Full-Color LED Displays

There is a mistaken perception regarding light-emitting diodes (LEDs) and the cost of LED signs, which goes something like this: The cost of these things keeps going down; just wait a few years longer, and the price will be even more affordable." While that is true in principle, it’s important to understand the reasons behind recent history and the realities pertaining to future pricing. What is an LED screen? LEDs are crystals encased in plastic shells, which, when charged with electricity, glow in a specific color, depending upon the chemical composition of the crystal. Full-color LED screens comprise an array of pixels laid out in a specific matrix of vertical columns and horizontal rows. Each pixel comprises the same number and type of diodes as the next and is equidistant from the adjacent pixels, which creates a uniform grid. To create the proper color balance, each pixel combines red, green and blue (RGB) diodes. The computer, or image processor, determines each diode’s degree of brightness at each moment to create the exact color for each pixel. Each diode in each pixel receives an update (refresh) many times per second. This constant state of changing information allows the overall display to change from one impression to the next — each pixel’s, and each diode’s, color intensity is updated as commanded by the programmer. Evolution of brightness Just as each pixel’s color changes by adjusting each diode’s degree of brightness, the display’s overall brightness reflects the individual pixels’ cumulative brightness. The earliest outdoor LED screens comprised larger pixels, with multiple diodes of each color necessary to achieve outdoor usage’s brightness requirements. As diodes have evolved, each color’s new generation has become brighter. LED engineers strive to reach "quantum efficiency" — the direct conversion of energy to light. With each efficiency increase, more light — and less heat — is generated. As with most developments, the greatest strides occurred initially. Recent efficiency gains have been more modest, as practical quantum efficiency approaches its maximum potential. Brightness as reflected in price Increased brightness has allowed a reduction in diodes per pixel. This hasn’t been uniform. Each color has its own lifecycle and corresponding quantity reductions. However, for each new pixel pitch, the number of necessary diodes to provide each pixel’s required brightness has declined…until now. Because there is a base number of diodes per pixel — one red, one green and one blue — necessary to create the palette of colors, the reduction, logically, stops at three. Currently, most pixels, up to and including the 25mm, use only three diodes. The 26- through 38mm pitches typically use four diodes. Therefore, those pitches can be reduced by only another 20%. However, note that the color green is least visible to the human eye. Although 60% of visible light is green, actual color must be over-created to see it. Consequently, we may never see a large-size pixel comprising only three diodes. Therefore, the dramatic cost reductions of reduced diodes are essentially over. Future cost reductions will come, but only as a consequence of diode-manufacturer price reductions as production costs go down. Cost reductions will level off dramatically over the next two to three years. An LED display’s life A full-color LED display’s projected life is 100,000 hours until half brightness. That formula is a scientific measure calculated for the diodes’ anticipated life. However, we have measured the oldest Las Vegas displays still used and have found them to be significantly brighter than the scientific evidence suggests. LED manufacturers project life based on numerous accelerated degradation tests, during which they both heat and over-drive the diodes until they expire. From the length of time in which the diodes survive under these conditions, manufacturers extrapolate backwards into an anticipated life under normal conditions. This methodology produced the 100,000-hour figure. I have heard recent discussions of displays anticipated to last five to seven years. That number is a gross underestimate. Using the scientific calculation of 100,000 hours, with the assumption of 24 hours per day, 365 days per year, the life would be 11.42 years. However, when you consider that the calculation assumes all diodes are on at full brightness, all of the time — a condition that never actually exists — the duration actually is much longer. In a typical still pictorial, few, if any, of the diodes are on at full brightness. During animation or video sequences, even fewer diodes are fully charged. Each incremental reduction in brightness for each hour of operation directly produces longer life. In addition, some ordinances require one second of dead time between each image. During these change times, all diodes are off. One second may sound trivial, but the California Department of Transportation (Caltrans) allows only still messages, each four seconds on, one second off. That 20% cumulative effect could increase life by up to 2.25 years. No one knows how long a full-color LED screen will last. The industry isn’t as old as the projected life. However, manufacturers’ conservative, scientific data is proving to be understated, compared to real-world figures. The bottom line Without question, the LED, as well as the displays from which it is produced, will continue to evolve. However, the dramatic cost reductions and image improvements of the last five years were a phenomenon spun from the genesis of an entirely new industry. Early LED assessments predicted a development curve, based on other, more-mature, semiconductor-device developments, that has proven itself almost precisely. That curve shows small, incremental advancement with corresponding cost reduction for the near future. Some LED screen providers actually believe that their systems will last 20 years in a routine sign application. While that may be optimistic, it might be closer to the truth than any of us actually knows.