The diode manufacturer picks and places the diode from the wafers into their packaging, completes the wire bonding (or wireless bonding), seals the package, and then sells these completed light-emitting diodes (LEDs) to display manufacturers.
Why should diodes be binned based on luminosity, 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.
Approximately 90 per cent of the market will bin based solely on brightness, as measured by millicandela (mcd), since the diodes tested to be brightest at the manufacturing stage will be the most efficient diodes in the field. Unless a display looks 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 36 mcd diodes down to the level of 26.9 mcd diodes. The 25 per cent reduction in brightness from 36 to 26.9 mcd 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 per cent.
The segment of the display market that bins for colour 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 colour wavelengths that can accommodate standardized colour spaces becomes the higher priority. For example, if a client requires an LED display that can hit the standard colour space of Rec.709 (standardizes the format of high-definition television, having 16:9 [widescreen] aspect ratio), 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 one within Rec.709.
What happens if diodes are not binned properly?
When targeting a specific colour space, improper binning can result in failure to deliver on a client’s request and a splotchy picture. When targeting maximum brightness and efficiency, improper binning will force one to drop the brightness levels across their entire display, leading to a less efficient product with worse greyscale quality. Every display will exhibit small inconsistencies over time, but negligent binning will exacerbate the issues stemming from these variations, 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. If done incorrectly, it can result in a proportional decline in greyscale quality. As the greyscale quality erodes, minor differences between shades of the same colour begin to fade. In other words, on a calibrated display, Coca-Cola red may no longer be distinct from other reds. This can be problematic for brands that have paid large sums of money to showcase a specific colour. Although, calibration smooths overall consistency, a display should not be calibrated too often because it diminishes brightness and thus greyscale, lessening the impact of the display over time.
