Application of high-brightness LED in green lighting engineering 1 Liu Xinfang Fang Fubo Chen Dingfang (Department of Logistics Engineering, Wuhan University of Technology, Wuhan 430063) The light emitted by the light pipe is the color matching principle of the three primary colors. The CIE chromaticity system is used to calculate the color of the three primary colors. The color ratio of the three primary colors; according to the principle of human visual inertia, the brightness of the LED is controlled by a pulse width modulation (PWM) control method, thereby realizing the control and adjustment of the gray level of the LED color to achieve the desired brightness effect.
0 Overview Light-emitting diodes (LEDs) are a wide range of electronic components. However, due to its low luminous efficiency and brightness, and fewer types of luminescent colors, LEDs were mainly used in the display of status, numbers and texts of instruments before the 1990s. In the early 1990s, LED chip manufacturing technology and packaging technology achieved significant development. The brightness of LEDs reached cd level, resulting in so-called high-brightness LEDs. The color of the light also covered the entire visible spectrum, greatly expanding the LED's Application scope and field. Many countries and regions in the world have begun to replace incandescent lamps and metal halogen lamps with ultra-high brightness LEDs, which are widely used as traffic lights, warning lights, sign lights, high-position brake lights, tail lights, turn signals and instrument panels on automobiles and cars. Lighting and display, etc. Some foreign companies have also produced LED bulbs, LED desk lamps and small flashlights. So far, the luminous efficiency of white LED has reached 1520lm/W, which is equivalent to or higher than that of incandescent lamps. It has become possible to replace incandescent lamps with LEDs.
Compared with incandescent lamps, LED solid lamps have long service life (up to 100000h), low energy consumption, high luminous efficiency, harmless to the environment, low maintenance costs, stable operation, etc., and are an ideal green lighting source. . Since the luminous flux of a single LED tube is relatively small, it is necessary to obtain a plurality of LED tubes to obtain an illuminance equivalent to an incandescent lamp or a fluorescent lamp, so the cost of LED solid lighting is relatively high. At present, solid-state lighting has not been fully developed, and the main factors that restrict the development of LED lighting are the brightness and development cost of LEDs. With the development of LED manufacturing technology and the continuous reduction of cost, LED lighting will become more and more popular. High-brightness LEDs have a broad development prospect and market for solid-state lighting.
1 color matching principle of high-brightness LED 1.1 Color matching principle The method of adjusting two or more colors to be visually the same as a certain color is called color matching. Colors can be mixed with each other. This mixing can be a mixture of color light or a mixture of dyes, and the results obtained by the two mixing methods are different. The former is called color addition and mixing, and the latter is color subtraction and mixing. The light of several colors stimulates the human visual organ at the same time or sequentially, and a new color feeling different from the original color is produced, which is a method of adding and mixing colors. The CIE-XYZ chromaticity diagram is used as the basis for the color calculation.
Bend, the color changes from green to blue-recorded, blue-recording stretches from 510nm to 480nm with a certain curvature, while the blue and purple bands are compressed to a shorter range at the tail of the spectral track connecting 400nm and 700nm The straight line is the spectral trace of the purple to red color spectrum that is not in the spectrum and the horseshoe shape formed at both ends of the connected spectral track includes all physically achievable colors, and the three primary color points of the coordinate system fall outside this area. That is to say, the chromaticity of the three primary color points is imaginary, and it is physically impossible to achieve the same. Any color that falls outside the spectral trajectory and the red end to the purple end straight line cannot be produced by real light. Various intermediate color chromaticity diagrams accurately represent the basic laws of color vision and the general rules of color mixing. This chromaticity diagram is also called a color mixture diagram.
2 color calculation According to Greensman's law, any two non-complementary colors mixed in any ratio will produce intermediate colors on the chromaticity diagram to represent any two colors (including complementary colors) mixed, the resulting color is two The three linearly independent colors of the color on the line segment of the endpoint are mixed, and the resulting color is the color within the triangle enclosed by the three points, as shown. The use of three-color LED color mixing is also based on this principle.
The main task of color matching is to determine the ratio of the three primary colors, that is, to determine the tristimulus value, thereby further determining the number of red, green, and blue LED tubes in comparison with the color combinations mentioned in the text, mainly referring to matching white light, that is, in red. When the green and blue primary color LED tubes reach the brightest, the mixed light is the selected white light, which is called white balance.
The chromaticity coordinates of the illumination of the arc tube are X(XR, yR, z*), Y(xg, yg, Zg), and Z(xb, yb, z,), respectively, expressed in the form of a useful matrix according to Greensmann's law. Therefore, the pulse width modulation mode controls the brightness of the LED, which is actually a schematic diagram of the color control. The curved surface in the figure represents the LED light intensity distribution! The eetoniePjD is lit up periodically, and it is periodically changed to substitute the formula (6) into the available standard on 1 week. That is to say, when X, Y, and Z are the three primary colors to match w, the brightness of the three primary colors or the ratio of light to light.
By calculating the chromaticity coordinates of the color matching, the total light intensity or the light-to-light ratio of the red, green and blue LED light-emitting tubes can be known from the unit of the chromaticity system, and various characteristic parameters of each light-emitting tube can be determined. The approximate number of LED tubes.
3LED lighting tube arrangement design The color ratio of the three primary colors determines the color obtained after color mixing, that is, the relative number of red, green and blue primary color LEDs determines the color of the mixed color. In practical applications, LEDs have a direct impact on the light that participates in the color mixing, and the effect is as shown. The position of the center of the red, green and blue colors in the figure is the position of the center of the light emitting the corresponding color LED.
The color mixing of the three-color LED is known, and the color mixing area is divided into a monochrome area, two monochrome mixed areas, and a three-color mixed area. The positions of the monochrome LEDs are different, and the area of ​​each color area is different. When the distance between the monochromatic LEDs is reduced, the area in which two or three monochromatic colors are mixed is large, and vice versa. Color matching with the three primary colors is to obtain another single color by mixing two or three primary colors, and it is required to avoid other colors when mixing colors. Therefore, in order to obtain a more ideal color matching effect, it is necessary to minimize the spacing between the LEDs participating in the color mixing, thereby greatly mixing the color regions.
According to Greensman's law, the color of the mixed color is determined by the relative proportion of the three primary colors, and the mixed color brightness is the sum of the color brightness before the color mixing. The light intensity of the LEDs is not equal, and the illumination produced by it is not uniform. It can be seen from the adjacent red, green and blue LEDs. When the adjacent red, green and blue LEDs are mixed, according to their mutual distance, the distribution of light intensity after color mixing can generate one or more peaks, so that the local The brightness is high and the mixed colors are significantly different from the surrounding. A part of the light from the nearby LED also hits the area, but because of its low intensity, it has little effect on the peaks and troughs. When a few adjacent LEDs produce a peak of intensity distribution, these LEDs can be regarded as one light-emitting unit, and the mixed color area can be regarded as illuminated by a plurality of such light-emitting units, and the mixed color result is also several. The result of illuminating the illuminating units. Therefore, when arranging the LED tubes, the LED light-emitting units determined by the relative quantity ratio of the three primary color LEDs calculated by the color matching are formed, and the light-emitting units are uniformly distributed and the distribution distance is appropriate.
There are two ways to control LED brightness by 4LED brightness control. One is to change the current flowing through the LED, and the other is pulse width modulation (PWM) to control the lighting time of the LED. In general, except for the large brightness of the red LED with current, the brightness of other LEDs will vary with the operating current. By controlling the operating current flowing through the LED, the brightness of the LED can be controlled over a wide range. However, current control is not easy to implement in software, so it is not suitable for digital control. The PWM method can be easily implemented in software, so it has a wide range of applications in digital circuits.
The degree becomes the length of the lighting time in the period, thereby realizing the change in the brightness of the LED. When the LED is in working state, its working current remains stable. When it is in the lighting state, its brightness is only related to the characteristics of the LED itself, and it is independent of time, so its brightness function is a constant. When the LED is periodically lit and extinguished, its brightness satisfies the formula (10). By changing the lighting time of the LEDs in the cycle, different brightness levels of the LEDs can be obtained, which are called different gray levels. In the working state, continuously changing the lighting time of the LED in the cycle period, continuous change of the gray level of the LED brightness is realized. By dividing the cycle period T into n equal parts, the gray level of the LED is correspondingly divided into n stages.
In order to make the LED change in the gray level, the human eye does not notice the brightness jump, that is, there is no flickering feeling, the LED light-off change frequency is greater than the critical frequency, and the cycle period is at least not more than 0.10.02s. The experiment proves that When the gray level of the LED is 256 levels, the red, green and blue primary colors are mixed without color, and the human eye feels the color is gradual. The 256-level gray scale corresponds to 256 levels of brightness, which corresponds to 256 kinds of color changes. The red, green and blue primary colors can produce 2 563 colors after mixing, which is called 24-bit color.
At present, the C-51 series single-chip microcomputer is widely used in the embedded control system. The external crystal frequency can reach 20MHz, and the machine cycle is 1 post, which can easily meet the critical frequency requirement. When the C-51 series MCU is used to control the red, green and blue LEDs with 256 levels of gray, the LEDs are divided into 256 levels; the monochrome control and color mixing control can be performed on the red, green and blue LED tubes. . When the three primary colors of red, green and blue are separately controlled to realize the generation of 24-bit color, the control method of the segmentation delay can be used to control the duty ratio. When color mixing is performed, the three primary colors are mixed at a certain brightness ratio, that is, mixed at a certain gray level, and the duty ratios corresponding to different gray level fixings are different, and the LEDs are lit at different times in the cycle period. The lighting time of the lowest gray level is t1, the lighting time difference between the middle gray level and the lowest gray level is t2, the difference between the highest gray level and the middle gray level lighting time is t3, and the extinguishing time of the highest gray level is T4, according to this time difference, the control of the LEDs of the three colors is realized.
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