Polarizers have been widely used in different types of displays to achieve different functions. Being an essential component in modern displays, the polarizer’s performance and functionality are crucial determinants of display quality. For instance, the importance of a polarizer is evident in organic light-emitting diodes (OLEDs) and liquid crystal displays (LCDs) where polarizers are used to convert the emitted light into linearly polarized light for further modulation. In most cases, only one polarizer is utilized in the emissive display of the structure such that the emissive display performance is affected by the transmittance variation of the polarizer. Despite this knowledge and its potential practical implications, most research on the application of polarizers on displays has focused on the light leakages of the crossed polarizers with little attention to the optical defects associated with single polarizers.
Currently, the iodine type polarizer, whose polarization selecting effects is based on the alignment of the iodine molecules, is the most used in displays. Due to its anisotropic structure, it exhibits obvious azimuthal transmittance variation. This transmittance variation is generally difficult to eliminate and has significant effects on the display quality, considering that emissive displays only have one polarizer. Consequently, it has additional effects of inducing the enlargement of the luminance mura induced by the cavity of the emissive displays. Therefore, it is important to prevent or minimize the azimuthal transmittance variation to improve display quality.
On this account, Dr. Chi Zhang, Dr. Rui Niu, Dr. Xiaoshuai Li, Professor Hongmei Ma and Professor Yubao Sun from Hebei University of Technology developed a multi-layer composite polarizer with a twisted nematic liquid crystal polymer (TN-LCP) polarizing layer structure. First, the azimuthal transmittance variation of the normal polarizer was illustrated using a newly introduced 3D iodide molecule distribution model. The model also guided the design of the multi-layer composite polarizer by providing a better understanding of its structure and polarization process. Next, the transmittance variations of the normal and multi-layer composite polarizers were measured, analyzed and compared, shown in Fig. 1. Finally, potential concerns related to low transmittance and color shifts were investigated and detailed. The work is currently published in the research journal, Optics Express.
The authors showed that the azimuthal transmittance variation of the multi-layer composite polarizer was way smaller (about one-fifth) compared with that of the normal polarizer. Further calculations showed the ability to adjust/tune the transmittance distribution of the composite polarizer to achieve various specific application requirements. The concerns regarding color shift and low transmittance of the composite polarizer were addressed by increasing the liquid crystal polymer layer thickness and reducing the polarization degree of its polarizing layers. These obtained results were validated experimentally.
In summary, the researchers proposed a multi-layer composite polarizer comprising two polarizing layers and a TN-LCP layer and successfully demonstrated its effectiveness in reducing the azimuthal transmittance variation ratio below that of the normal polarizer. The introduced iodine model provided a good explanation of the differences in the transmittance variations. Overall, the multi-layer composite polarizer exhibited superior results to the normal polarizer. In a statement to Advances in Engineering, Professor Yubao Sun said that the composite polarizer is a promising candidate for reducing the viewing angle defects of emissive displays.
Zhang, C., Niu, R., Li, X., Ma, H., & Sun, Y. (2021). Twisted nematic liquid crystal polymer-based multi-layer composite polarizer with low azimuthal transmittance variation. Optics Express, 29(26), 43720.