Another mode of early LED failure occurs in the form of color shift. Color shift involves a significant change in the spectral output of a light source, resulting in a change in the color temperature and colour rendering properties. Color shift can occur temporarily due to operating conditions or permanently as a result of physical changes to LED packages. In the case of a permanent and noticeable shift in color, it can be considered a mode of parametric failure since the specifications guaranteed by the manufacturer are no longer met.
The vast majority of LED lighting products available on the market are of the PC (phosphor coated) package type. The degree and direction of colour shift depends on the mechanism of package degradation, which in turn depends on the particular variant of PC LEDs used by the manufacturer. One known LED phosphor coating method has, with aging, demonstrated curling of the phosphor relative to the LED chip, causing a shift toward blue. Another coating method has been observed to cause a shift toward yellow as high temperatures cause air gaps to open up between the phosphor coating and the LED chip, called 'delamination'.
The newest generation of LED packages have modified earlier PC methods to address curling and delamination issues, greatly improving color stability. Color shift as a result of other factors such as aging of primary or secondary optics materials and of the driver itself however, means that colour shift may still occur within the rated lifetime of a product.
So far, color shift in LED lighting is likely to only be an issue in the case of lighting critical applications, such as museum display and medical lighting. A DOE LED lighting project implemented in the Smithsonian was what brought attention to the issue of color shift, where the degree of change was observed to be consistent across all lights of the same type, even those from different manufacturers. It has so far been observed that for a given LED type, the change in light color will depend on operating lifetime, thus lights of the same age can be expected to show no color difference relative to each other.
Back to FAQ index