Explanation of professional terms often encountered in LED displays

LED is the English abbreviation of light emitting diode, Chinese name: light emitting diode. LED light emitting diode is composed of III-IV group compounds in the element spectrum, such as GaAs (gallium arsenide), GaP (gallium phosphide), GaAsP (phosphorus arsenide) Gallium) and other semiconductors, the core of which is a PN junction. Therefore, it has the I-N characteristics of the general P-N junction, that is, forward conduction, reverse cut-off and breakdown characteristics. In addition, under certain conditions, it also has luminescent properties. Under forward voltage, electrons are injected into P region from N region, and holes are injected into N region from P region. Part of the minority carriers (minor carriers) entering the opposing region recombine with the majority carriers (multiple carriers) to emit light. Assuming that the luminescence occurs in the P region, the injected electrons recombine directly with the valence band holes to emit light, or are first captured by the luminescent center and then recombine with the holes to emit light. In addition to this luminescent recombination, some electrons are captured by the non-luminescent center (the center is near the middle of the conduction band and the dielectric band), and then recombine with the holes. The energy released each time is not large and cannot form visible light. The greater the ratio of the luminescent recombination amount to the non-luminescent recombination amount, the higher the luminous efficiency.

 

The advantages and specialties of LED and its application on the display screen

The luminous color and luminous efficiency of LEDs are related to the materials and processes used to make 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 with a certain brightness, and the brightness can be adjusted by voltage (or current), and it is resistant to shock, vibration, long life (100,000 hours), and high luminous efficiency. , So in the large-scale display equipment, there is no other display method that can match the LED display method. Putting red and green LEDs together as a pixel is called a two-color screen color screen; putting red, green, and blue LED tubes together as a pixel is called a three-color screen or a full-color screen. . Usually, for the convenience of engineering installation, multiple image points are made 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 reinforced iron box, the box also accommodates the power supply, control system, cooling system and other devices, and has the functions of waterproof, dustproof, lightning protection, shockproof and so on; An iron box with display modules and systems constitutes the entire led display.

 

Grayscale

Regardless of whether LEDs are used to make a single-color, two-color or three-color screen, to display an image, the luminous brightness of each LED that constitutes a pixel must be adjustable, and the fineness of the adjustment is the gray level of the display screen. The higher the gray level, the more delicate and colorful the displayed image, and the more complex the corresponding display control system. Generally, an image with 256-level grayscale has a very soft color transition, while a color image with 16/32/64-level grayscale has a very clear color transition boundary. Therefore, color LED screens are currently required to be made into 256/16384 grayscales. The color combination and color excess realized by this grayscale level far exceed the human eye’s ability to distinguish colors.

 

Resolution

Refers to the display terminal’s ability to process and display the screen in the horizontal and vertical directions, 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.

 

Optical terminology

A. Luminous flux: The symbol of luminous flux is the amount of light emitted by the φ light source in unit time, the unit is lumens (lumin), and the symbol is lm;

B. Luminous intensity: The symbol of luminous intensity is the currency value of the luminous flux dφ contained in the small solid angle of the light source in a given direction and the solid angle dQ, and the unit is canterla (cd) 1cd=1000mcd.

C. Brightness: luminous, the symbol is the ratio of the luminous intensity of the L light source at a small solid angle in a given direction to the orthographic projection area on a plane perpendicular to the given direction. The unit is canterla per square meter (cd/m2)

D. Light efficiency: unit lumens/watt Lm/w, indicating the ability of electric light sources to convert electrical energy into light, expressed by dividing the emitted luminous flux by the power consumption.

 

Point spacing P

The distance between the physical centers of any two adjacent pixels, another way of calling this distance as the luminous diameter φ of the pixel; the smaller the dot pitch, the better the picture of the display screen when viewed at close range; the more the dot pitch When it is large, the optimal observation distance increases, and the luminous intensity of the LED also needs to be appropriately increased.

 

Color temperature

When the color of the light emitted by the light source is the same as the color of the light radiated by 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]. Ambient effect of color temperature and light color >5000K Cool (bluish white) Cold atmosphere 3300-5000K Intermediate (white) refreshing atmosphere <3300K Warm (reddish 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 the corresponding primary color information of several adjacent pixels in a time-division multiplexing manner. Taking a four-pixel dynamic pixel in the common form of 2R+1G+1B as an example, one pixel is divided into four independent LED units. Each LED unit reproduces the corresponding primary color information of four adjacent pixels in a time-division multiplexing manner. Generally, the LEDs are evenly distributed at equal intervals. Advantages (Taking four-pixel dynamic pixel technology as an example) Virtual pixel (pixels that do not exist physically, but are actually achievable) increase the density to 4 times; the effective visual pixel density can be increased by a maximum of 4 times. Disadvantage This technology adopts the uniform distribution of LEDs with equal spacing, so the spacing between the LEDs constituting each pixel presents a state of maximum dispersion. Compared with the concentrated distribution of LEDs, the color mixing performance of the pixels is slightly worse; the visual brightness of the display screen is weaker under the same physical brightness. Since the time division multiplexing method is adopted for each LED, the information of the adjacent four pixels is cyclically scanned, so the phenomenon of unclear handwriting will appear when displaying the text of a single stroke. The virtual pixel technology is suitable for viewing distances greater than 2048 times the physical pixel pitch P of the display screen.

 

Afterimage technology

In the display system, when the displayed information is scrolled in a certain direction at a certain speed, the feature of human vision persistence is utilized; a series of moving, non-physically existing pixels will be generated between two adjacent pixels. virtual pixels, thereby increasing the resolution of the display. It is generally used in the display of text bar screens.

 

Nonlinear grayscale correction technology

When the gray level is raised to a higher level, the human eye is extremely sensitive to the low brightness range, but cannot clearly distinguish the high brightness level, resulting in a larger difference between the human eye’s actual ability to distinguish brightness and the linear gray level of the measuring instrument. Therefore, it is necessary to perform nonlinear visual correction on the LED light-emitting device, compress the bottom brightness level difference and expand the high brightness level difference, so that the actual displayed gray level difference conforms to the physiological vision of the human eye. This method will increase the computational difficulty 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 junction 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 the various points of the display screen work. The temperature difference will cause the luminous intensity of each LED to be different, which will affect the color uniformity of the display screen, and even cause some LEDs to work in the abnormal working range and cause premature aging and damage. When the constant current drive technology is used, as long as the constant current is determined within the rated working range of the LED (its I/V characteristic is close to a straight line), the luminous intensity of the LED can be basically not affected by the working voltage, working temperature and its own parameters. Influence, so as to ensure the uniformity of LED display brightness and chromaticity. Medium and high-end LED full-color displays should use constant current drive technology.

 

Adaptive brightness adjustment technology

When the display screen works in different working environments (such as day, night, morning, evening, overcast, rain, sunlight, etc.), the led display screen will automatically adjust the luminous intensity of the display screen according to the light intensity of the environment to obtain the best brightness. and contrast to meet people’s visual effects.