Thermal property of LED refers to thermal resistance and PN junction temperature. Thermal resistance is the ratio of temperature difference in channel to its power consumption. Resistance and junction temperature are two important parameters which will impact an influence on LED High Bay photoelectric properties. There are two main methods for measuring LED junction temperature: using infrared microscope and micro-thermocouple to measure the surface temperature of LED chips. This value is approximately equal to LED junction temperature. Another method is to calculate temperatures of PN junctions according to the current-temperature relations. The later is more accurate than the former and has been widely used.

Reliabilities of LED Lights include electrostatic sensitivity characteristic, service life and environment properties. Electrostatic sensitivity characteristic is the maximum electrostatic voltage that LED can endure. Some LEDs have higher resistivity and longer distance between positive electrode and negative electrodes. When electrostatic charge accumulated to a certain amount on both ends, PN junction will be breakdown. In more serious conditions, LED will be destroyed directly. Thus electrostatic sensitivity test for LEDs is necessary before installing them onto lighting boards.

 

Service life of LED Projects is calculated though another parameter: light attenuation, need long-term tests. In order to obtain an average value, scientists need to choose LED samples randomly. Through long-term observation and statistics to estimate the average working life of LEDs.

 

Environment characteristics of the LED need to be tested in simulated conditions. Like high/low temperature impact test, humidity cycling test, salt spray test, dust test, irradiation test, etc.

 

All LED testing methods mentioned above have strict international standards. As for electrical property tests, final reports should contain curve graph for forward bias voltage, backward bias voltage, forward bias current and backward bias current in accordance with related standards. Tests for luminous flux should using 2π structures. Light intensity should be measured in conditions provided by CIE-127. Spectrum tests, thermal property tests, service life tests all have their own standards.

 

LED has been developed more than ten years in China, there still lacking of scientific and effective standards. Standards for LED testing methods can eliminate differences that exist in LED testing in different regions. In other words, it can help realizing the unification of the commodity. The testing results can truly reflect the overall level of Chinese LED industry.

 

Using integrating method, LED lighting sources will automatically absorb rays, affecting the final results. In order to remove this influence, technicians need to use auxiliary lamps. Once obtained the luminous flux, we can use it to get the lighting efficiency of LED Lights. Testing methods for radiation flux and radiation efficiency are quite similar to luminous flux and lighting efficiency.

Light intensity and light distribution are two relatively complicated compare with other LED testing methods. Spot lighting sources have even light distribution in space. Sensors in different distances and using different apertures will have same results. However, the light distribution of LED STREET LIGHT is uneven. Measuring results will be various depending on measuring distances and diameters of apertures. Standard CIE-127 gives out a series of reference parameters in different measuring conditions.

The spectral parameters include peak wavelength, bandwidth and spectrum power distribution, etc. signal light LED only has one wavelength (spectral properties can only be presented by peak wavelength and bandwidth), while white LED spectrum is a combination of multicolored rays (spectral properties can only be presented by power distribution). We can also calculate color parameters though the LED spectrum power distribution. LED spectrum power distribution can be obtained though spectrophotometers, measuring various rays separately.

 

Switching feature of LED Products can be defined as the optical/electrical/color changing properties at the moment of power charge and power lose. Through them we can obtain the working conditions and physical attributes at these moments, helping us to learn the damages caused by opening/closing the switch to LEDs. Designers also can improve driver module though this parameters.

 

Color properties of LEDs include chromaticity coordinates, dominant wavelength, color purity, color temperature and CRI, etc. Color properties are rather important for white LEDs. Currently, there are two LED testing methods for color properties: Spectrophotometry and Integration. Spectrophotometry can analyze the LED spectral power distribution by using monochromator, then obtain the corresponding color parameters though Chroma Weighting Function. Integration method directly measure chromaticity in the use of specific filters along with photo detectors. Spectrophotometry is more accurate and more complicated than Integration method. Both of them need to measure color parameters with the aid of high-precision instruments. Scientists are looking for cheaper and more accurate methods.

 

LED Street Light has been widely used in industrial area, commercial area and residential area. Lacking of an identical standard in LED testing methods is always the biggest problem for LED industry. Once promoting a new product, manufacturers can only set the parameters though some theoretic values. The most common example is the working time. Most of product specifications will set this parameter as 50000 hours. As a matter of fact, this is a theoretic value rather than testing result, calculated though another parameter (light attenuation) in ideal conditions. In practices, it is impossible for any LED products to constantly work for nearly 6 years without any defects. LED manufacturers and research institutions have a lot of disagreements in setting parameters. Customers often complained that their LED products cannot work in the described conditions (insufficient light intensity, higher temperature). This situation, to a large extent, restricted the development of domestic LED industry. In following sections, we are going to discuss some international standards for LED testing methods: electrical property, optical property, switching feature, color property, thermal property and reliability.

 

Electrical Property: LED is a kind of uni-polar PN junction diode. The voltage-current characteristic can be divided into two parts: forward bias and backward bias. Thus electrical properties of LED high bay include maximum forward current, maximum forward voltage, backward current, backward voltage. LEDs must work under a normal current (theoretically 20mA). We can also obtained maximum working power tough these electrical properties. LED electrical properties should be measured under constant voltage and constant current, by using multimeter.

Optical property: similar to other lighting sources, LED optical properties include luminous flux, lighting efficiency, radiation flux, radiation efficiency, light intensity, light distribution and spectral parameters, etc. We will introduce the testing methods of these parameters in detail.

Luminous Flux and Lighting Efficiency: there are two different ways in measuring luminous flux: integrating sphere method and variable angle photometric method. The later is the most accurate measuring method, but costs much more time. Thus, in normal conditions, scientists and technicians tend to use integrating sphere method to measure luminous flux of LEDs. Integrating sphere method also has two different measuring structures: 1. LED is settled in the middle of the sphere; 2. LED is settled on the edge of the sphere.