**1. Imaging of Surveillance Cameras**

The spectral power distribution with a central wavelength of 830 to 950nm and a half-peak bandwidth of around 40nm falls within the near-infrared spectrum, which is within the perceptible range of standard CCD and CMOS sensors. Consequently, real-time monitoring is possible both during the day and at night. As any object above absolute zero (-273°C) emits infrared radiation, CCD or CMOS sensors can detect both visible and infrared light during the day. Based on the principles of light refraction, longer wavelengths exhibit smaller refractive indices, while shorter wavelengths exhibit larger refractive indices. Therefore, when visible and infrared light simultaneously enter the camera lens and are refracted by the lens optics, visible light and infrared light form images on different target surfaces. The imaging of visible light produces color images, while the imaging of infrared light produces grayscale images. Once the image of visible light is optimized through lens focus adjustments, the so-called lens back focus adjustment and focusing, infrared light creates a virtual image on the same target surface, thereby impacting image color and quality.

**2. Single Optical Filter**

Optical filters, while shaping light and restoring authentic image colors, also filter out infrared light. Consequently, during situations when there is no visible light at night, imaging is not possible, rendering night vision useless. To address this, dual-peak single optical filters have been developed and utilized, providing cameras with night vision capabilities. However, these single optical filters, though cost-effective and suitable for both day and night use, can introduce disturbances into image color restoration during the day. This phenomenon is due to the broadened wavelength frequency, which allows some natural infrared light from daytime environments to enter CCD or CMOS sensors. As a result, during daylight, certain colors, such as green plants turning pale, red pictures becoming light red, and black changing to purple, may experience color deviations (especially noticeable in sunny outdoor conditions). Moreover, to achieve acceptable performance during both day and night, the curve of the optical filter is challenging to optimize fully. At night, due to the filtering effect of the dual-peak glass, CCD or CMOS sensors cannot utilize all available light, leading to the occurrence of "snowflake" artifacts and reducing the image clarity and low-light performance of infrared cameras.

**3. Dual Optical Filters**

The technology of dual optical filters goes by various names in different regions: IRCUT in Europe and the US, ICR in Taiwan, and dual optical filter switcher in Mainland China. The IRCUT dual optical filter switcher allows regular day-night cameras to use different filters for daytime and nighttime operation, effectively addressing the issues stemming from the limitations of dual-peak single optical filters. The IRCUT dual optical filter switcher comprises an infrared cutoff low-pass filter, a full-spectrum optical glass, a dynamic mechanism, and an outer casing. A circuit control board triggers the switching and positioning of the switcher. During ample daylight, the control board commands the switcher to position the infrared cutoff filter, allowing CCD or CMOS sensors to restore authentic colors. At night when visible light is insufficient, the infrared cutoff filter automatically moves away, allowing the full-spectrum optical glass to engage. In this mode, the sensor can detect infrared light from infrared lamps, allowing CCD or CMOS sensors to utilize all available light, significantly enhancing the night vision performance of infrared cameras and producing clear, natural images.

While the application of IRCUT dual optical filters markedly improves the performance during both daytime and nighttime, some issues and challenges existed historically due to factors such as technology and awareness. Not all products from every factory with IRCUT dual optical filter switchers were mature and effective. The quality of IRCUT dual optical filter switchers depends on two key aspects: the quality of the filters and the structural design. The infrared cutoff degree, transmittance, and light shaping effects of the filters directly impact the image clarity and color of CCD and CMOS sensors. IRCUT dual optical filter switchers with all-metal casings are not only resistant to high temperatures and deformation but can also accommodate larger and heavier lenses. They can efficiently dissipate heat, reducing the impact of temperature on image sensors and ensuring stable image quality.

Color deviations in surveillance camera images can also result from incorrect installations. For example, aligning two high-quality infrared cameras to shoot at each other or placing a camera with four lights inside a small box for testing may lead to a pale purple image. In these instances, switching the mode manually would display normal behavior. Cameras first sense the illuminance through a light-sensitive resistor to activate infrared lights and then switch the IR-CUT. In the case of testing inside a box with infrared lights turned on, the IR-CUT does not switch, and the camera remains in daytime mode, resulting in the reflection of weak infrared light, which might appear as pale purple.

Incorporating CMOS chips, most optical filters are now dual filters. However, image color deviations may still occur. This can be attributed to the power supply, especially in centrally powered projects. Insufficient power supply can cause improper filter switching, particularly when infrared is enabled during the day. In such cases, if the infrared cutoff filter remains active due to inadequate power supply, image quality and color may be negatively affected.

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