LED Display Lighting

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 neighboring 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 3×1 correction coefficients (3 coefficients for each pixel). A full color corrected screen will use 3×3 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.