1. It can filter interference light.
2. The infrared signal can be amplified.
When we do not use the Fresnel Lens, the detection radius of the pyroelectric infrared sensor at room temperature is less than 2 meters, and it is very unstable and easily interfered by external light. Only when used with a Fresnel Lens can it be used to its maximum effect. When combined with Fresnel Lens, the detection radius of the sensor can reach 20 meters.
The Fresnel Lens is made of polyethylene plastic sheet (HDPE). The common color is translucent, milky white or black. However, it is transparent to infrared rays with a wavelength of about 7-14um, only for this 7-14um Infrared light passes through the range. Most of its shape is hemispherical, the lens is divided into multiple parts in the horizontal direction, and each part is equally divided into several different areas in the vertical direction. Each equal part of the uppermost part is a lens unit, which is composed of concentric circles, and the center of the concentric circles is in the lens unit. Each equal part of the middle and lower halves is also a separate lens unit, which is also composed of concentric circles, but the center of the concentric circles is not in the lens unit. Multiple lens units are combined into a lens group unit. In theory, the more lens units and the denser the sensitivity, the higher the cost of the mold.
When light passes through these lens units, the visible and blind areas of light and dark will be formed. Since each lens unit has only a small viewing angle, the inside of the viewing angle is the visible area, and the outside of the viewing angle is the blind area. Any two adjacent lens units are separated by a blind zone and a visible zone, and they are intermittent without overlapping and crossing. In this way, when the Fresnel Lens is placed in a proper position in front of the sensor, once the moving human body appears in front of the lens, the infrared radiation from the human body will form an alternating shadow area (blind area) and a bright area (visible area) on the sensor after passing through the lens, so that the surface temperature of the sensor will change continuously and the electric signal will be output.
The Fresnel Lens can not only form visible and blind areas, but also has a focusing effect. Its focal point (that is, focal length) is generally about 4 mm to 30 mm. In practical application, the distance between Fresnel lens and sensor (that is, focal length) should be adjusted according to the actual situation or the information provided. Generally, the lens is fixed 4 to 30mm directly in front of the sensor.