|Terminology of LED display screen wall panels|
What is the principle of LED displays and LED lighting
LED is the abbreviation of light emitting diode, Chinese name: light-emitting diode. LED light-emitting diode is composed of III-IV compound in element spectrum, such as GaAs (gallium arsenide), GaP (gallium phosphide), GaAsP (phosphorus arsenic) Made of semiconductors such as gallium, the core of which is the PN junction. Therefore, it has the I-N characteristics of a general P-N junction, that is, forward conduction, reverse cutoff, and breakdown characteristics. In addition, it has luminescent properties under certain conditions. At the forward voltage, electrons are injected into the P region from the N region, and holes are injected into the N region from the P region. A part of the minority carriers (small children) entering the other area is combined with the majority carriers (multiple sub-) to emit light. Assuming that luminescence occurs in the P region, the injected electrons directly composite with the valence band holes to emit light, or are first captured by the luminescent center and then condensed with the holes. In addition to this luminescent composite, some electrons are captured by the non-luminous center (this center is near the middle of the conduction band and the intermediate band), and then recombined with the holes, and the energy released each time is not large, and visible light cannot be formed. The greater the ratio of the amount of luminescent composite to the amount of non-luminescent composite, the higher the luminous efficiency.
LED's advantages and applications on the display
The color and luminous efficiency of LEDs are related to the materials and processes for making LEDs. Currently, red, green and blue are widely used. Due to the low operating voltage of the LED (only 1.5-3V), it can actively emit light and have a certain brightness. The brightness can be adjusted by voltage (or current), and it is resistant to shock, vibration and long life (100,000 hours). Therefore, in large display devices, there is currently no other display mode that rivals the LED display mode. The display screen that puts the red and green LEDs together as a pixel is called a two-color screen color screen; the display panel that puts the red, green and blue LED tubes together as one pixel is called a three-color screen or a full-color screen. . Usually for the convenience of engineering installation, a plurality of image points are formed into a standard dot matrix form of 8*16/16*16/16*32/32*32 on the PCB circuit board, which is called a display module: in order to enhance the display The structural strength of the screen, the display module will be installed on the iron box with enhanced strength, the box also houses the power supply, control system, cooling system and other devices, and has waterproof, dustproof, lightning protection, shockproof and other functions; An iron box with display modules and systems forms the entire LED display.
Regardless of whether a single-color, two-color or three-color screen is made with an LED, the brightness of each LED that is required to constitute an image must be adjustable, and the degree of fineness of the adjustment is the gray level of the display. The higher the gray level, the finer the displayed image and the richer the color, and the more complex the corresponding display control system. Generally, the image of 256 gray scales has a very soft color transition, while the color image of 16/32/64 gray scale has a clear color transition boundary. Therefore, color LED screens are currently required to be 256/16384 gray scales. The color combination and color overtones of this gray level have far exceeded the color resolution of the human eye.
Refers to the ability of the display terminal to process and display the picture in the horizontal and vertical directions. It is usually expressed by the product of the number of effective pixels in the horizontal direction and the number of effective pixels in the vertical direction, that is, the total number of effective pixels.
A, luminous flux: luminous flux symbol is the amount of light emitted by the φ light source per unit time, the unit is lumens (lumin), the symbol is lm;
B. Luminous intensity: The luminous intensity is the value of the luminous flux dφ and the solid angle dQ contained in the small angular angle of the light source in a given direction, in units of Cantera (cd) 1cd=1000mcd.
C. Luminance: Luminous, the symbol is the ratio of the luminous intensity of the L source in a given direction at a small solid angle to the orthographic area on a plane perpendicular to a given direction. The unit is Cantera per square meter (cd/m2)
D. Light efficiency: Unit Lumens per watt Lm/w, indicating the ability of an electric light source to convert electrical energy into light, expressed by the amount of luminous flux divided by the amount of electricity consumed.
Point spacing P
The spacing between the physical centers of any two adjacent pixels, the other is called the spacing as the pixel's luminous diameter φ; the smaller the spacing, the better the picture of the display when viewed at close range; the more the spacing When the time is large, the optimal observation distance is increased, and the luminous intensity of the LED needs to be appropriately increased.
When the color of the light emitted by the light source is the same as the color of the black body at a certain temperature, the temperature of the black body is called the color temperature of the light source, and the unit is Kelvin [K]. Atmosphere effect of color temperature and light color >5000K Cool (white with blue) Cold atmosphere 3300-5000K Middle (white) refreshing atmosphere <3300K warm (with red white) stable atmosphere
Virtual pixel technology (also known as LED multiplexing technology or pixel decomposition technology)
Split a pixel into several independent LED units. Each LED unit reproduces corresponding primary color information of a plurality of adjacent pixels in a time division multiplexed manner. Taking a four-pixel dynamic pixel of 2R+1G+1B in the usual form as an example, one pixel is split into four independent LED units. Each LED unit reproduces corresponding primary color information of four adjacent pixels in a time division multiplexed manner. Generally, the LEDs are uniformly distributed with each other at equal intervals. Advantages (four-pixel dynamic pixel technology as an example) Virtual pixels (physically non-existent, but actually achievable pixels) density is increased by a factor of four; effective visual pixel density can be increased by a factor of four. Insufficient in this technique, since the spacing of the LEDs is evenly distributed, the spacing between the LEDs constituting each pixel exhibits a maximum discrete state. Compared with the centralized distribution of LEDs, the color mixing performance of pixels is slightly worse; in the case of the same physical brightness, the visual brightness of the display is weak. Since each LED is time-division multiplexed, the information of adjacent four pixels is cyclically scanned, so that unclear characters may appear when displaying a single stroke of text. The virtual pixel technology is suitable for viewing distances greater than 2048 times the physical pixel pitch P of the display.
In the display system, when the displayed information scrolls at a certain speed in a certain direction, the feature of the human eye persistence is utilized; a series of moving, physically non-existent between the adjacent two pixels is generated. Virtual pixels to increase the resolution of the display. Generally used in the display of text bars.
Nonlinear gray scale correction technique
When the gray level is raised to a higher level, the human eye is extremely sensitive to the low brightness difference, and the high brightness level difference cannot be clearly distinguished, resulting in the actual resolution of the human eye to the brightness and the linear gray level of the measuring instrument. Difference, this requires non-linear visual correction of the LED lighting device, compressing the bottom brightness level difference, and expanding the high brightness level difference, so that the actual display gray level difference conforms to the physiological vision of the human eye. This method will increase the difficulty of computing and system complexity, and is an advanced video processing technology.
Constant current drive technology
When the LED is driven by constant voltage, due to the nonlinear characteristics of the PN section of the LED, the current passing through it is extremely sensitive to the applied voltage. At the same time, the specific parameters of each LED are different due to process factors, and there are various points when the display is working. The difference in temperature will cause the intensity of each LED to be different, affecting the uniformity of the display, and even cause some LEDs to work in an abnormal working range and cause premature aging and damage. When the constant current driving technology is adopted, as long as the constant current is determined within the rated working range of the LED (its I/V characteristics are close to a straight line), the luminous intensity of the LED can be substantially prevented from being affected by the operating voltage, operating temperature and self-parameters. The effect of ensuring the brightness and chromaticity of the LED display. High-end LED full-color displays should use constant current drive technology.
Adaptive brightness adjustment
When the display works in different working environments (such as 昼, night, morning, evening, yin, rain, sunlight, etc.), the LED display will automatically adjust the luminous intensity of the display according to the ambient light intensity to obtain the best brightness. With contrast to meet people's visual effects.
Abbreviation for English red, green, blue (ie red, green, blue).