Fresnel Light Trap Structures for Smoke Detection Backscatter Reduction
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Solution Overview
Problem
Existing smoke detection units with measurement chambers suffer from excessive backward scattered light, which interferes with the photodiode's ability to detect smoke reliably due to the extreme backward scattered light arrangement, causing the light intensity to exceed the alarm level and rendering reliable smoke detection impossible.
Innovation Solution
The implementation of Fresnel light trap structures within the measurement chamber, which are designed to follow the compact-design principle of a Fresnel stepped lens, aligning the Fresnel axis with the optical transmission axis of the light emitter and photosensor to reduce the backscatter cross-section and position the Fresnel point at the light exit of the emitter, thereby minimizing the basic pulse interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional light-absorbing angular structures are used in the measurement chamber, then the chamber provides shielding from direct ambient light, but the backward scattered light from LEDs exceeds the alarm level and interferes with smoke detection
Solution Approach 1:
The measurement chamber cover is segmented into multiple concentric Fresnel light trap structures (first, second, and third structures) with different geometric configurations. Each segment serves to trap light from specific angles, collectively reducing backward scattered light while maintaining ambient light shielding.
Solution Approach 2:
The patent introduces a vertical dimension by positioning the Fresnel light trap structures at different heights above the circuit board (first structure at 5-15mm, second at 15-25mm, third at 25-35mm). This multi-level arrangement creates overlapping light traps that effectively intercept backward scattered light paths without blocking ambient light from reaching the photodiode.
2Measurement precision
If the measurement chamber is designed with extreme backward scattered light arrangement (160°), then smoke detection sensitivity is maximized, but the basic pulse from LED reflections overwhelms the photodiode signal
Solution Approach 1:
The Fresnel light trap structures act as intermediary elements between the LEDs and the photodiode. These structures intercept and trap backward scattered light before it reaches the photodiode, allowing the extreme backward scattered light arrangement to maintain its smoke detection sensitivity while eliminating the overwhelming basic pulse interference.
Solution Approach 2:
The patent converts the harmful backward scattered light into a beneficial effect by using it to illuminate the Fresnel light trap structures, which then trap and redirect the light away from the photodiode. The light that would otherwise be harmful interference is instead used to fill the measurement chamber with diffusely scattered light that enhances smoke detection while maintaining a clean signal.
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
This design significantly reduces the backscatter cross-section and basic pulse interference, enhancing the reliability of smoke detection by aligning the Fresnel light trap structures with the optical axes of the light emitter and photosensor, allowing for effective smoke detection without overwhelming the photodiode with excessive light levels.
Implementation Method 1
angular, sharp-edged, light trap structures (FL) are shaped in such a way that they follow the compact-design principle of a Fresnel stepped lens (FLI)
Implementation Method 2
the (Fresnel) light trap structures (FL), analogous to a focal point (BP) of a Fresnel lens (FLI), have a Fresnel axis (FA) and are aligned on a common Fresnel point (FP)
Data Source
AI summary
Various embodiments of the teachings herein include a measurement chamber for a smoke detection unit of a smoke detector. The measurement chamber may include: at least a passage for smoke to be detected; and a measurement chamber cover with angular light trap structures on an inside of the measurement chamber. The angular light trap structures are shaped to follow the compact-design principle of a Fresnel stepped lens.