Abstract: Solid-state lighting is the dominant direction of future lighting, and LED lighting is the main component of solid-state lighting and is an ideal next-generation lighting device. The development of the LED industry is inseparable from the light color measurement technology. Starting from the development requirements of LED lighting, the article introduces the light color measurement of LED lighting.

Keywords: LED lighting; light color measurement; lighting evaluation;

    1 Introduction 

    As a new light-emitting body, LED has the advantages of high electro-optical efficiency, small size, long life, low voltage, energy saving and environmental protection. It is the first choice for a new generation of lighting. The development of LED has attracted widespread attention at home and abroad, and new products and new technologies have emerged one after another. In recent years, the LED industry has developed rapidly, with increasing luminous efficiency and increasing brightness. Nowadays, LED has been widely used in many occasions, especially the continuous progress of white light LED technology, which has gradually popularized its application in the field of lighting.

    2. The working principle of LED

    Light-emitting diode (LED) is a solid device that can convert electrical energy into light energy. Its structure is mainly composed of PN junction chips, electrodes, and optical systems. The basic working principle of LED is an electro-optical conversion process. When a forward bias is applied to both ends of the PN junction, due to the reduction of the PN junction barrier, the positive charge in the P area will diffuse to the N area, and the electrons in the N area will also move toward the N area. The P region diffuses, and the accumulation of unbalanced charges is formed in the two regions at the same time. Because the minority carriers generated by current injection are relatively unstable, for the PN junction system, the unbalanced holes injected into the valence band must recombine with the electrons in the conduction band, and the excess energy will radiate outward in the form of light. The greater the energy difference between electrons and holes, the higher the photon energy produced. The energy level difference is different, the frequency and wavelength of the generated light will be different, and the color of the corresponding light will be different. 3. The light parameters of LED

    3.1 Luminous flux

    Luminous flux is the amount of light emitted by the light source per unit time, that is, the effective equivalent of the radiant power (or radiant flux) that can be felt by the human visual system. The symbol of luminous flux is Φ, and the unit is lumens (Lm).

    According to the spectral radiant flux Φ (λ), the luminous flux can be determined by the following formula:

    Φ=Km■Φ(λ)gV(λ)dλ

    In the formula, V (λ)-relative spectral luminous efficiency;

    Km—the maximum value of the spectral luminous efficacy of radiation, in Lm/W. In 1977, the Km value was determined by the International Committee of Weights and Measures to be 683Lm/W (λm=555nm).

    3.2 Light intensity

    The luminous intensity I of a light source in a given direction is the quotient of the luminous flux dΦ transmitted in the cube corner element of the light source divided by the cube corner element dΩ, namely:

    I=■

    The unit of luminous intensity is candela (cd), 1cd = 1Lm/1sr. The sum of the light intensity in all directions in space is the luminous flux.

    3.3 Brightness

    The brightness L at a certain point on the light-emitting surface of the light source is the luminous intensity of the face element dS in a given direction divided by the quotient of the face element’s orthographic projection area on a plane perpendicular to the given direction, namely:

    L=■

    The unit is candela per square meter (cd/m2).

When the luminous surface is perpendicular to the measuring direction, cosθ = 1.3.4 Illumination

    The illuminance E of a point on the surface is the quotient of the luminous flux dΦ incident on the panel containing the point divided by the area of the panel dS. which is: 

    E=■

    The unit is Lux (Lx), 1Lx=1Lm/m2.

    3.5 Other parameters

    The light parameters of LEDs also include: spectrum, chromaticity coordinates, dominant wavelength and color purity, color temperature and correlated color temperature, color rendering and color rendering index.

    The necessity of 4LED light color measurement

    4.1 Avoid hazards

    LED is different from traditional lighting, it has the characteristics of point light source, high brightness, narrow beam output and so on. When LEDs are used in lighting appliances, if the light exit angle is not strictly controlled, strong glare will be produced. Some high-brightness LED products may even cause light radiation hazards to the human body. Light color measurement can provide guidance for the safe use of LEDs.

    4.2 Promote the development of LED industry

    The light color measurement of LED can provide a large amount of experimental data, which can be used as a standard for judging whether LED products are qualified or not, and can provide a basis for improving the design and manufacturing of LEDs.

    5LED light color measurement method

    5.1 Measurement of luminous flux

    5.1.1 Integration method

    Measure the light intensity of the LED in various directions, and then calculate these light intensity values to obtain the total luminous flux of the LED (as shown in Figure 3).

    5.1.2 Integrating sphere method

    The integrating sphere is also called the luminous sphere, which is a hollow and complete spherical shell. The inner wall is coated with a white diffuse reflection layer, and the points on the inner wall of the ball are evenly diffused. The illuminance generated by the light source at any point on the spherical wall is a superposition of the illuminance generated by multiple reflections. According to the principle of integration, the illuminance at any point on the sphere is proportional to the luminous flux of the light source. Therefore, the luminous flux of the lamp under test can be obtained by comparing the standard lamp with known luminous flux with the lamp under test, as shown in Figure 4(a).

    However, due to the difference in physical structure and properties (such as absorption) between the standard lamp and the lamp under test, the test results need to be corrected when the integrating sphere method is used to test the luminous flux. The auxiliary lamp method can be used, as shown in Figure 4(b) .

    5.1.32π solid angle luminous flux test

    When using the integrating sphere method to test the LED luminous flux, there is also a test structure (as shown in Figure 5), which is called the forward luminous flux test or 2π solid angle luminous flux. This test does not test the total luminous flux of the LED, but people often confuse it with the total luminous flux of the LED.

    5.2 Light intensity measurement

    For the LED light intensity test, CIE-127 specifies two test conditions, as shown in Figure 6 and the table.

    5.3 Brightness

    The test of LED brightness is usually used in the process of testing the brightness of LED chips and evaluating the safety of LED light radiation. The test generally adopts the imaging method, and the microscopic imaging can be used to measure the chip test, as shown in Figure 7.

    5.4 Illuminance measurement Strictly speaking, illuminance is not actually an optical parameter of LED. Illuminance is the optical quantity that indicates the degree of illumination of the illuminated surface, and it does not make much sense to test the illuminance of a certain point or a certain surface of a single LED, because in general Under the circumstances, the lighting of the actual situation is completed by multiple LEDs.

 5.5 Measurement of other parameters

    Other parameters such as chromaticity coordinates, dominant wavelength and color purity, color temperature and correlated color temperature, color rendering and color rendering index can be tested with a correlated colorimeter or a spectrometer.

    6LED's light radiation safety test and evaluation

    In recent years, the light radiation safety of LEDs has received more and more attention, so this article will also give a brief introduction to this.

    Zhejiang University Tricolor Instrument Co., Ltd. first carried out the research on optical radiation safety testing in China, and jointly drafted the national standards for photobiological safety of incoherent light sources with the National Electric Light Source Testing Center. At present, Zhejiang University Tricolor has made new progress in the light radiation safety testing of LEDs. It has carried out lighting LED testing work for Phillips, and has completed the LED radiation safety testing system with China's independent intellectual property rights. The following is a test case we did to introduce the process of LED light radiation safety test and evaluation.

    6.1 The tested white LED

    6.1.1 LED working conditions

    The lamp current is 0.417A, the voltage is 12V DC, and the power is 5W.

6.1.2 Physical map and spectral distribution

    6.2 Tests of tourist sources

    As shown in Figure 9, we tested the white light LED with the external source test system. These 9 pictures are the source images of the external source taken at an interval of 10° in the direction of 0° (right) and 90°. In theory, we should test and evaluate light safety in various directions, but for the sake of simplicity, this article only tests and evaluates the situation in the 0° direction (because the light output is the strongest in this direction), and other angles can be deduced by analogy. The 0° direction indicates that the size of the sightseeing source is a round spot with a diameter of 2.5 cm.

 6.3 Test condition 1

    In accordance with the requirements of IEC-60825, first perform the test according to test condition 1. The white light LED exit hole is 2m away from the light receiving port, and the diameter of the light receiving port is 50mm.

    Analysis of test results:

    Type 1 AEL calculation

    400～600nm photochemical hazard: not exceeding the specified AEL

    400～700nm thermal hazard: does not exceed the specified AEL

    700～1400nm AEL: Not exceeding the specified AEL

    Conclusion: The optical radiation output of this white light LED measured under test condition 1 does not exceed the AEL regulations of class 1 laser products.

    6.4 Test condition 2

    The white LED light exit hole is 100m away from the light receiving port, and the diameter of the light receiving port is 7mm.

    Analysis of test results:

    (1) Calculation of Type 1 AEL

    400～600nm photochemical hazard AEL: exceeding the specified AEL

    Conclusion: The white light LED exceeds the AEL regulations on photochemical hazards of Class 1 laser products, and does not belong to Class 1 products. (2) Calculation of Type 2 AEL

    400～700nm AEL: does not exceed the specified AEL

    700～1400nm AEL: Not exceeding the specified AEL

    Conclusion: The white light LED does not exceed the AEL regulations of Class 2 laser products under test condition 2.

 6.5 Irradiance or radiance test conditions

    In the above-mentioned test process, the test structure of test condition 2 is the same as that of the irradiance or radiance test condition, and the 1M type AEL refers to the regulations of the type 1 AEL. Therefore, from the above analysis, the light radiation output of the white light LED exceeds that of the 1M type AEL. .

    6.6 Conclusion

    Based on the above analysis, it can be seen that the tested white light LED products belong to the second category of LED products, and looking directly at the LED for a long time will damage the retina of the human eye. However, under normal circumstances, human eyes will not cause damage to human eyes due to natural avoidance reactions such as blinking.

    From the above introduction, it can be known that the testing and evaluation of the safety of LED light radiation is a very complicated process involving many aspects. However, the testing and evaluation of the safety of LED light radiation has gradually become an indispensable content in the testing of LED products. For example, all LED products currently sold to Europe must undergo strict compliance with EN-60825 (equivalent to IEC-60825) standards. test. Therefore, we should increase research input in this field.

    7. LED light color measurement and lighting evaluation

    The ultimate goal of LED lighting is to obtain the best lighting effect, and judging the quality of the lighting effect depends on the light color measurement of the LED.

    7.1 Evaluation of current lighting

    The current lighting evaluation is mainly based on several main optical basic quantities: luminous flux, brightness, light intensity and illuminance, among which the first three basic quantities are mainly used for the evaluation of the light source, and the illuminance is the evaluation of the light radiation action surface of the light source.

    These optical basic quantities are obtained based on the biophysical characteristics of the human eye, and are also the result of a spectral luminous efficiency function V(λ) approved by the International Commission on Illumination (CIE) and the corresponding radiance measurement weighted integral, which satisfies the following relationship:

    Photometric (λ) = KmgV (λ) Radiometric (λ)

    In the formula, Km—683Lm/W.

    7.2 Existing problems and shortcomings

    The biophysical mechanism of the human eye is very complex, and the response to light stimuli is different in different situations. It can be roughly divided into photopic vision, scotopic vision and mesovision. Among them, the mesovision is particularly complex, so the above-mentioned spectral light efficiency is not accurate. The ground represents the actual effect of lighting. As shown in Figure 10, the stimulus value of the same light source to human eyes is significantly different under photopic vision and scotopic vision. In addition, the current domestic lighting evaluation lacks consideration of photobiological safety, which may cause varying degrees of harm to the human body.

    7.3 Solution

    7.3.1 Test methods and instruments

    The full spectrum method is used to measure the spectral distribution of the LED, and then a spectrometer is used for spectroscopic testing.

    7.3.2 Evaluation method

    (1) Lighting effect According to different visual conditions, different spectral light efficiency functions are used to weight the LED spectral distribution data to obtain the actual lighting effect of the light radiation of the measured light source under the corresponding visual conditions. For example, scotopic vision uses a scotopic spectrum optical efficiency function, photopic vision uses a photopic spectrum optical efficiency function, and intermediate vision uses an intermediate vision spectrum optical efficiency function.

    (2) Photobiological safety

    According to the type of hazard, use the corresponding effect function to weight the LED spectral distribution data to get the actual hazard effect

In short, LED lighting evaluation should meet the actual biophysical needs of the human body.

    8 Summary and Outlook

    (1) The development of the LED industry is inseparable from the LED light color measurement technology. In order to adapt to the LED lighting application, there should be new breakthroughs in the future light color measurement technology.

    (2) LED lighting and light color measurement should be people-oriented, and everything should be oriented to meet the actual requirements of human beings.

    (3) Current tasks: ① Strengthen the research of basic biological theories related to the human body, promote cross-disciplinary cooperation; ② Integrate with international standards, speed up the formulation and implementation of regulatory documents, and strive and actively participate in internationally renowned CIE, IEC, etc. Organized activities; ③Increase scientific research investment and develop light and color measuring instruments with independent intellectual property rights; ④Strengthen the research of various testing technologies, train professional testers, and accelerate the improvement of the equipment level and testing capabilities of quality inspection institutions.

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