In the realm of security systems, infrared detectors stand among the most vital technologies for intrusion detection. To thoroughly understand their applications and performance, it is essential to grasp the fundamental concepts behind sensors, detection principles, types of infrared detectors, key technologies such as dual-technology and curtain detection, as well as best practices for installation and maintenance.

1. What is a Sensor?

At the heart of any intrusion detection system lies the sensor, a critical component functioning as a physical transducer. Sensors are engineered to convert physical quantities—such as force, pressure, displacement, vibration, temperature, sound, and light intensity—into electrical signals that can be readily processed. These signals typically manifest as variations in voltage, current, resistance, or capacitance.

The transformation from physical input (x) to electrical output (y) follows a specific transfer function, represented by the equation y = f(x). Ideally, this function captures only the intended signals; however, real-world environments introduce disturbances such as air pressure changes, temperature fluctuations, vibrations, and electrical noise. High-quality sensors are meticulously designed to minimize the impact of such interferences.

Sensor outputs are categorized into two primary types:

• Analog Signals: Continuously variable signals, such as the current produced by a photodiode in response to changing light intensity.
• Digital Signals: Binary outputs derived by comparing analog signals against a preset threshold. When the signal exceeds this threshold, it indicates an intrusion (“1”); otherwise, it indicates a non-intrusion (“0”).

Certain sensors, such as reed switches, inherently produce digital signals without requiring analog-to-digital conversion.

2. What is a Fresnel Lens?

A Fresnel lens serves two principal purposes in infrared detection:

• Focusing: It refracts or reflects infrared radiation onto the Passive Infrared (PIR) sensor, concentrating the energy for more effective detection.
• Segmenting Detection Zones: It divides the monitored area into multiple bright and dark regions. As a moving object passes through these zones, it generates alternating infrared signals, significantly enhancing motion detection capabilities.

Modern Fresnel lenses are often fabricated from lightweight plastics and feature finely engineered microstructures that maximize infrared transmission while minimizing signal loss.

3. Principle of Passive Infrared (PIR) Detectors

Passive infrared detectors operate by sensing infrared radiation naturally emitted by objects without actively transmitting any signals themselves. Every object radiates infrared energy according to its temperature. In a monitored environment, stable backgrounds—such as walls, furniture, and terrain—emit consistent infrared levels. When a human body enters the field of view, it disrupts this equilibrium, creating detectable infrared variations.

Fresnel lenses channel these variations onto the PIR sensor, which then converts them into electrical signals to trigger an alarm.

(1) Advantages of PIR Detectors:

• Highly energy-efficient.
• Excellent indoor performance.
• Simple and reliable operation.

(2) Key Parameters for PIR Detectors:

• Detection Range: Typically 6–12 meters for standard indoor models.
• Detection Angle: Ranges from 60° to 180°, depending on the lens design.
• Sensitivity Adjustment: Some models offer manual sensitivity tuning to accommodate specific installation environments.

4. Principle of Active Infrared Detectors

Active infrared detectors comprise an emitter and a receiver. The emitter projects a modulated infrared beam toward the receiver, forming an invisible protective barrier. When an intruder crosses the beam, the signal is interrupted or altered, prompting the system to trigger an alarm.

Active infrared systems offer substantial flexibility, as protective grids or lines can be customized to suit different environments. They are ideal for securing perimeters, open spaces, and entranceways.

Types of Active Infrared Detection:

• Single-Beam Detectors: Suitable for short-range or low-security applications.
• Multi-Beam Detectors: Employ multiple infrared beams arranged in a grid pattern, offering higher security levels by making it harder for intruders to bypass detection.

6. What is Dual-Technology Detection?

Dual-technology detectors integrate passive infrared sensing with microwave sensing. Utilizing the Doppler effect, microwaves are emitted at a specific frequency and reflect back at a different frequency when encountering a moving object. By simultaneously analyzing both the infrared and microwave signals, the detector can confirm an intrusion with greater accuracy.

Advantages of Dual-Technology Detectors:

• Significantly reduced false alarm rates.
• Enhanced detection accuracy, particularly in challenging environments.
• Greater flexibility in sensitivity adjustments to match various application needs.

Dual-technology detectors effectively address issues like the “body-temperature effect,” where ambient temperatures closely match human body heat, thus maintaining high sensitivity without sacrificing reliability.

7. What is Triple-Technology Detection?

Outdoor environments pose greater challenges for intrusion detection due to factors like strong winds, intense sunlight, and small animal movements. Triple-technology detectors address these challenges by combining two or more infrared sensors in a dual-view window structure, ensuring that only genuine intrusions trigger alarms.

(1) Features of Triple-Technology Detectors:

• Resistance to high-intensity light (over one million Lux).
• Immunity to strong winds and environmental interferences.
• Extended detection perimeters, enhancing security before intruders reach a building.

(2) Typical Application Scenarios:

• Large open spaces.
• Building perimeters.
• Critical infrastructure facilities.

8. What is Dual-Curtain Technology?

Dual-curtain detectors employ two distinct infrared beams: an inner curtain (A) and an outer curtain (B). When a person moves from inside to outside, Curtain A is triggered first, followed by Curtain B—recognized as normal movement, resulting in no alarm. Conversely, if an intruder approaches from outside, Curtain B activates before Curtain A, immediately triggering an alarm.

Installation Tips for Dual-Curtain Detectors:

• Mount vertically at entry points such as windows and balconies.
• Ensure there are no obstacles blocking the curtain fields.
• Adjust sensitivity based on the expected traffic flow to minimize false alarms.

9. What is Quad-Curtain Technology?

Quad-curtain technology builds upon dual-curtain systems by incorporating advanced digital signal processing (DSP) and sophisticated vector analysis. By analyzing motion vectors more precisely, quad-curtain detectors achieve superior direction recognition, offering enhanced resistance to false alarms and greater overall detection accuracy.

Ideal Applications for Quad-Curtain Detectors:

• Financial institutions.
• Museums.
• Government facilities requiring high-precision perimeter protection.

10. Environmental Considerations for Infrared Detector Installation

Proper installation and environmental awareness are critical to maintaining infrared detector performance:

(1) Active Infrared Detectors:

• Indoor Use: Reliable and stable operation.
• Outdoor Use: Susceptible to degradation from dust, rain, and fog.
• Maintenance: Regularly clean emitter and receiver lenses to maintain signal clarity.
• Mounting: Install on stable surfaces to prevent misalignment due to vibrations.

(2) Passive Infrared Detectors:

• Obstacle Management: Ensure the monitored area is free of solid barriers, as infrared radiation cannot penetrate solid objects effectively.
• Heat Source Avoidance: Avoid placing sensors near heaters, radiators, or windows exposed to direct sunlight.
• Optimal Positioning: Install detectors to maximize lateral movement across their field of view, enhancing detection sensitivity.

(3) Additional Best Practices:

• Use shielded cables for wired installations to reduce electromagnetic interference.
• Employ weatherproof housings for outdoor applications.
• Perform comprehensive walk tests after installation to verify detection range and sensitivity.

11. Installation Standards for Active Infrared Detectors

To achieve effective installation of active infrared detectors, adhere to the following guidelines:

• Install at a minimum height of 20 cm, considering an intruder’s body width.
• Maintain a maximum distance of 20 cm from the tangent point when installing near curved surfaces.
• Ensure that the detector’s center remains within 20 cm of the fence or protected structure edge.
• Mount detectors securely to prevent movement or vibration-induced misalignment.
• Keep the protective area free from obstructive vegetation or debris.
• Precisely align emitters and receivers using optical alignment tools, and periodically recalibrate as needed.

Step-by-Step Installation Procedure:

• Select the optimal mounting location based on environmental conditions.
• Securely install the emitter and receiver on stable surfaces.
• Use a laser or optical tool to achieve precise beam alignment.
• Test beam stability and verify alignment using the detector’s built-in indicators.
• Conduct walk tests across the protected area to ensure consistent and reliable detection.

12. Maintenance Guidelines for Infrared Detectors

Routine maintenance is essential for sustaining peak performance:

• Inspect and clean lens surfaces monthly using a soft, dry cloth.
• Recheck alignment and recalibrate active infrared detectors every six months.
• Test PIR detectors under varying lighting and temperature conditions to ensure stable sensitivity.
• Replace any damaged lenses or sensors promptly to maintain full security coverage.