光学低通滤波器(optical Low Pass Filter,OLPF)

   当我们在使用智能手机、直播相机、拍照数码相机，数码摄像机，监控摄像头等CCD或CMOS光感应元件取像时，由于CCD&CMOS的像素是离散的，所以根据奈奎斯特抽样定理CCD&CMOS所能分辨的最高空间频率是它的空间采样频率的1/2即奈奎斯特极限频率。若图像的空间频率高于奈奎斯特极限频率在传感器上高频部分将被反射到基本频带造成图像周期频谱交叠即混频现象,即出现莫尔条纹(如下图),导致图像质量降低，不符合人眼感观，所以需在CCD&CMOS前加上光学低通滤波器(Optical Low Pass Filter,OLPF)，以提高图像质量，因其光学低通滤波器(Optical Low Pass Filter,OLPF)有如下几个优点，所以被广泛应用于光学拍照&摄像&直播相机等领域

   When using smartphones, live streaming cameras, DSLR digital camera, camcorders, surveillance cameras, etc., which utilize CCD or CMOS light-sensitive elements for imaging, the pixels of CCD and CMOS are discrete. According to the Nyquist sampling theorem, the highest spatial frequency that CCD and CMOS can resolve is half of their spatial sampling frequency, known as the Nyquist limit frequency. If the spatial frequency of the image exceeds the Nyquist limit frequency, the high-frequency components will be reflected to the baseband on the sensor, causing the image's periodic spectrum to overlap, known as aliasing, which results in moiré patterns (as shown in the figure below). This leads to a decrease in image quality and does not meet human visual perception standards. Therefore, an optical low-pass filter (OLPF) is added before the CCD and CMOS to improve image quality. The OLPF has several advantages, making it widely used in optical photography, video recording, and live streaming cameras.

莫尔条纹示意动画(Moir é stripe animation)	光学低通滤波器工作原理示意图(Schematic diagram of the working principle of optical low-pass filter)

      1.因为光学低通滤波器(Optical Low Pass Filter,OLPF)是消除混频(莫尔条纹)现象的有效的手段，一般由一片或多片不同切割角度的石英晶体晶片胶合组成，固定 在CCD&CMOS传感器的前面。其工作原理为当目标图象信息的光束经过光学低通滤波器(Optical Low Pass Filter,OLPF)后产生会发生双折射反应（分为寻常光o光束和异常光e光束，我们俗称为分光点）。根据CCD&CMOS像素尺寸的距离和感光面积计算出抽样截止频率，同时也可根据特定公式计算出o光和e光分开的距离，从而决定石英晶体晶片的切割角度及厚度以改变入射光束将会形成差频的目标频率，达到减弱或消除低频干扰条纹的目的(特别是彩色CCD&CMOS出现的伪彩色干扰条纹)。

   Since optical low-pass filters (OLPF) are an effective means of eliminating moiré patterns, they typically consist of one or more quartz crystal wafers bonded together at different angles, fixed in front of CCD and CMOS sensors. The working principle involves the light beam carrying target image information passing through the OLPF, causing birefringence (splitting into ordinary light (o-ray) and extraordinary light (e-ray), commonly referred to as splitting points). Based on the distance between CCD and CMOS pixel sizes and the photosensitive area, the sampling cutoff frequency is calculated. Additionally, using specific formulas, the distance between the o-ray and e-ray can be determined, which then decides the cutting angle and thickness of the quartz crystal wafer to alter the incident light beam's resulting beat frequency, thereby weakening or eliminating low-frequency interference stripes (especially pseudo-color interference stripes in color CCD and CMOS).

OLPF原材料，石英晶体(俗称:水晶) OLPF raw material, Quartz Crystal	OLPF原材料，蓝玻璃 OLPF Raw Material, Blue Glass

       2. 除了要考虑光学低通滤波器(Optical Low Pass Filter,OLPF)的频率特性以外，还应考虑它的光谱特性,因为CCD&CMOS光感应元件拍摄彩色景物时能侦测到人眼所不能侦测的红外线部份，会导致D.S.P无法算出正确颜色而出现颜色失真,不符合人眼感观。

   In addition to considering the frequency characteristics of the optical low-pass filter (OLPF), its spectral characteristics should also be taken into account. This is because CCD and CMOS light-sensitive elements can detect infrared parts that the human eye cannot see when capturing colored scenes, which may cause the DSP to fail to calculate the correct colors, resulting in color distortion that does not match human visual perception.

OLPF组成层数示例（Example of OLPF composition layers）：

       3.所以必须将人眼无法侦测的红外线部份除去,故一种方法为在Optical Low Pass Filter,OLPF表面镀上红外截止膜（IR-CUT Filter），将其红外线部份反射掉，只留符合人眼条件的(通常为420~680nm)可见光部份通过,我们通常称为反射式滤光片;另一种方法即是为Optical Low Pass Filter,OLPF配上一片蓝玻璃（我们通常称为吸收式滤光片）,因其蓝玻璃本身材料特性为吸收红外线（而不是反射式可能造成图像诡影的产生），因此于入射光角度对于光谱特性的影响较小，图像色彩还原性更佳，使影像呈现的色彩更符合人眼的感觉,还您一个真实的绚丽多彩视觉盛宴!

        Therefore, it is necessary to remove the infrared part that the human eye cannot detect. One method is to coat the Optical Low Pass Filter (OLPF) surface with an infrared cutoff filter (IR-CUT Filter), which reflects the infrared part and only allows the visible light part that meets the human eye conditions (usually 420-680nm) to pass through. This is commonly referred to as a reflective filter. Another method is to pair the Optical Low Pass Filter (OLPF) with a blue glass filter (commonly known as an absorptive filter). Due to the material properties of the blue glass itself, it absorbs infrared light (unlike the reflective method, which may cause ghosting in images), thus having a smaller impact on spectral characteristics at different incident angles, resulting in better color reproduction and making the image colors more consistent with human visual perception, providing you with a truly vibrant visual experience!

OLPF常用截止型光谱(OLPF commonly used cut-off spectroscopy)：