The function and selection of crystal lenses
As is well known, both CCD and CMOS are image sensors that use silicon photodiodes for light to electricity conversion. They are composed of individual photoreceptors (CELL), which require direct light to enter. Light that enters obliquely will interfere with adjacent photoreceptors, resulting in color drift (false color), which requires adjustment of the light. We utilize the physical polarization properties of crystals to retain the direct part, reflect and refract the oblique part of the incoming light, avoiding the oblique light from affecting the adjacent photosensitive points. However, oblique light has different angles, and a crystal can only handle oblique light from one direction. In theory, the more layers of crystal pieces stacked in different directions, the better the effect of solving color drift (false color). However, considering actual needs and costs, generally only 1 to 3 crystal pieces are used to solve horizontal, vertical, and 45 ° C problems; The problem of color drift (false color) in corners. There is a saying that goes, '...'; Two piece” 、 “ Three piece” Filter, where IRCoating film or blue glass is used to filter out infrared light, while crystal is used to adjust the light. ARCoating coating is also required on the crystal sheet to increase transmittance.
Crystal polishing is a physical process, and different brands, specifications, models, and pixels of CCDs, as well as N and P systems, require the thickness of the crystal to vary according to the photosensitive point on the CCD, and cannot be used incorrectly. For example, 8.8× 8.2× The 2.85mm three-layer filter is specifically designed for SONY405CCD, and people often mistakenly use the 1.08mm crystal for SONY405CCD, etc.
Application and shortcomings of single filter
The filter not only trims the light and restores the true color of the image, but also filters out infrared radiation. Therefore, in the absence of visible light at night, imaging is impossible, and there is no night vision function. To solve this problem, a single filter with high bimodal values was developed and applied, which gave this camera night vision function. However, this type of single filter, although low-cost, can be used both day and night. Due to the opening of wavelength frequencies, during the day, some of the infrared light in natural light can also enter CCD or CMOS and interfere with image color reproduction, such as green plants turning gray white, red paintings turning light red, black turning purple, etc. (especially in outdoor environments with sunlight). In order to comprehensively consider the effects of day and night, it is difficult to fully adapt to the curve graph of the filter. During the day, there are still some infrared lights that interfere with image color reproduction; At night, due to the filtering effect of the bimodal glass sheet, CCD or CMOS cannot fully utilize all the light, resulting in snowflake spots and reducing the image clarity and low illumination performance of the infrared camera.
IRCUT dual filter technology
The dual filter technology has different names in different places, and is called IRCUT in Europe and America; Taiwan, China is called ICR; Chinese Mainland calls it a dual filter switch.
The IRCUT dual filter switcher allows ordinary day and night cameras to operate with different filters at night and during the day, effectively solving the problem of dual peak single filter day and night that cannot be balanced.
The IRCUT dual filter switcher consists of an infrared cut-off low-pass filter, a full spectrum optical glass, a power mechanism, and a housing. It is switched and positioned through a circuit control board. When the daytime light is sufficient, the circuit control board drives the switch to switch and position the infrared cut-off filter, and the CCD or CMOS restores the true color; When there is insufficient visible light at night, the infrared cut-off filter automatically moves away, and the full spectrum optical glass begins to work. At this time, it can sense the infrared light of the infrared lamp, allowing the CCD or CMOS to fully utilize all the light, greatly improving the night vision performance of the infrared camera, and the entire image will be clear and natural.
The application of IRCUT dual filter technology has greatly improved the performance both at night and during the day. However, due to various factors such as technology and understanding, IRCUT dual filter switches have had various problems in the past, and not all factory products of IRCUT dual filter switches are mature and effective. The quality of the IRCUT dual filter switcher depends on two major aspects: the filter and the structure.
The infrared cut-off degree, transmittance, and light shaping effect of the filter directly affect the image clarity or color of CCD and CMOS. The IRCUT dual filter switch with an all metal casing is not only resistant to high temperatures and deformation, but also capable of carrying larger and heavier lenses. It can also better transfer heat, reduce the impact of temperature on the imaging chip, and ensure stable image quality. Different filters need to be used during the day and night, so their mechanical properties are very important.
There are various driving methods and mechanisms for IRCUT dual filter switches, mainly divided into inductive coil type and motor type, each with its own advantages.
Inductive coil type can be divided into coil motion or magnet motion, as well as linear motion or pendulum motion. The coil driving method has a small volume and low cost, but the driving force is small and the self-locking ability is poor. The lifespan of the coil movement is short, while the lifespan of the magnet movement is long. The pendulum movement method has relatively good self-locking ability, but cannot drive large-sized filters.
The motor type can be divided into gear tooth strip type, worm gear type, and spiral pair type. The spiral pair has a compact structure, high power, and strong self-locking force, making it a good choice.