Introduction
Curtain lenses, also known as long-range or corridor lenses, are specialized Fresnel lenses that create a narrow, elongated detection pattern. They are essential for applications requiring long-distance detection.
How Curtain Lenses Work
Unlike wide-angle lenses that spread zones across a broad area, curtain lenses concentrate detection zones into a narrow “curtain” – typically a vertical sheet extending outward. This creates a virtual wall that detects anyone passing through it.
Zone Pattern
A typical curtain lens creates multiple detection zones arranged vertically:
- Near zones: Close to the sensor (few meters).
- Mid zones: Intermediate distances.
- Far zones: Maximum range (up to 30-40 meters).
As a person walks through the curtain, they sequentially cross these zones, generating a characteristic signal.
Advantages of Curtain Lenses
- Long range: Up to 30-40 meters, compared to 10-15m for wide-angle.
- Narrow beam: Reduces false triggers from side movement.
- Defined crossing point: Ideal for perimeter security.
- Direction detection: With two sensors, can detect crossing direction.
Applications
Perimeter Security
Curtain sensors mounted on walls or fences create an invisible barrier. Anyone climbing over or cutting through triggers the alarm.
Hallways and Corridors
A single curtain sensor can cover a long hallway, detecting anyone walking through.
Doorways and Gates
Creating a detection curtain across an entrance.
Industrial Conveyors
Detecting objects passing on a conveyor belt.
Choosing a Curtain Lens
When selecting a curtain lens, consider:
- Range required: Lenses are specified for maximum distance.
- Beam width: Typically 5-15° horizontal.
- Number of zones: More zones provide better coverage but may increase cost.
- Compatibility: Must match your sensor’s package (TO-5, SMD, etc.).
Installation Tips
- Mount at appropriate height (typically 2-3m).
- Aim the curtain across the protected area, not along it.
- Ensure no obstructions in the curtain path.
- Test with a person walking through at various points.
Limitations
- Narrow field means multiple sensors needed for wide areas.
- May miss motion that doesn’t cross the curtain (e.g., someone already inside).
- Requires precise alignment.
Case Study: Warehouse Perimeter
A warehouse installed curtain sensors every 10 meters along the perimeter wall. Each sensor created a vertical curtain. Intruders climbing the wall were detected with 98% reliability.
Conclusion
Curtain lenses extend the range of PIR sensors dramatically, making them suitable for perimeter security and long corridor applications. Proper installation is key to performance.