Nine parameters to evaluate the pros and cons of LED
The LED has a typical PN junction volt-ampere characteristic. The current passed directly affects the LED brightness and the PN series-parallel connection. The characteristics of the respective LEDs must match, and the reverse electrical characteristics must also be considered in the AC operating state. . Therefore, they must be tested for forward current and forward voltage drop at the operating point, as well as parameters such as reverse leakage current and reverse breakdown voltage.
2. LED luminous flux and radiant flux
The total electromagnetic energy emitted by an LED in a unit of time is called the radiant flux, which is the optical power (W). For the LED light source for illumination, the more concerned is the visual effect of illumination, that is, the amount of radiant flux emitted by the light source that can cause the human eye to perceive, called the luminous flux. The ratio of the radiant flux to the electrical power of the device represents the radiation efficiency of the LED.
3, LED light intensity distribution curve
The light intensity distribution curve is used to indicate the distribution of the light emitted by the LED in all directions of the space. In lighting applications, the light intensity distribution is the most basic data when calculating the illumination uniformity of the working surface and the spatial arrangement of the LEDs. For a LED whose spatial beam is rotationally symmetric, it can be represented by a curve of the plane of the beam axis; for an LED with an elliptical beam, the curve of the two vertical planes of the beam axis and the elliptical axis are used. For asymmetrical complex graphics, it is generally represented by a plane curve of more than 6 sections of the beam axis.
4, the spectral power distribution of the LED
The spectral power distribution of an LED represents a function of the radiant power as a function of wavelength. It determines both the color of the luminescence and its luminous flux and color rendering index. Generally, the relative spectral power distribution is represented by the text S(λ). When the spectral power drops to 50% of its value along both sides of the peak, the difference between the two wavelengths (Δλ=λ2-λ1) is the spectral band.
5, LED chromaticity coordinates
For the three primary colors red (R), green (G), blue (B), x = R / (R + G + B), y = G / (R + G + B), z = (R + G + B) . Since x+y+z=1, it is only necessary to give the values of x and y to uniquely determine a color, which is what is commonly called a chromaticity diagram. If x and y are used as the plane coordinate system, the x and y values of various colors in the natural world are measured by the colorimetric experiment method, and the chromaticity diagram is obtained by drawing the x and y values in the coordinate plane. Any point on the tongue of the chromaticity diagram represents the hue of light of a certain wavelength, and any point in the curve represents the color of a certain mixed light that can be seen by the human eye. The color temperature values of the color coordinate values of the feature points A., B, C, D, and E of the region are shown in the following table.
6, LED color temperature and color rendering index
For a light source whose illumination color is basically the same as LED, the apparent color of the light source can be accurately expressed by the chromaticity coordinates, but the specific value is difficult to be associated with the custom light color feeling. People often refer to the light-colored orange-red color as “warm color”, and the more blazing or slightly blue-colored ones are called “cold color”. Therefore, it is more intuitive to use color temperature to indicate the light color of the light source.
7, the thermal performance of LED
The improvement of LED luminous efficiency and power for lighting is one of the key issues in the current development of the LED industry. At the same time, the PN junction temperature of the LED and the heat dissipation problem of the housing are particularly important, and are generally expressed by parameters such as thermal resistance, case temperature, and junction temperature.
8, LED radiation safety
Currently, the International Electrotechnical Commission (IEC) equates LED products with semiconductor lasers for radiation safety testing and demonstration. Because LED is a narrow beam, high-brightness light-emitting device, considering its radiation may be harmful to the human eye retina, the international standard specifies the limits and test methods for effective radiation for LEDs used in different occasions. Radiation safety for lighting LED products is currently implemented as a mandatory safety requirement in the European Union and the United States.
(1) Average life: The time taken for a batch of LEDs to illuminate at the same time, when the proportion of non-bright LEDs reaches 50% after a period of time.
(2) Economic life: Under the condition of considering LED damage and attenuation of light output at the same time, the comprehensive output is reduced to a certain proportion of time, which is 70% for outdoor light source and 80% for indoor light source.