Visual display signs have become commonplace in sports stadiums, arenas, and public forums throughout the world. The signs are typically very large, often measuring several hundred feet in size. Because of their immense size, the signs must be assembled and installed on-site using a series of smaller panels, which are themselves further comprised of a series of modules. The modules are internally connected to each other by way of a bus system. A computer or central control unit sends graphic information to the different modules, which then display the graphic information as images and text on the sign.
Each module in turn is made up of hundreds of individual light-emitting elements, or “pixels.” In turn, each pixel is made up of a plurality of light-emitting points, e.g., one red, one green, and one blue. The light-emitting points are termed “subpixels.” During calibration of each module, the color and brightness of each pixel is adjusted so that the pixels can display a particular color. The adjustment to each pixel necessary to create a color is then stored in software or firmware that controls the module.
Although each module is calibrated before leaving the factory, the individual pixels often do not exactly match each other in terms of brightness or color because of manufacturing tolerances. Furthermore, the electronics powering the various modules have tolerances that affect the power and temperature of the subpixels, which in turn affect the color and brightness of the individual pixels. As the sign ages, the light output of each subpixel may degrade. Because the degradation is not uniform for each color of subpixel, or even for each subpixel of the same color, the uniformity and color point of the sign will degrade over time. This can cause color shifts, visible edges around individual screen modules, and pixel-to-pixel non-uniformity.
Accordingly, the assembled visual display sign needs to be recalibrated periodically to maintain the ability to display colors clearly, uniformly, and accurately. However, the immense size of most visual display signs makes recalibration of the sign in a testing center impossible. Likewise, it is not cost-effective or practical to disassemble the sign in the field and bring in the individual modules to a testing center for recalibration.
On-site measurement and calibration provides its own challenges. For example, at a typical American football field the scoreboard may be 200 meters from a suitable measurement location. The requirement to measure subpixels that may only be a few millimeters in size from a distance of 200 meters requires high-powered, specialized optics. Another problem with on-site measurement is the extraction and management of the massive amount of data that must be collected, stored, and used for calculation of new correction factors. A typical display sign will have well over two million subpixels that must each be measured and recorded.