Research on extinction characteristics of polaroids in near-infrared region

In the research of detecting extinction ratio of polarimeter, some significant characters are discovered. One polaroid has perfect extinction ratio in the visible, but its extinction ratio becomes weak in near infrared (NIR) region, and the transmittance of a pair of vertical polaroid becomes strong in NIR region. This phenomenon has been found in several other polaroids as well. The transmittance increases greatly in NIR even if the crystal Glan-Taylor prism polarimeter with good polarization is used. The extinction ratio of the polaroid changes from 3:100 in visible to 98:100 in NIR. More experiments have shown that the weakening of extinction ratio and the enhancement of transmittance of a pair of vertical polaroid in NIR are closely associated with the intensity of incident light. When the light intensity increases, the effect becomes clearer. The transmittance exceeds the theoretical results from classical physical optics, reaching 98%. Our results provide two aspects of applications. First, because the extinction ratio of the polaroid is related to the wavelength of incident light, the polaroid can be used in definite spectral range. Second, a pair of vertical polaroid can perfectly filter the visible light, while making NIR light transmit. This research result can provide some conveniences at IR spectrum, IR photography, IR imaging and IR detection.