What is colour stability?
Color stability also known as chromaticity stability describes the ability of a light source to maintain its color properties over time.
The change of lighting color properties, i.e. colour shift over the life is another mode of early LED failure. 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.
Note that color shift is generally independent of lumen depreciation, although they can be related.
Why and where is colour stability important?
The importance of colour stability varies depending on the application: it is unavoidable for a museum or a retail shop, less important for streets and big areas lighting. The colour stability is also important when more devices are used (for instance the case of the “wall washer”) in order to light up a wall, or for those applications where the evaluation of the lighted objects colour assumes particular relevance, such as in a day hospital or in a factory manufacturing foodstuff.
What causes colour shift?
Colour stability can vary based on LED lamp or luminaire product design with several factors affecting the resulting performance.
Factor 1 - LED Packages:
Materials and manufacturing methods used in LED package production are known to contribute to color point stability over time. The primary factor that contributes to LED color performance is operating temperature. Depending on the design of the LED package, the phosphor layers may settle, curl, delaminate, or otherwise change the number of photons that are converted to white under high operating temperatures.
Factor 2 - Luminaire Construction:
As with LED packages, the materials and methods used to assemble luminaires can affect the color shift of the product. Materials in the optical path, such as silicones or plastics may discolor over time. In addition, materials such as glues or chemicals may diffuse into the LED package and affect chromaticity stability.
Factor 3 - Application:
The environment in which the luminaire is operated can adversely affect the color point stability. Contaminants such as Sulfur is present in the environment around us (i.e. Automobile exhaust fumes) that come into contact with a luminaire during its operation can cause degradation of reflective surfaces in the LED package and luminaire that will accelerate color shift. Temperature fluctuations during operation may also intensify degradation mechanisms for some LED products. Temperature again is an important factor.