Brief Introduction to the Light Color Measurement Method of Power-Type White Light LED Devices

With the rapid development of semiconductor lighting industry, LED has become the most popular and eye-catching light source in the world with its advantages of rich light color, energy saving, high light efficiency, low voltage and long life. In particular, the luminous efficiency and brightness of LED are greatly improved in recent years, and LED will surely become the most widely used lighting source. Nowadays, LED has penetrated into a large number of applications, especially with the continuous advancement of white light LED technology, the application of LED in the field of lighting has become more and more popular.

Ordinary lighting LEDs are white light LEDs. There are two main technical approaches to achieve white light: the first is to use the blue light emitted by the LED to excite the phosphor, and then mix it with the "yellow light" emitted by the phosphor to finally obtain white light; the second is to use the combination of red, green and blue primary color LED chips to generate white light.

LED is a solid device that can convert electrical energy into light energy. Its structure is mainly composed of PN junction chip, electrode and optical system. 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 region will diffuse to the N region, and the electrons in the N region will also diffuse to the P region, and at the same time, an accumulation of unbalanced charges will be formed in the two regions. Since the minority carriers generated by current injection are unstable, for the PN junction system, the unbalanced holes injected into the valence band will 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 energy of the generated photons. Different energy level differences will generate different frequencies and wavelengths of light, and the corresponding colors of light will be different.

However, of the electrical energy injected into LED devices, only about 15% of the electrons and holes are radiatively recombined to emit photons, while about 85% of the electrical energy is converted into heat energy. The thermal performance of LED directly affects its luminous efficiency, wavelength, forward voltage drop, reverse breakdown current and device life. Traditional Φ3mm and Φ5mm LED devices have little impact on thermal performance due to their small driving current and low heat generation. In recent years, with the rapid development of power LED devices, the impact of LED thermal performance has become increasingly obvious and has attracted widespread attention.

1. Common LED light color parameters

1. Luminous flux

Luminous flux is the amount of light emitted by a light source per unit time, that is, the effective equivalent of the radiant power (or radiant flux) that can be sensed by the human visual system. The symbol of luminous flux is Φ, and the unit is lumen (lm). Based on the spectral radiant flux Φ (λ), the luminous flux can be determined by the following formula.

Where V(λ) is the relative spectral luminous efficiency, Km is the maximum value of the spectral luminous efficiency of radiation, and the unit is lm/W. In 1977, the International Committee for Weights and Measures determined the Km value to be 683 lm/W (λm=555nm).

2. Light intensity

The luminous intensity of a light source in a given direction is the quotient of the luminous flux dΦ transmitted by the light source within the solid angle element dΩ in that direction divided by the solid angle element, that is,

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

3. Color parameters

Color parameters mainly include spectrum, chromaticity coordinates, dominant wavelength and color purity, color temperature and correlated color temperature, color rendering and color rendering index, as shown in Figure 1.

The "white light" LED that has entered industrial production has an uneven spectral mixture, and its color distribution, like the light intensity distribution curve, changes with distance and angle.

2. Parameters and Method Difficulties of LED Thermal Performance Test

The thermal performance parameters of LED mainly refer to junction temperature, thermal resistance, transient change curve, etc. The junction temperature of LED refers to the temperature of PN junction, and the thermal resistance of LED generally refers to the thermal resistance between PN junction and shell surface. Junction temperature is a parameter that directly affects the working characteristics of LED, and thermal resistance is a parameter that indicates the heat dissipation performance of LED. Studies have shown that the lower the thermal resistance of LED, the better its heat dissipation performance, and the corresponding LED light efficiency is generally higher and the life is longer. The key to detecting thermal characteristics lies in the accurate measurement of LED junction temperature. There are generally two methods for testing LED junction temperature: one is to use infrared temperature measurement method to measure the temperature of LED chip surface and regard it as the junction temperature of LED, but the accuracy is not enough; the other is to obtain PN junction temperature through temperature sensitive parameter (TSP), which is the most common LED junction temperature test method at present. Its technical difficulty lies in the high requirements for test equipment.

The junction temperature has a significant impact on the light output of the LED. The higher the temperature, the smaller the operating voltage of the LED being tested, and the luminous flux and optical power gradually decrease.

3. Light color measurement method

1. Luminous flux

There are currently two methods for measuring luminous flux, one is the integrating sphere method and the other is the distribution photometer method.

The integrating sphere method is a commonly used method. It uses the forward emission characteristics of LEDs. Some integrating spheres use a 2p structure for measurement. In order to eliminate spectral matching errors, a spectral radiation analyzer is used as the receiving probe. In order to eliminate the influence of junction temperature changes, an LED test holder with a constant temperature function is used to clamp the LED sample (Figure 2). The integrating sphere method has the advantages of fast speed and no influence of external stray light. However, due to the influence of the screen, seams, openings in the sphere wall, spraying effects and other factors in the integrating sphere, the response of the integrating sphere is uneven, and the uncertainty is large for measuring narrow beam light sources such as LEDs.

Figure 2 Integrating sphere test system produced by Instrument systems of Germany

Therefore, for high-precision measurement requirements, the distribution photometer method is often used for measurement (Figure 3). The distribution photometer method has the advantages of uniform response and high measurement accuracy, but it is affected by factors such as stray light interference. In addition, considering the non-uniformity of the color space of white light LEDs, when the distribution photometer method is used to measure the luminous flux, a spectral radiation analyzer can also be used as a measurement probe to eliminate the spectral matching error of the probe. Moreover, the use of a spectral radiation analyzer can not only measure the light intensity distribution of the measured LED, but also measure the color distribution.

Figure 3 Small goniophotometer produced by Instrument systems, Germany

2. Light intensity

Taking into account the special characteristics of LEDs, such as narrow beams and the inability to simply apply the inverse square law of distance, CIE has specified the average light intensity parameters of LEDs (Figure 4). Simply put, the average light intensity of LEDs is measured under two conditions, namely CIE-A conditions and CIE-B conditions.

3. Color parameters

Simple color parameters such as chromaticity coordinates can be measured by either a colorimeter or a spectroradiometer, but parameters such as the color rendering index must be measured by a spectroradiometer. Using a spectroradiometer can eliminate spectral matching errors and measure all color parameters.

4. Test environment conditions

In LED testing, constant current is usually used for power supply, and geometric conditions and ambient temperature conditions are also extremely important. If you do not pay attention to the plane being measured when measuring the light intensity distribution curve, or if the mechanical axis alignment is wrong even if it is the same plane, there will be greater measurement errors.

In addition, LED light source is a light source with strong temperature dependence. The rise in temperature may cause the red shift of the LED emission peak wavelength and the rapid attenuation of light output. Therefore, it is necessary to maintain effective control of the ambient temperature (using a constant temperature integrating sphere) and monitor the temperature of the LED itself during LED testing.

V. Conclusion

In view of the particularity of power white light LEDs, scientific and reasonable measurement methods must be used to eliminate the differences in LED testing among various laboratories, strengthen communication between laboratories, cultivate professional testers, improve the equipment level and detection capabilities of quality inspection agencies, and make the measurement results more scientific, comparable and repeatable.