If a display manufacturer were to indiscriminately purchase a run of packaged diodes from a diode manufacturer without any thought to the variation of optical and electrical specifications between the diodes, the display product they eventually manufactured would exhibit inconsistent performance. To avoid the outsized impact on display performance differences between diodes can have, diode manufacturers, at the request of display manufacturers, sort newly created diodes through a classification process that groups together like diodes based on three separate metrics. Display manufacturers then purchase only those diodes that fit their parameters. This diode sorting process is referred to within the industry as “binning,” and the three metrics commonly used to “bin” diodes are brightness, color wavelength, and forward voltage. But what impact does strict binning actually have on performance? Do the differences between diodes really matter? What happens if you don’t bin? To answer these questions, we’ll first have to answer a few more.
Why Do Diodes Differ?
Though the diodes within an LED display can seem identical to the naked eye, every diode is not made equal, and one inconsistent diode can compromise an entire display. How is this possible when every diode used in a display is created using the same process? The answer to this question can be understood through a simple analogy. Imagine spraying a flat surface with spray paint, using a precise automated system that aims to coat the surface equally. Even though you may limit the number of variables and strictly control the operations of the system, you are still going to find small inconsistencies and variations. The same is true with the diode manufacturing process. Each diode made in a production run will exhibit tiny differences from the others on its silicon wafer, even though they were all created with the same materials, ostensibly under the same conditions. Variations between diodes on separate wafers will be even more prevalent. The diode manufacturer will then pick and place the diode from these wafers into their packaging, complete the wire bonding (or wireless bonding), seal the package, and then sell these completed LEDs to display manufacturers.
Why Bin Based on Lumosity, Wavelength, or Voltage?
Most display manufacturers will only bin based on brightness or wavelength – forward voltage is largely ignored in the binning process since it can be controlled and adjusted on its own by a display’s driver chip. Each of these measurements come with an acceptable tolerance ratio.
Roughly 90% of the market will bin based solely on brightness, as measured by millicandelas since the diodes tested to be brightest in the manufacturing stage will be the most efficient diodes in the field. Lest a display look splotchy or imbalanced, the brightness for these diodes, once they are in place, must be set to the same level. For example, a manufacturer purchasing diodes from their supplier’s highest performing bin may have to decrease 36mcd diodes down to the level of 26.9mcd diodes. This 25% reduction in brightness from 36 to 26.9 can seem steep but it is a paltry drop when compared to the millicandela measurements of diodes from lower-performing bins, which could see a diminishment of more than 50%.
The segment of the display market that bins for color wavelength is largely focused in the cinema space or professional video playback. For clients in non-cinema applications, the uniform efficiency of the diode is the most important consideration, so brightness is prioritized. In the cinema space, where brightness is less of a factor, having diodes with color wavelengths that can accommodate standardized color spaces becomes the higher priority. For example, if a client requires an LED display that can hit the standard color space of Rec.709, a bin of blue diodes with a wavelength range between 464 and 468 will deliver diodes outside of the Rec.709 standard, while a bin with a range between 462 and 466 will keep you within Rec.709.
What Happens if you Don’t Bin Properly?
When targeting a specific color space, improper binning can result in a failure to deliver on a client’s request and a splotchy picture. When targeting maximum brightness and efficiency, improper binning will force you to drop the brightness levels across your entire display, leading to a less efficient product with worse grayscale quality. Every display will exhibit small inconsistencies over time, but negligent binning will exacerbate the issues stemming from these inconsistencies, necessitating repeated calibration. This corrective action contributes to eroded performance and decreases display lifespan.
Before completing a display product, display manufacturers perform a smoothing process called calibration. Sophisticated optical equipment gathers data on every pixel within a display, and then runs that data through analyzing software to generate a set of coefficients to apply to each diode. Once applied, these coefficients attune the performance of every diode in the display down to the levels of the worst-performing diode. This process improves the consistency of the display, but it reduces the brightness. When not done correctly it can result in a proportional decline in grayscale quality. As grayscale quality erodes, the slight differences between shades of the same color begin to fade. In other words, on a calibrated display, Coca-Cola red may no longer be distinct from other reds. That’s a problem for brands who have paid good money to showcase their specific color. Ultimately, calibration is a good thing because it smooths overall consistency, but a display should not require it too regularly because it does diminish brightness and thus grayscale, lessening the impact of your display over time.
Does Binning Have any Downside?
The more specific your binning requirements, the smaller percentage of given manufacturing run you are purchasing. That requires a provider to perform multiple runs, which adds cost. This is especially true when binning for both brightness and wavelength, as the more specific your bin requirements get, the more production runs you require the manufacturer to perform. Binning to hit the Rec. 709 color space can increase costs 20-30% while hitting cinema-quality DCI-P3 color space can increase costs by 50% or higher as bins to that standard can account for less than 5% of a total production run. These increased costs are worth paying to deliver a display product that is consistent, high performing, and long-lasting. After all, a chain is only as strong as its weakest link, and a display is only as good as its worst diode.