One issue raised frequently by lighting designers is the variation in color within batches of white LEDs. The human eye is highly sensitive to differences in hue, and can also perceive differences in intensity as a difference in color. The eye is particularly sensitive to edges, for example where two beams of slightly different color tints are projected side by side onto a wall.
Gordon Routledge of UK company Lumidrives described some of the challenges of using white LEDs for illumination, starting with product binning. Older binning methods tend to use correlated color temperature (CCT), but LEDs with the same CCT can have a different color tint depending on their position relative to the blackbody locus. Suppliers have now started to use new binning strategies that reduce the variability of LEDs within each bin, but this remains an issue.
Routledge pointed out that the problem is less acute on relatively small projects requiring small numbers of LEDs, which can all be obtained from the same bin, and in applications such as edge illumination, where mixing effectively averages out product variations. In large-scale applications, such as attempting to produce a consistent color for signage for a particular brand that will be seen in many locations, color matching can be a major challenge. The problems are most acute in applications where the LEDs are viewed directly, side by side without mixing.
Color Kinetics' Kevin Dowling emphasized the problem of color variation. "For those of us buying large quantities of LEDs, it's very frustrating not to be able to buy all the devices from the same bin," he said.
LED manufacturers require constructive criticism on this subject, as Keith Scott from Lumileds pointed out. "LED makers need constant feedback from their customers regarding binning requirements, otherwise they will produce bins that work best for them in terms of maintaining high yield," he said.