Color and Luminance Uniformity Correction for LED Video Screens

LED video screens are amazing. They are bright enough to see in broad daylight; they can stand up to severe weather conditions, they last almost forever, and they consume far less power than CRT technology. If only they weren’t so blotchy! This article explains the causes of luminance and color non-uniformity in LED screens, and what screen manufacturers can and are doing to fix these problems.

Have you ever seen a LED screen that looks like grandma’s patch-work quilt, with each module displaying a different brightness and color? Or, have you ever seen a screen with pixel-to-pixel variances so high that it appeared like you were looking through a dirty window? The cause of these problems is luminance and color non-uniformity

Uniformity Problems in LED Screens
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.

What’s a Screen Manufacturer to Do?
There are several methods screen manufacturers can use to handle non-uniformity issues. The first method is to ignore the issue altogether. The second method is to buy the LEDs from manufacturers in highly binned lots. Thirdly, they can adjust the current to try to achieve luminance uniformity and finally, they can use PWM correction coefficients to achieve a high degree of both luminance and color uniformity. Let’s explore each of these methods.

Do Nothing
This may seem like an apathetic approach to the problem, but it is exactly the method that many screen manufacturers take when addressing this issue. Instead of improving uniformity they focus on brightness and price hoping that the uniformity issues will never come up. Manufacturers with this approach will rarely show you their screen with a flat color. Instead, they would rather show fast moving video clips that hide the problems.

Binning
In response to the fact that LED manufacturers cannot control the manufacturing process to produce more uniform LEDs they can resort to the process of binning, or sorting the LEDs into bins according to luminance and color. Binning is a time consuming and costly process and the tighter the bins the more expensive it is to do the binning. Industry standards for bin sizes as published by the LED manufacturers on their web sites are set at 25 to 40% difference in luminance and 5nm (wavelength) difference in color.

When using binned LEDs as the only means of uniformity correction it is important to know the sensitivity level of the human eye to color and luminance differences. The human visual system is much more adept to distinguish edges than it is to distinguishing slowly changing light conditions. For this reason, the module-to-module differences are much easier to distinguish than pixel-to-pixel differences due to the fact that there is a hard edge between two modules. The human eye can distinguish a module-to-module difference of 3% in luminance and a 1-2 nm difference in color. Pixel-to-pixel thresholds are more on the order of 5% in luminance and 3-4 nm in color.

Current Adjustment
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 module by adjusting a variable resistor (three actually: one each for red, green, and blue) on the power supply or on the module itself. 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. Also, this method can be used only for module-to-module level differences and not for pixel-to-pixel level differences due to the very large number of pixels in a screen.

Pulse Width Modulation (PWM) Correction
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.

For example, study Figure 3 below. Notice that the amplitude of the pulse (current applied) is always either full on or full off. The heights of the pulses are always either off or maximum current. The brightness of the display is determined solely by the temporal duration of the pulses. A bright display (90% brightness) will have pulses that are 9 times longer than a dim display (10% brightness). The brightness of the display is controlled only by how long the display is turned on, not the current that runs through the LEDs.

PWM Uniformity Correction
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. (See Figure 4 below.)

The PWM correction process works like this. Suppose you design a video screen where the green color should display at 1000 nits and with a color of Cx= 0.32, Cy = 0.64 when the green video signal is 50%. To do this you would select LEDs that have a maximum output of 2000 nits and a wavelength of about 556 nm. However, due to LED-to-LED variances, when you apply the 50% green video signal, the actual luminance and color is most likely to be something different than the target values as shown in the figure below. The error values are calculated with reference to the target values of luminance = 1000 nits and Cx=0.32, Cy = 0.64.

Now, to fix this problem using PWM correction coefficients, it is necessary to measure the LED and then compute a coefficient matrix that will apply the proper adjustments. In the example portrayed in the figure below, the green signal will be reduced to 92.3% and a little red and blue will be added. The result is that both the luminance and the color will be very close to the target values when a 50% green video signal is applied. The correction coefficients shown (0.13, .923, .027) are unique for this particular pixel. In order to correct green for each pixel in the screen, each pixel needs a set of green correction coefficients.

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. The final process looks like the Figure 5 below (note that the video out value for green is a bit higher than the previous example, this is because the green correction values for the red and blue color have been added). Each pixel has a unique set of 9 correction factors. The video signal (50% gray in the example shown in Figure 5) is multiplied by the 3x3 correction matrix to determine the pulse widths of each color channel for the pixel. As the video progresses, the video signal will change, but the coefficients will not, they remain the same for each pixel regardless of the video signal.

In order to implement PWM uniformity correction, the screen manufacturer must be able to do the following.

1. Measure the luminance and color of each pixel displayed with fully saturated red, green, and blue.
2. Generate a unique set of 3x3 correction coefficients (or 3x1 coefficients if only luminance is being corrected).
3. Store the coefficients in the video controller.
4. Use FPGA electronics to multiply the video signal by the PWM correction coefficients when video is being displayed on the screen.

This is a lot of work, but fortunately for the screen manufacturers, turn-key solutions have been developed to automatically perform all the steps above. In order for this process to work, the screen manufacturer needs to modify the electronics of the video controller in order to store the PWM correction coefficients and to use them to modify the video signal. Screen manufacturers can make these electronic modifications in house, or they turn to third-party companies to provide the solution for them.

How Good is PWM Uniformity Correction?
PWM uniformity correction is the undisputed best method for fixing uniformity problems in LED screens. When a screen has been calibrated using PWM correction methods, it is not possible to see the module and panel boundaries and the image is crisp and clear. When performed correctly, the modules vary in luminance by less than 3% and in chromaticity by less than 0.003. This type of quality is required to produce screens for the top venues such as world class sports stadiums, which demand very high uniformity performance. Additionally, the PWM method is the only one able to make adjustments for each pixel in a large screen, or to perform pixel-based calibration tune-ups in the field after the LEDs have dimmed over time.

State of the Industry
PWM uniformity correction first became available several years ago. Older screens may not have the necessary electronics to store and use the PWM coefficients. Now many screens sold to first-tier venues (such as world class sports stadiums, or screen spectaculars) use PWM for uniformity correction. Also, screens sold as advertisement signs posted along highways should always use PWM uniformity correction since they must display static messages (video messages are not allowed in most places for highway signs). Since static displays are most likely to show screen non-uniformities, uniformity correction is essential for highway signs.

Questions to Ask Your LED Screen Vendor
The following list of questions can be used to determine the type and level of uniformity correction provided by your screen vendor.

Can I see the screen in Flat White with no video?
The best way to look for uniformity problems is to view the screen with nothing moving. Flat white is the best since it is a combination of all colors. If you have time, ask to see the screen in the component colors also. Look for variance in luminance and colors between the modules. Look for bright or dark lines between the modules and panels. Look for pixel-to-pixel variation. Finally, use a spectraradiometer to measure the color and brightness of the screen.

What method do you use to improve luminance and color uniformity
The most important thing is to find out is what your screen manufacturer does to fix the inherent non-uniformity issues found in all LEDs. The method that yields the best results is the PWM correction method, and it often produces a screen that is cheaper than extreme binning methods, because the screen manufacturer does not need to purchase such tight binning lots from the LED manufacturers in order to achieve uniformity. Also, it enables the user to recalibrate the screen as the LEDs become dimmer over the course of several years.

What binning levels do you Use?
If a screen manufacturer uses LED binning as their only source of uniformity control it is important to understand what binning levels they use. Ask them the binning levels for luminance and color. Remember that the human eye can begin to distinguish 3% luminance difference and a 2nm wavelength color difference if the difference is between two modules with a hard edge. When comparing pixel to pixel differences the human eye is less sensitive but can distinguish a 5% luminance and 3nm color difference between nei***oring pixels.

Are correction coefficients used for each pixel or only for each module?
For best results, correction coefficients should be provided for each pixel. Correction coefficients for the module can reduce or eliminate the patchwork quilt effect that shows modules with different brightness and color, but it cannot completely remove the boundary differences between different modules and will do nothing to eliminate the dirty window effect caused by pixel-to-pixel uniformity differences

How many correction coefficients are used for each pixel?
It is possible to produce correction only for luminance and white point. A screen of this type will use 3x1 correction coefficients (3 coefficients for each pixel). A full color corrected screen will use 3x3 coefficients (9 coefficients for each pixel).

Are the PWM coefficients stored in the modules in addition to storing them in the video controller?
The coefficients need to be stored in the video controller in order to perform the video signal processing; however, it is also very convenient to store them in a type of flash memory on the module. This not only provides a backup for the coefficients, but it also enables the module to automatically update the coefficient database in the video controller when it is moved or replaced.

Is it possible to perform PWM uniformity correction on-site?
On-site correction becomes necessary when the video screen has aged, or when a catastrophic failure causes loss of coefficient data. If the only correction remedy is to dismantle the screen and send it back to the manufacture for recalibration, this will be a very expensive proposition both in screen down time and shipping charges. On-site calibration is the best answer for this problem. An on-site crew should be able to come to your venue and provide the necessary calibration without any dismantling. It is wise to ask whether or not the screen manufacturer can correct on-site upfront to avoid finding out too late you will need to tear down the screen and send it back to the factory if a uniformity problem arises.