrates four groups of filters: red, green, blue, and transparent. The light intensity is collected through a photodiode and converted into a pulse output by a circuit. It is used to measure the luminous spectrum of the above five color LEDs, and the measurement results are compared with those measured by the monochromator to determine the accuracy of the color sensor measurement. The spectral characteristic curves measured by the monochromator and the color sensor are shown in Figures 9 and 10, and the luminous components measured by the color sensor are shown in Figure 11. From the measurement results, it can be seen that the color sensor is basically consistent with the results measured by the monochromator in the working range of 480~635nm.

　　Peak wavelength is an important parameter to describe spectral characteristics. Experiments show that the peak wavelengths of red, blue and green LEDs are within the range of the main wavelength, as shown in Table 3.

　　3.4 Relationship between LED light power and current

　　Optical power refers to the radiation power that can be felt by the human eye, that is, the relationship between the axial light intensity of the LED and the forward injection current I. Since many LEDs are often used in a product, the luminous brightness of each LED must be the same or in a certain proportion to present a uniform appearance, so it is very necessary to test this performance. In the experiment, from the beginning of the LED light emission, the current is gradually increased, and the corresponding value P of the optical power meter is recorded, and the test result PI relationship is obtained as shown in Figure 12. It can be seen from the figure that the larger the current, the stronger the light emission. Through fitting, a set of currents can be found that can make the luminous power of the five LEDs the same.

　　3.5 Relationship between LED luminous efficiency and current

　　Luminous efficiency refers to the ability of a luminous body to convert absorbed energy into light energy when it is excited. It is an important parameter to characterize the function of a luminous body. The experiment measured the illuminance E of 5 kinds of LEDs. Using the formula

　　Calculate the luminous flux Φ. In the formula, S is the hemispherical area of ​​the illuminance meter receiver, n is the luminous angle of the LED, and the distance d between the hemispherical area of ​​the illuminance meter and the illuminance meter receiver is measured to satisfy πd2=S. Figure 13 is a calculation principle diagram, and then use η=Φ/P (P=UI) to calculate the luminous efficiency η. Finally, the relationship curve between the luminous efficiency and the current is fitted as shown in Figure 14. It can be seen from Figure 14 that the luminous efficiency of the white square, green square, white circle, red square and blue square decreases in turn, that is, their ability to convert electrical energy into light energy decreases in turn. Theoretically, the luminous efficiency of these five LEDs has a trend of increasing first and then decreasing. As the current gradually increases, the probability of non-radiative recombination of electrons injected into the luminous area increases, which leads to a decrease in luminous efficiency.

　　4 Conclusion

　　Using conventional experimental instruments, an LED test system was built. The system has a clear principle and a simple measurement method. It is highly operable and has a large measurement range. The normal luminous voltage of the five LEDs measured in the experiment is 1~4V, the threshold voltage is 1.5~3V, and the luminous wavelength is consistent with the main wavelength; the square LED is more directional than the round one; each colored LED is not a single color light, but their peak wavelength is consistent with the main wavelength. The test results show that the spectrum tested by the color sensor is consistent with the result measured by the monochromator; the luminous flux measurement method can achieve simple measurement without the use of instruments such as integrating spheres; within a certain range, the optical power of the five LEDs increases with the increase of current, but their luminous efficiency increases first and then decreases with the increase of current.