Color Shift Investigations for LED Secondary Optical Designs: Comparison between BPA-PC and PMMA

• Inconsistent degradation of wavelength–dependent transmittance induces the decrease of the blue/yellow light intensity ratio and thus gives rise to the color shift toward the yellow field, which is the color shift mechanism of BPA-PC.
• Even for the non-aged BPA-PC, the transmittance varies with wavelength, which caused the change of intensify ratio of blue light to yellow light, leading to color change.
• Oxidation plays a key role in the degradation of transmittance, which is in correlation with the discoloration of thermally-aged BPA-PC materials.

Recently, color shift of LED-based lighting products has attracted much attention due to its increasing impact on the field application. However, significant research investigations on the color shift mechanisms are not publically available especially for important transmission materials used for secondary optical design. In this paper, broadly used such commercial materials (BPA-PC and PMMA) are experimentally investigated on the color shift effects during aging. Besides this, color shift mechanisms of degradation of transmittance are also studied. Results revealed: (1) Inconsistent degradation of wavelength–dependent transmittance induces the decrease of the blue/yellow light intensity ratio and thus gives rise to the color shift toward the yellow field, which is the color shift mechanism of BPA-PC; (2) Even for the non-aged BPA-PC, the transmittance varies with wavelength in the visible light field due to the chemistry of the material, which caused the change of intensify ratio of blue light to yellow light in the SPD, leading to color change in perception; (3) Oxidation plays a key role in the degradation of transmittance at around the peak wavelength of the blue light field, which is in correlation with the discoloration of thermally-aged BPA-PC materials. By contrast, for the PMMA specimen aged up to 3000 h, oxidation was neither occurred at 85 °C, nor with additional exposure to blue light, nor even with additional humidity of 85%RH.