Passive Infrared Detector Aperture Stop Fresnel Lens
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Solution Overview
Problem
Passive infrared detectors with broad fields of view often trigger nuisance alarms due to detecting innocent passersby or street traffic, and narrowing the field of view to reduce these alarms results in poor detection performance and blind spots, especially near the lens, with existing solutions being costly and complex.
Innovation Solution
A compact, low-cost passive infrared detector configuration using a dual-element pyroelectric detector with an aperture stop and a Fresnel lens array of reasonable focal length, minimizing nuisance alarms while maintaining detection performance by creating a narrow sheet of protection across openings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the focal length of the lens is increased to narrow the field of view, then nuisance alarms are reduced, but detection performance deteriorates and blind spots appear near the lens
Solution Approach 1:
The patent divides the optical system into multiple components: a primary lens for focusing and an aperture stop positioned at the focal plane to segment the field of view. This segmentation allows the system to maintain a reasonable focal length while creating a narrow detection zone by blocking specific angular ranges, thus reducing nuisance alarms without creating blind spots.
Solution Approach 2:
The aperture stop acts as an intermediary element positioned at the focal plane of the lens. It mediates between the broad field of view captured by the lens and the narrow detection zone required to avoid nuisance alarms, allowing selective passage of infrared energy from specific angular ranges while maintaining proper focus on the detector.
2Object-affected harmful factors
If multiple pyroelectric detectors and sophisticated circuitry are used to achieve a narrow detection zone, then nuisance alarms are reduced, but device complexity and cost increase
Solution Approach 1:
The patent extracts the field of view control function from the detector array and circuitry approach and implements it through a simple aperture stop in the optical path. This removes the need for multiple detectors and complex electronic processing while achieving the same narrow detection zone effect through optical means alone.
Solution Approach 2:
The patent replaces the mechanical/electronic approach of using multiple detectors and sophisticated circuitry with an optical solution using a lens and aperture stop. This substitution simplifies the system by using passive optical elements instead of active electronic components to achieve narrow detection zone control.
3Device complexity
If a single cylindrical lens is used with a dual-element pyroelectric detector, then the system is simple and low-cost, but the field of view is overly broad causing nuisance alarms
Solution Approach 1:
The patent introduces an aperture stop that can be dynamically adjusted or positioned to control the field of view. This allows the simple single-lens system to achieve variable detection zones, enabling the user to narrow the field of view to eliminate nuisance alarms while maintaining system simplicity and low cost.
Solution Approach 2:
The aperture stop serves as an intermediary element added to the simple single-lens system. It mediates between the simple optical structure and the requirement for a narrow field of view, allowing the system to maintain its simplicity while effectively reducing nuisance alarms through controlled angular acceptance.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The configuration effectively reduces nuisance alarms while maintaining detection performance by using an aperture stop and Fresnel lens array, providing a narrow field of view without the high cost and complexity of multiple detectors or sophisticated circuitry.
Implementation Method 1
a Fresnel lens array disposed on the second side of the window
Implementation Method 2
optics to focus thermal infrared energy
Implementation Method 3
an aperture stop disposed on the second side of the window
Implementation Method 4
a pyroelectric detector onto which the optics focus
Implementation Method 5
detecting the movement of an infrared emitting body
Data Source
AI summary
An infrared detector (301) is provided which comprises a pyroelectric detector (303) having first and second sensing elements (345), an aperture stop (311), and a Fresnel lens array (305). The detector may be used as a passive infrared sensor unit for detecting intrusion into large openings.