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Large LED Display screen applications in advertising Published:2018-09-03
LEDs and led video displays: An Overview
 
LEDs are plastic capsules containing a specific chemical compound on a microscopic wafer that emits light when subjected to an electrical current. The emission of light depends on electrons flowing between the anode and cathode within an LED chip, and the color of the visual emission depends on the materials utilized.
 
Putting LEDs together forms a pixel. By definition, a pixel is the "luminous dot" present on every full-color LED display. Typically, this luminous dot is formed by the three basic LED colors: red, green, and blue (Fig. 2). When these LEDs are clustered together, the combined light emission appears white to the human eye.
 
Hence, a pixel represents an individual dot in a video display and has its own color and brightness attributes. Combining these pixels yields an LED display panels with a specific resolution (Fig. 3). The resolution is the total number of pixels in a display, usually specified by its horizontal and vertical pixels (e.g., a standard video signal has a native resolution of 640 x 480 pixels). The higher the number of pixels, the greater the ultimate resolution of the display.
 
Fig. 1: Bank of America's LED display in Times Square, New York City.
 
One of the most important characteristics of an LED screen wall is the distance between the centers of the nearest pixels. This is called the pixel pitch and is expressed in millimeters. Currently, the average pixel pitch for outdoor screens is 12–30 mm, while it is 3–12 mm for indoor screens. The pixel pitch is a defining factor of a large LED display because the closer the pixels, the closer the minimum viewing distance can be in order for the image to appear smooth. Accordingly, as the distance between pixels decreases, the cost of the screen per square area increases. Therefore, the pitch determines both the image definition and cost of the screen. A small pixel pitch results in higher resolution and cost; a large pixel pitch results in lower resolution and cost.
 
Clients who purchase LED displays tend to opt for a smaller pixel pitch and, therefore, a higher image quality. However, this must be carefully investigated. The higher the number of pixels on the LED screen (i.e., lower pitch), the higher the cost.1 This cost factor always needs to be balanced with the required display resolution for good perceived image quality. In fact, the human eye is not able to recognize small details from long distances. So, the designer should always question the need for a higher-resolution LED screen if the observers will not be able to appreciate its superior visual performance.
The image on a large outdoor LED display is visible from a long viewing distance. However, the closer the observer gets to the display, the more unpleasant and uncomfortable the viewing experience becomes. To best determine the pixel pitch required for a given LED display, a minimum viewing distance must be defined. This minimum estimated viewing distance is based on the visual acuity that is normally taken to be approximately 1 minute of arc (1/60th of a degree) for a typical viewer with 20/20 vision.
 
Technically speaking, visual acuity is a measure of the eye's spatial resolving power and indicates the angular size of the smallest detail that the human visual system can resolve. For example, if two thin, bright parallel lines are viewed on a dark background, the visual acuity of the eye will define how close together the lines can be before the eye sees them as a single bright line rather than two separate lines. This is defined in terms of angle rather than distance because the distance between the lines in inches (or millimeters) will depend on how far away they are: if they are 1 mm apart, the lines can easily be separated at a distance of 15 cm, but probably not at a distance of 10 m.
 
This is relevant to LED displays because if the separation between pixels exceeds this angle, the observer will be able to make out the individual pixels as pixels rather than simply seeing a smooth image. If the pixel separation is significantly smaller than this angle, the typical eye cannot actually resolve any more detail, so the extra resolution is wasted. Based on this 1 minute of arc angle, a list of minimum viewing distances for various pixel pitches can be easily compiled by multiplying the pitch by approximately 3400. For example, a 10-mm pitch results in a minimal viewing distance of 34 m. In reality, this minimum calculated viewing distance, based on the theory of visual acuity, is a very conservative estimation of a "minimum viewing distance." It could, in fact, be divided by 3 or 4 while still being acceptable as the viewing distance. As the observer comes closer to the display, the pixelation will not be bothersome – personal preference plays a big role in defining what each viewer perceives as being the minimum viewing distance.
 
Pixel pitch is only one factor that influences the image quality of LED screens advertising. It is important to understand that behind the LEDs is an electronic system that will actually drive the diodes and make sure an excellent image is displayed. How this system has been designed and how it will perform video processing will make a huge difference in the perceived image quality. Parameters such as gamma correction, available colors, or refresh rate will all have an impact on the visual performance of the display.
 
In addition to resolution, display uniformity, or the variation in light intensity over the surface of the display, must be considered. The LED display contains thousands of single diodes (usually hundreds of thousands) that vary significantly in terms of exact color emission and luminous efficiency. To address this problem, the LEDs are screened and sorted during the manufacturing process according to specific brightnesses and color characteristics in a process called "binning." Each diode characteristic is then measured by robotic equipment during production and balanced to obtain overall color uniformity throughout the entire LED wall.
 
Fig. 2: Blue-, red-, and green-light-emitting diodes (LEDs).
 
Fig. 3: The process of forming a full-color LED display from R-G-B LEDs.
Advantages and Challenges
 
Very large and bright video displays have traditionally been difficult to accomplish. In fact, for screens of this size, there have been only two viable technologies: cathode-ray-tube (CRT) and LED.
 
Traditional CRT-based solutions, such as the Sony JumboTron, were launched in the 1980s and were mainly used in sporting arenas and entertainment venues. Each pixel of these displays is made up of at least three tiny CRTs – one red, one green, and one blue. By varying the brightness of each of these, any color can be created. Each of these CRTs is similar to a tiny television picture tube, except that it is only producing the intensity of one picture dot and the entire display is made up of hundreds of thousands of CRTs rather than one. The end result is a large, bright video image.
 
LED displays work on exactly the same principle, except that the tiny CRTs are replaced by LEDs. However, LED displays enjoy distinct advantages over similarly sized CRT displays: they consume far less power and are considerably lighter and occupy much less volume. Additionally, in the late 1990s, LED technology advanced to the point of surpassing CRTs in resolution and brightness.
 
Today, LED technology is the technology of choice to display a good image under any lighting condition – even in direct sunlight. High brightness is the main advantage of LEDs, and it is therefore widely used in outdoor environments or at auto shows where there is plenty of ambient light. Technologies such as front and rear projection, plasma, or LCD suffer today from a lack of brightness as well as contrast and perform quite poorly under high-ambient-light conditions.
 
Another advantage of an LED display is its mechanical structure. Assembling an LED display is simply a result of linking smaller modules together. This modular system makes LED displays suitable for any type of installation (walls, roofs, temporary structures for specific events, etc.), enables easy maintenance, and does not set any limits in shape, size, and resolution. The typical module size for an indoor LED display produced by Barco is about 45 x 45 cm, while it is about 90 x 70 cm for an outdoor LED display.
 
In addition, LED technology provides a much wider color gamut (providing more accurate color fidelity). And LED technology by nature is very stable, demonstrating long lifetime and reliability: Even for operation 24 hours a day, the displays last for many years with little maintenance.
 
Of course, many challenges still exist. Producing and maintaining a display with uniform color and brightness is not an easy task. During manufacturing, LEDs are binned by - emission color into different ranks. The composition of LEDs from different batches in one display will lead to color differences in the total tiled image because the color characteristics of the individual LEDs are not identical. In order to achieve a uniform image over the complete LED display, a color calibration process on each individual LED must be performed during manufacturing. However, over the display's lifetime, non-uniformity appears because of differential aging of the LEDs. Red, green, and blue LEDs have different decay curves, which implies that the color uniformity of a LED display cannot be guaranteed over time without human intervention. Re-calibration of an older display may be required to improve the perceived uniformity of the LED display over its lifetime.
 
Controlling the thermal stability of the LED die is also necessary in order to maintain the performance and stability of the LED display. The LED produces light that is actually absorbed into the adjacent die, the mounting substrate, or other surfaces of the LED assembly. This absorption results in an increased thermal loading of the entire LED assembly. This heat must be addressed in order to obtain maximum light output and reliability. Additionally, any light that is emitted outside of the system is not useable and only adds to the heat and overall power loading. Controlling this absorption and maximizing the thermal efficiency to extract heat from the die are all critical to increasing the light output and usability of the LEDs. Another challenge that results from higher thermal loading is that of color shift. As the P-N junction changes temperature, the output wavelength of the light can shift over time. This color shift obviously impacts the color point for that color, but also impacts the white point for the system since each of the colors are mixed to create white. Fundamentally, in order to stabilize this color shift, the LEDs must either be run at a lower power or thermal stability must be maintained.
 
Despite these challenges and the high power consumption of LED displays, LEDs have captured a major portion of the market. LEDs are currently the dominant technology used in the largest sunlight-readable displays.