what’s the main cause of uniformity problems in LED displays
Saturday, August 23rd, 2008LED fullcolor display are amazing with it’s bright enough to see in broad daylight;stand up to severe weather conditions and pwoer less. But as two sides of a coin, LED have her biggest disadvantage—-Uniformity of Luminance and Color.
The root cause of luminance and color uniformity problems in LED screens is the LEDs themselves. Modern manufacturing processes for LEDs produce LEDs that vary greatly in both brightness and color. For example, when you apply the same electrical current to two green LEDs produced as part of the same batch, the brightness may vary by as much as 50% and the wavelength may vary by as much as 15-20 nanometers. These differences are very noticeable and LEDs that vary by this much should not be used in the same video screen.
By contrast, televisions, which rely on phosphors to produce brightness and color, do not have this problem. The phosphors in each pixel produce nearly the same brightness and color when hit with the same amount of energy from the cathode ray gun. Since the pixel response in CRT devices is very repeatable, the uniformity of CRTs is very high. (See the first line of pixels in Figure 1 below.)
LED screens, however, have two problems as mentioned above. First, the brightness of each LED varies widely even though they are driven by the same voltage and current. (See the second line of pixels in Figure 1, where the brightness of each LED is represented by different size dots.) Second, the colors of the LEDs are also quite variable. (See line 3.) When you add these two problems together (See line 4), you can see why achieving uniformity in a LED screen is so difficult. Imagine viewing a TV where the phosphor dots are different sizes and different colors. It would not be a pretty sight, just like some LED screens you may have seen.
Another cause of non-uniformity in LED screens is that the LEDs get slightly dimmer as they are used (see Figure 2). 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.
