Color Retention
Similar to the conditions that contribute to lumen depreciation, excessive thermal stress and the migration or interaction of impurities and moisture at the phosphor layer are the main causes of color maintenance failures in COB LEDs. COB LEDs typically exhibit color shift behaviors in the blue to green range. A chromaticity shift in the blue direction results from a decrease in phosphor quantum efficiency brought on by thermal deterioration or chemical reaction. When LEDs are overdriven, the phosphor operates over the saturation flux level, which can cause the chromaticity shift to shift toward the blue. The presence of moisture may result in a reduction in the peak wavelength of red phosphors, which initially results in a blue shift. When the binder and phosphor are improperly mixed and dispersed, the phosphor will settle and precipitate, which will also result in blue shift. A shift in the green direction will be introduced by photo-oxidation of the phosphor matrix. A spectrum shift of the LED emission and a decline in efficacy may also be caused by the MCPCB's reflecting white coating degrading.
