Making LED video screens look their best WHITE PAPER

Transcription

1 Introduction to LED Screen Correction services based on Radiant Imaging s industry leading PM- LEDC application system Making LED video screens look their best WHITE PAPER NE Alder Crest Drive, Suite 100 Redmond, WA USA (T) (F) ,

2 [MAKING LED VIDEO SCREENS LOOK THEIR BEST] 2 Why LED Screen Correction? When LED video screens are not properly adjusted for luminance (brightness) and color uniformity, every imperfection is visible. Modules do not match, pixel-to-pixel variation results in uneven images, and colors are wrong shifted away from true white and often oversaturated. These problems can occur anytime during the life of the screen, either because of manufacturing variations, or aging of the screen LEDs, or module replacements. It is often felt that as these LED video screen performance issues must be accepted as a natural limitation of LED technology. But thanks to advances in LED screen correction technology, this is no longer true! By properly implementing pixel level correction, almost any LED video screen can achieve: uniform pixel and module luminance, uniform pixel and module color, seamless boundaries between modules, an accurate color gamut. As a result, the screen image is optimized. To understand how this can be accomplished, it is first necessary to understand the causes of LED screen brightness and color non-uniformities. Based on this, we will describe a proven process for LED screen correction, along with the results that can be expected. Field-proven methods for optimizing the visual appearance of LED video screens are available today, and can be applied to almost any LED screen

3 [MAKING LED VIDEO SCREENS LOOK THEIR BEST] 3 Understanding Non-Uniformity in LED Screens LED video screens are an amazing display technology. They are bright enough to see in broad daylight, withstand severe weather conditions, last almost forever, and consume far less power than traditional display technologies. However, variations in individual LED performance and in the design of LED video screens too frequently results in pixel-to-pixel or module-to-module luminance and color non-uniformity that is clearly perceivable by viewers and that is seen as a quality failure. Understanding the causes of luminance and color nonuniformity in LED screens is important to understanding what can be done to fix these problems. Variations in individual LED performance The root cause of luminance and color uniformity problems in LED screens is the brightness and color variations of the LEDs themselves. Modern manufacturing processes for LEDs produce LEDs that can vary greatly in both brightness and color. For example, when applying 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 nanometers. These differences are easily noticeable to the human eye 1. LED screens visual appearance problems Patchwork mismatches between modules Dirty window effect because of pixel variations Poor white balance Over saturated colors 1 LEDs that vary by this much should not be used in the same video screen. While LED screen correction methods will compensate for variations in LED brightness and color, the quality of the final result will be determined by the having a reasonable distribution of LED properties across the LED screen.

4 [MAKING LED VIDEO SCREENS LOOK THEIR BEST] 4 Performance changes due to LED aging Another cause of non-uniformity in LED screens is an aging effect: LEDs get progressively dimmer as they are used. Blue LEDs dim the fastest over time and the red LEDs dim the slowest. This divergence in performance degradation results in color migration over time for individual LED screen pixels. Therefore, even if an LED screen is perfectly uniform when it leaves the factory, after about one year of usage the LED screen will be perceivably less uniform. Environmental effect on LED performance Furthermore, LED luminance and color are known vary with environmental conditions. LEDs that are matched or corrected on one day may not match those matched or corrected in exactly the same manner on another day. Generally this is not an issue for screens that are assembled from modules corrected or matched at the same time. This becomes a problem, however, when modules need to be replaced in a screen. Either LED aging or environmental variations during manufacturing will lead to significant and visible lack of uniformity if a new module is inserted into a screen without performing a calibration. This is true of both modules that are corrected to match a prescribed color gamut, and modules that are assembled from tightly binned LEDs. Therefore LED screen correction is required to achieve brightness and color uniformity both when the screen is initially built and as a regular maintenance activity. LED Screen Correction Concepts The brightness and color correction process as applied to a single LED screen pixel takes advantage of the fact that the LED pixel is comprised of different, individually controllable color channels. By controlling the proportions with which these colors are mixed, the apparent color and brightness of the LED pixel can be carefully regulated to meet performance targets. Step 1: Establish Design Objectives, for target luminance, color coordinates, and color temperature. These targets should be based on the performance that the LEDs can support.. Step 2: Measure the actual LED performance. Generally an LED screen pixel consists of 3 LEDs: one

5 [MAKING LED VIDEO SCREENS LOOK THEIR BEST] 5 each of Red, Green, and Blue either individually or as separate LED die in the same LED. Other color combinations also exist and can be corrected as well. By measuring the individual color channels the baseline performance of the LEDs is established. Step 3: Determine Errors Due LED Variation. Due to LED performance variation the achieved luminance and color is very likely to be noticeably different than the target values. The error values are calculated with reference to the target luminance and color coordinates. Step 4: Determine Pixel-specific Correction Coefficients. Correction coefficients are computed for each LED screen pixel in such a way as to correct the luminance and the color coordinates. The key is that color and luminance are corrected by using all three (or more) colors in appropriate mixtures. Because of the relative distance of the human observer from the screen, they will perceive the mixed color of the LED pixel rather than the individual LED color channels. Luminance is additive so pixel brightness is the sum of the individual color channel brightness and the final color is determined by how the individual color channels are mixed. The computed correction coefficients are then specific to each individual LED screen pixel. Step 5: Apply the Correction Coefficients. Once computed for all of the pixels in the LED screen, the correction coefficients are then activated in the video controller and applied to the incoming video stream. Additional Considerations The discussion to this point addresses the basic issues in LED module and screen correction. For simplicity in explanation, several significant factors were mentioned only briefly, but they are worth considering more deeply. White Balance: By displaying a White screen (with Red, Green, and Blue full on), capturing a color image, and determining the luminance and chromaticity of each pixel, the Radiant Imaging VisionCAL System can further refine the correction coefficients so that a pure white display exhibits a very high degree of uniformity and achieves the target color temperature.

6 [MAKING LED VIDEO SCREENS LOOK THEIR BEST] 6 Aging effects: The luminance of individual LEDs degrades over time and different colors - red, green and blue of LEDs degrade at different rates. This means that screens will noticeably drift out of calibration after as little as 6 months of operation. The effect will be an overall decrease in brightness and also a drift and randomization in the color of each LED screen pixel. Usually in this case the correction coefficients will need to be updated to return the LED screen to optimal performance. This correction will need to be performed on-site at the screen location. LED Screen Correction Systems Based on over 5 years of experience correcting tens of thousands of LED screens and modules, Radiant Imaging has developed an LED video screen correction method that is fast, accurate, and cost-effective and which results in outstanding appearance. Radiant Imaging s LED video screen correction process measures the luminance and chromaticity of each LED in a target module or screen using a Radiant Imaging ProMetric Series imaging colorimeter for precision and speed. This pixel-level data is used to calculate correction factors for each pixel and module that will allow them to achieve user defined targets for color and brightness uniformity. The calculations take into account screen geometries and human visual perception. These correction factors are uploaded to the screen video controller and applied to the incoming video signal, modifying the video in real time to adapt to the exact performance of that LED video screen! The LED video screen correction process is fast and simple, whether for a full screen or just a panel, because the correction software manages the entire process from measurement to uploading the correction coefficients to verifying performance. The Radiant Imaging correction system, VisionCAL, has three components: An imaging colorimeter to collect detailed, pixel-

7 [MAKING LED VIDEO SCREENS LOOK THEIR BEST] 7 level, data on screen appearance for various test patterns. Application specific software to controls the measurement process, performs automated analysis of screen performance and calculates the optimal correction coefficients. A control interface to the module or screen controller provides automated control of the module or screen during testing and automates correction coefficient transfer to the controller to complete the correction process. VisionCAL can be used to calibrate any size LED screen ranging from a small module to the biggest stadium screen. It provides precision correction with module-to-module luminance corrected to +/- 3% and module-to-module color corrected to +/-.003 delta u v. Radiant Imaging offers both the VisionCAL system and VisionCAL-based VisionTUNE screen correction services to screen manufacturers, installers, and owners. VisionTUNE Screen Correction Services When Radiant Imaging engineers perform a VisionTUNE LED screen correction we follow a well-defined correction process. We baseline the performance of the system with all corrections and adjustments turned off just determining the raw performance capability of the screen; then we establish a target color gamut and luminosity this can be defined by the screen owner or user 2, or optimized based on the measured data; generate the correction coefficients appropriate for the target color gamut; and then upload the correction coefficients to the panel/screen video controller. After the correction coefficients are implemented, the brightness and color uniformity of the LED screen or module is verified. In some cases LED non-linearity may require a second iteration for the best result, or a second module-level correction may be run to make module boundaries as seamless as possible. 2 It is possible that arbitrarily defined or even optimum luminance and color targets may not be achievable by all of the pixels in the LED screen. Based on the baseline measurements Radiant Imaging engineers will be able to determine how well they can be achieved.

8 [MAKING LED VIDEO SCREENS LOOK THEIR BEST] 8 LED screen correction improves: Luminance uniformity Color uniformity White balance Color gamut accuracy When performing an LED screen correction job, the Radiant Imaging engineer will arrive with all of the necessary measurement systems, including the imaging colorimeter, all necessary calibration equipment, and a control PC operating the VisionCAL software. REQUIRED ON-SITE PREPARATIONS: An area from which to perform the measurements, preferably directly in front of the LED screen, with an unobstructed view. This area should be about 10 meters away for every meter that the screen is high. (Some variability in this is permitted and can be compensated for.) Access to power at the location where the measurement is set up for the imaging colorimeter, the control PC, and any ancillary equipment. Standard power is sufficient. Access to the LED screen controller sufficient to display the required test patterns usually full Red, full Green, full Blue and full White screens. Also the ability to upload correction coefficients to the LED screen controller or other hardware, and to enable or disable them. The screen installer is usually able to provide this access. Sufficient control over the measurement environment to be able to turn off or block, to the extent possible, all ambient and other lighting during the measurement process. Once the screen correction process begins, the following sequence of events will take place. Auto-registration of the physical location of each LED in the panel: Upon initialization of the LED screen correction, a registration image of the LED module or screen is captured and then searched to locate a predetermined key LED in each LED screen pixel. The VisionCAL software uses both the registration image and the known geometry of the module to establish a grid across the screen with the location of each LED device identified. This registration process is able to locate each LED even if the module or panel has been rotated or poorly aligned. Sequential display of red, green and blue LED screen patterns and capture of a color image of each: Using these images and the previously recorded locations of each LED, the software determines each LED s luminance and chromaticity (Cx and Cy) and records this information in the measurement database. Computation of the correction coefficients for each pixel or module based on the target values set by the user: The correction coefficients are then uploaded to the

9 [MAKING LED VIDEO SCREENS LOOK THEIR BEST] 9 DUT. At this point the panel has been corrected and the process can stop. However, there are several optional steps that the user may choose to implement. Optional white balance correction: Further refinement of the correction coefficients results in improved uniformity for an all white display. Large areas of white are common in many images and displaying this areas uniformly white presents the greatest challenge for an LED screen. When displaying a pure white screen, a lack of uniformity is easily apparent for uncorrected LED screens. Another correction sequence can improve this. Optional module level correction: This step is similar to the previous steps except that the images integrate the luminance and color data of an entire module and adjust the correction coefficients of each pixel by the same value. This step can be applied to module arrays or an entire screen. It is done to remove or mitigate module boundary artifacts. Optionally capture images of the corrected LED screen or module. This data is used to determine if the LED screen performance meets any predefined pass/fail criteria. This data should also be recorded in a results database referenced by the module serial number and retained for future analysis or reference.

10 [MAKING LED VIDEO SCREENS LOOK THEIR BEST] 1 0 Radiant Experience and FAQs Radiant Imaging VisionTUNE Services have been applied to LED screens indoors and outdoors, in stadiums, arenas, public areas, along highways, and more. The VisionCAL System has been used to correct literally thousands of LED video screens around the world. Frequently Asked Question 1: Why is an imaging colorimeter required to do the screen correction? Could a high-end DSLR camera be used instead? The answer is No, because to accurately measure the luminance and color of the individual LEDs requires a calibrated measurement system. Also, to avoid losing data a full-frame CCD with individual measurements of the RGB color channels (via a color filter wheel) is needed. Finally, the imaging colorimeter also provides a high dynamic range measurement that enables extremely fine variations in luminance to be detected. Frequently Asked Question 2: Why is a special program required to do the measurements? Can the correction factors be calculated more simply from the individual LED measurements? Because of the complexities of human visual perception, the calculation of the correction factors involves a lot more than a simple mathematical computation. The human eye is more sensitive to variations in blue than in red, it is more sensitive to discontinuities in brightness and color that occur along a straight line (at the boundary between modules) rather than randomly, and is sensitive to effects of reflections at the edge of the LED screen. All of these factors, and more, related to human visual perception and LED screen physical design are integrated into VisionCAL calculations and have been demonstrated to yield a higher quality visual result. Frequently Asked Question 3: What if my screen does not support correction coefficients? Some LED screens both new and already installed do not support integrated correction coefficients. For such cases Radiant Imaging offers the VisionLINK correction processor. VisionLINK applies VisionCAL generated correction coefficients in real-time to the video feed to the LED screen.

11 [MAKING LED VIDEO SCREENS LOOK THEIR BEST] 1 1 Frequently Asked Question 4: How do I maintain the appearance of my LED video screen as it ages? Radiant Imaging recommends using VisionTUNE on a regular basis over the life of the screen to maintain pixel level uniformity. For LED video screens that are in constant use, this may be on an annual basis. For any LED screen, this should be done when the screen appearance has noticeably degraded. When just replacing modules with spares in an ordinary maintenance activity, a simpler method is available using Radiant Imaging s VisionMATCH module matcher. Contact Information For more information on VisionTUNE, VisionCAL, VisionLINK or VisionMATCH please contact: vision@radiantimaging.com NE Alder Crest Dr., Suite 100 Redmond, WA USA (T) (F)