7 reasons why LED displays require point-by-point correction:
What is the point-by-point correction?
Point-by-point correction is a technology used to improve LED displays’ brightness uniformity and color fidelity. This is achieved by collecting data on the brightness (and chromaticity) of each pixel (or primary color sub-pixel area) on the display and assigning a correction coefficient for each primary color sub-pixel or correction coefficient matrix for each pixel. This information is then fed back to the display screen’s control system, which applies the correction coefficient to achieve a differential drive for each pixel (or primary color sub-pixel). This process results in a pure and delicate picture with truly restored colors.
The basic principle of point-by-point correction.
The point-by-point calibration requires the cooperation of two systems: the “control system” and the “point-by-point calibration system”. The point-by-point calibration system generates correction coefficients, while the control system applies those coefficients. Both systems are indispensable to the calibration process.
To perform point-by-point calibration, the calibration system uses a professional camera to image the LED display, obtain the brightness and color of each LED light, generate a set of unique calibration coefficients for each pixel, and then send the calibration coefficients to the control system for storage and curing. During operation, the control system multiplies the image content of each pixel by the correction coefficient at a high speed, completing point-by-point correction. Point-by-point correction can be divided into point-by-point brightness correction and point-by-point chroma correction.
Point-by-point brightness correction is normally used to correct LED luminous intensity. LED displays consist of pixel arrays, with each pixel comprising red, green, and blue LEDs. The brightness of the LEDs is controlled by the pulse width of the control system, and the combination of different brightnesses of red, green, and blue LEDs forms various brightness and colors as required. To achieve better brightness uniformity, for LED lights whose brightness is higher than the target value, the brightness can be reduced by properly compressing the control pulse width to reach the target value.
Point-by-point chromaticity correction is based on the basic principle of chromaticity compensation. The primary color is compensated by the other two primary colors, and color adjustment is realized through color mixing. For example, if the red light of a certain pixel is too red (i.e., the wavelength is too long), the red light of the pixel can be dimmed while slightly lighting up the green and blue lights that should not be on. The number of green and blue bands is determined through image acquisition, recognition, processing, and calculation. In this way, after color mixing, the human eye perceives the red light as less red.
Point-by-point chroma correction has several advantages over point-by-point brightness correction, which are as follows:
- Point-by-point chromaticity correction can achieve higher brightness and chromaticity uniformity (point-by-point luminance correction cannot solve the problem of chromaticity non-uniformity);
- In point-by-point chromaticity correction, smaller brightness sacrifice can be obtained. For a general display screen, point-by-point brightness correction must sacrifice about 15% to 20% of brightness, while point-by-point chromaticity correction only needs to sacrifice 5% to 8% of brightness. This advantage is particularly critical for those displays that have been used for many years and whose brightness has been attenuated to a relatively low level. Why does the LED display need point-by-point correction? Ideally, if the brightness and chromaticity of the light emitted by each pixel on any angle on the display are always consistent, then the display can be considered uniform. In the actual production process, the uniformity of the display screen will be affected by many factors, so the uniformity of the uncorrected LED display screen cannot reach the ideal level, and there is a significant gap in image quality compared with other flat panel display technologies. Many factors lead to poor uniformity of LED displays, such as:
Discrete inherent defects of the LED itself:
(1) The LEDs provided by lamp manufacturers have differences in brightness and chromaticity; because even among LED lamps of the same batch, there will still be about 10% brightness deviation, coupled with differences in driver ICs, So the display is inherently flowery.
(2) There are differences in the spatial distribution characteristics of LED luminous intensity, which will lead to inconsistencies in the attenuation value of the luminous intensity of different LEDs at any viewing angle relative to the optical axis direction.
Numerous other factors are introduced in the display manufacturing process:
(1) the Flatness of module assembly;
(2) The flatness of cabinet assembly; since the current sheet metal processing accuracy can no longer meet the assembly accuracy requirements of small-pitch LED products, there will inevitably be bright and dark lines between the cabinet and the module.
(3) The flatness of the mask and the discreteness of the ink color;
(4) Inhomogeneity of heat distribution inside the module;
(5) When the LED is welded, the axis direction deviates from the normal direction of the display screen, especially the control of the optical axis direction of the in-line LED There are bright and dark blocks on the display screen due to crooked lights or mixed use of two batches.
Therefore, all LED screens have the problem of unsatisfactory uniformity to a certain extent when they leave the factory. Why does the LED display need point-by-point correction? When the LED display screen runs for a certain period, all LED light-emitting tubes will show brightness attenuation, and the attenuation curves of the three primary color tubes are not consistent. Therefore, their brightness will also decrease compared with the brightness before leaving the factory. However, the difference in the photoelectric characteristics of each LED light-emitting tube causes relative errors in the reduction level of their brightness. Therefore, when the display screen is used for some time, the light-emitting tube will show different degrees of brightness attenuation, making the display uneven between pixels, and then compared with the screen at the beginning of the factory, the entire image will show a granular display state, uniformity It will further deteriorate, and it is manifested by a large number of pockmarks, bright and dark spots, and even mosaics on the display screen, commonly known as the “flower screen” phenomenon. In theory, due to the attenuation of LED light-emitting tubes and changes in other factors such as ambient temperature, it is almost inevitable that the performance of a very good display screen at the factory will deteriorate. It is impossible in engineering to disassemble the installed LED display and transport it back to the factory for calibration. Given these characteristics, display manufacturers must perform on-site calibration to ensure that the display function at the factory is maintained throughout the entire life cycle of the display. There are some problems caused by the use of the day after tomorrow, and when the LED display is used for some time, due to the difference in the light decay of individual LEDs and other external factors,
Currently, display screens are a multi-million dollar investment primarily used for commercial advertisements and performances that require high-quality images. The theoretical lifespan of an LED display is 100,000 hours, but in reality, after running for about 5,000 to 10,000 hours, display uniformity deteriorates, causing the commercial value to decrease. After 15,000 to 20,000 hours, the commercial value is almost completely lost, resulting in a great waste of social resources.
For rental users, the requirement for mixed-use of multiple batches of cabinets and cabinets with different service periods occurs from time to time, requiring random splicing. Both of these requirements need the LED display to be corrected on the production line.
Whether before leaving the factory or after a period of use, point-by-point correction technology allows users to vastly improve display uniformity, significantly improving image quality in a very short time and at a low cost. Applied before leaving the factory, point-by-point calibration is a means of quality improvement, enhancing competitiveness, and expanding profit margins. After a period of use, point-by-point calibration can prolong the “eye-pleasing life” of the LED display, creating more business value for users and reducing the waste of resources.
There are two common misunderstandings about point-by-point correction: Misunderstanding: Only old screens need correction, and new screens do not. Solution: Due to the inherent ugliness of the display screen caused by differences between the LED and driver IC, calibration is necessary at the factory, especially for small pitch displays, where the current is lower than 5mA, resulting in greater differences.
Myth: Calibration damages the display and cannot restore the original brightness. Correction: To achieve good uniformity, the brightness of overly bright lamps needs to be “pressed down”. For new screens, it only needs to be pressed by 5% to 10%. Calibration only “compresses” the brightness of the screen and does not “sacrifice” intelligence. For example, if the brightness of a screen is 5000cd and calibrated by 10%, the brightness after calibration will be 4500cd. If calibration is canceled, the brightness will immediately return to 5000cd. Therefore, for this screen, whether calibrated once, twice, or 100 times, the brightness will always be 4500cd after calibration.
