The role of security camera filters
Filter out infrared rays: Color CCDs can also sense infrared rays, which can cause D.S.P to be unable to calculate the correct color. Therefore, a filter needs to be added to separate the infrared part of the light, so only color CCDs need to be equipped with filters, black and white are not needed
Coating methods and blue glass can be used. Coating methods include vacuum coating and chemical coating. Chemical coating involves immersing a quartz plate in a solvent for electroplating, which is low-cost but has uneven coating thickness and is prone to detachment. Vacuum coating uses vacuum evaporation, which is uniform and not prone to detachment, but has high cost. The above is called IR coating, which aims to filter out infrared rays. In addition, AR coating coating is also added to increase the light transmittance, because when light passes through different media (such as entering the quartz plate from air), it will produce partial refraction and reflection. With AR coating, the filter can achieve a penetration rate of 98-99%, otherwise it will only have a penetration rate of 90-95%. This has an impact on the sensitivity of CCD. The impact is that blue glass is used, and blue glass is used; Absorption” IR Coating filters infrared rays through reflection, while IR Coating filters infrared rays through reflection. However, reflected light can easily cause interference. If we only consider filtering infrared rays, blue glass is a better choice. However, as mentioned earlier, glass cannot adjust light, so there is a so-called "blue glass plus quartz plate"; Two piece” Filter The blue glass is used to filter infrared rays, while the quartz plate is used to adjust light, so only AR coating is needed on the quartz plate
Trimming the incoming light: Because CCD is composed of individual photoreceptors (CELL), it is best for the light to be directly incident. However, in order to avoid interfering with adjacent photoreceptors, the light needs to be trimmed. Therefore, the filter is not made of glass, but a quartz plate, which uses the physical polarization characteristics of quartz to retain the direct part of the incoming light and reflect off the oblique part, avoiding affecting the adjacent photosensitive points
By utilizing the physical polarization properties of quartz, the incoming light is retained in the direct part and reflected off the oblique part, but can only be adjusted in one direction. Typically, cameras only consider horizontal resolution, so only horizontal adjustments are made to the light. Therefore, when applying filters, the direction must be correct and not reversed. What if vertical light also needs to be adjusted? It's simple, just stick two pieces together and turn one of them 90 degrees, so there's this kind of thing called 'rdquo'; Two piece” One filter is used for horizontal trimming, one for vertical trimming, and one for IR Coating to filter infrared radiation. What about the more advanced ones? It's just two pieces of quartz sandwiching a piece of blue glass, which gives us various advantages; Three piece” Commonly seen in imported machines from Japan