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What is pixel by pixel correction technology?

Date:2016-07-22 

Luminance and color uniformity are most important characters for led display. The root cause of luminance and color uniformity problems in LED screens is the LEDs themselves. First, the brightness of each LED varies widely even though they are driven by the same voltage and current. Second, the colors of the LEDs are also quite variable. Third, the LEDs get slightly dimmer as they are used. The blue LEDs dim the most and the red LEDs dim the least, but the biggest problem is that individual LEDs dim differently over time. So, even if an LED screen were perfectly uniform when it left the factory, it would loses its uniformity as the LEDs dim, and after about 2 years of usage it would begin to look quite non-uniform. When you add these three problems together you can see why achieving uniformity in a LED screen is so difficult.



What’s an LED display Manufacturer to do to keep the uniformity? There are several methods can use to handle non-uniformity issues. The first method is to buy the LEDs from manufacturers in highly binned lots. Secondly, they can adjust the current to try to achieve luminance uniformity and thirdly, they can use PWM (Pulse Width Modulation) correction coefficients to achieve a high degree of both luminance and color uniformity. The second and third methods are what we called PIXEL BY PIXEL CORRECTION technology.



The brightness of LEDs is determined by the amount of DC current that flows through the P/N junction. More current produces a brighter LED. Unfortunately, however, adjusting the current will also change the color of the LEDs. LED manufacturers can adjust the current applied to each pixel by adjusting a variable resistor for each leds on the module. This method can be used to make sure that the modules all have the same brightness, but it cannot be used to adjust the color differences. What’s more, if two modules were the same color before adjusting the current, they would not be the same color after the adjustment. This method is taking too much time by taking record to the brightness of each leds and adjusting the current individually.



Pulse Width Modulation (PWM) is a widely used technique to control the brightness of LEDs. It can be used to process the video signal and also to perform uniformity correction. PWM is used instead of varying the current because changing the current of the LEDs would also change the colors as explained above. PWM works by flashing the LEDs either full on or full off at a very high rate. The flashes are so fast that the human eye cannot notice them and the duration of the individual flashes (pulse width) determines the perceived brightness. PWM uniformity correction works by modifying the pulse widths to compensate for LEDs that are naturally brighter or dimmer or display a different color. In a non-corrected system, the video signal is turned into pulse widths by the video controller and then sent to the LED drivers to flash the LEDs. In a corrected system, the pulse widths are multiplied by correction coefficients before being sent to the LED drivers. The actual process is a bit more complicated. Each pixel not only needs green correction, but red and blue correction as well. This means that a little bit of green and blue need to be added to red and a little bit of red and green need to be added to blue.



However, pixel by pixel correction has high requirement on the accuracy of the equipment. Too much tolerance will also lead to the non-uniformity of led display.