Overview of Measurement Technology of LED Optical Parameters

1. Introduction

In the past 30 years, the technology of LED light-emitting diodes has experienced a rapid development process. LED has been widely used for its inherent advantages such as high efficiency, long life, low loss, vibration resistance, and fast response speed, such as indicator lights, display screens, traffic lights, etc., and is currently exploring its application in the field of lighting. Since my country has not yet established LED photometric standards, it has seriously affected the improvement of LED product quality and the unification of measurement values, and has affected international trade and international exchanges and promotion of LED technology. Therefore, although the quality and variety of LED products in my country have improved in recent years, there is a significant gap compared with advanced foreign countries. Under this situation, we have embarked on a detailed study of the measurement of LED optical parameters and established national LED photometric standards.

2. Introduction of LED light parameters

The important aspects of LED optical parameters are: luminous flux, luminous efficiency, luminous intensity, light intensity distribution, and wavelength.

2.1 Luminous efficiency and luminous flux

Luminous efficiency is the ratio of luminous flux to electrical power. Luminous efficiency characterizes the energy-saving characteristics of the light source, which is an important indicator to measure the performance of modern light sources.

2.2 Luminous intensity and light intensity distribution

The luminous intensity of LED is a measure of its luminous intensity in a certain direction. Since the luminous intensity of LED varies greatly at different spatial angles, we have studied the luminous intensity distribution characteristics of LED. This parameter is of great practical significance and directly affects the minimum viewing angle of LED display devices. For example, if the distribution range of the selected LED single tube is very narrow, the audience facing the display at a larger angle will see a distorted image. In addition, traffic signs also require a large range of people to be able to recognize them.

2.3 Wavelength

For the spectrum characteristics of LED, we mainly look at whether its monochromaticity is good, and we should pay attention to whether the main colors of red, yellow, blue, green, white LED are pure. Because in many occasions, such as traffic lights, the color requirements are relatively strict. However, it is observed that some LED signal lights in my country now have a blue green color and a dark red red. From this phenomenon, it is very necessary and meaningful for us to conduct special research on the spectrum characteristics of LED.

3. LED photometric measurement principle

3.1 Light intensity measurement method

Install and debug the light intensity standard lamp, LED and silicon photodiode equipped with V (λ) filter on the optical stand, especially strictly adjust the position of the filament, the light emitting part of the LED and the position of the receiving surface. First, calibrate the silicon photodiode with the light intensity standard lamp, C = E/S, where Es = IS/(d2s), ds is the distance between the standard lamp and the receiver, Is is the light intensity of the standard lamp, and Rs is the response of the standard lamp. Es = C & #183; Rt, where Et is the illumination of the LED under test, Rt is the response of the LED under test, then the light intensity It of the LED is: It = Et & #183; d2t, where dt is the distance between the LED and the receiving surface. For LED, its light emitting surface is in the shape of a round cap, and the light distribution is very special, so at different measuring distances, the light intensity value will change, deviating from the inverse square law of distance. Even if the measuring distance is fixed, the light intensity value will change due to the different receiving areas of the receiver. Therefore, in order to improve the measurement accuracy, the measurement distance and the size of the receiving area should be relatively fixed. For example, the measurement distance is 316mm according to CIE recommendation, and the receiving area is fixed to 10×10mm. At the same measurement distance, the light intensity of the LED varies with the angle of rotation, so in order to obtain the best value, it is best to read the maximum reading Rt.

3.2 Luminous flux measurement method
Luminous flux measurement is carried out on the turntable of the angular photometer. The LED is installed on the turntable. The turntable rotates 90 degrees around the vertical axis on its horizontal plane, and the LED rotates 360 degrees around the photometric axis on the vertical plane. The control of the rotation angle on the horizontal and vertical planes is achieved by a stepper motor. The turntable moves freely on the guide rail. When measuring the standard lamp, the turntable should leave the guide rail. During measurement, the large turntable rotates around the vertical axis on the horizontal plane with a step angle of 0.9°, 90° in the positive direction and 90° in the reverse direction. The LED itself is also rotating. At each horizontal angle, the signal is collected every 18° on the vertical plane. After rotating 360°, a total of 20 data are collected. The total luminous flux is calculated according to the following formula. If the large plate rotates 0°～90°, the small plate rotates 0°～360°. However, when the large plate rotates 0°～90°, the LED installation may be uneven (asymmetric) and cause errors. Therefore, the best solution is to rotate the large plate -90°～0°～90°, and the small plate still rotates 0°～360°. The average of the illuminance values ​​at the two angles with equal absolute values ​​in the range of 0°～90° and -90°～0° on the large plate is taken as the value within 0°～90°. The second method of measuring the total luminous flux of LED is the integral method. The advantage of this method is that it is simple and easy to use, but the measurement accuracy is not high. The calculation method of the total luminous flux of LED is as follows: first calculate the luminous flux Φs of the standard lamp (light intensity value Is) entering the integrating sphere at a distance of 1 from the incident window of the integrating sphere (incident window area A), Φs = Is ·A/I2, read the photocurrent signal is on the receiver, then place the LED on the window, read the corresponding receiver photocurrent signal it, then the total luminous flux Φ of the LED is: where K is the color correction coefficient.

3.3 LED spectral power distribution measurement method:

The purpose of measuring the spectral power distribution of light-emitting diodes is to master the spectral characteristics and chromaticity of LEDs, and secondly, to correct the photometric values ​​of the measured LEDs. When measuring the spectral power distribution of LEDs, the following points should be noted. First, when comparing with the standard spectral irradiance, since the spectral radiance of the standard lamp is much stronger than that of the LED, in order to avoid this problem, it is best to add a neutral filter in front of the standard lamp to make its spectral radiance close to that of the LED. The spectral width of LEDs is very narrow. In order to accurately depict the spectral distribution profile of LEDs, it is best to use a monochromator with a narrow wavelength width for measurement, and the wavelength interval is preferably 1 nm. The spectral power distribution Et of LEDs is calculated according to the following formula. In the formula, i is the response of the standard lamp at wavelength i; E is the spectral power distribution of the standard lamp; i is the response of the LED at wavelength λ. The formula for calculating the color coordinates of LED is: The color coordinates are: x=X/(X+Y+Z) y=X/(X+Y+Z) The dominant wavelength and color purity of the LED can also be calculated.

4. Conclusion

At present, the national photometric standard device for LED has established and calibrated a considerable number of standard LED tubes. In this process, we have accumulated a lot of experience. After that, we will transfer the value to the measurement departments at all levels and all LED production units, and conduct unified supervision. This will promote the improvement of the quality level of LED in my country, and will also provide a guarantee for the research and development of the country's high-tech field.