Conical Light Trap for Smoke Detector Stray Light Control
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Solution Overview
Problem
Scattered-light smoke detectors face challenges in reducing stray light reflections from chamber walls and external sources, which increase over time due to dust buildup and can lead to false alarms, especially in dusty environments.
Innovation Solution
The smoke detector employs an optical bench assembly with a prism and lens to redirect the light beam directly onto the photodetector, and a light trap with a tapered cone shape to absorb unscattered light, minimizing reflections and dust accumulation, while a serpentine air path entry system prevents external light and large dust particles from reaching the detector.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the chamber walls are made black to reduce reflections, then stray light is reduced, but dust accumulation on the walls increases reflected light over time
Solution Approach 1:
The harmful function of the chamber walls (reflecting light) is extracted and transferred to a dedicated light trap component. The light trap is a separate structure with highly reflective internal surfaces that actively directs stray light away from the detection path, while the main chamber walls can be simpler black surfaces that guide smoke to the detector without bearing the burden of preventing all reflections.
Solution Approach 2:
A light trap intermediary structure is introduced between the light source and photo detector. This light trap acts as a mediator that captures and redirects stray light through reflective surfaces and geometric design, preventing it from reaching the detector while allowing smoke particles to pass through to the sensing region.
2Adaptability or versatility
If air passages are opened to allow smoke entry, then smoke detection is enabled, but external light can enter through the same passages
Solution Approach 1:
The air intake system is segmented into separate functional zones: external air passages with light-blocking design that prevent light entry while allowing smoke ingress, and an internal chamber region where the optical detection occurs. The light trap is positioned to specifically address the interface between these zones, capturing any light that might leak through the air passages before it reaches the detector.
3Object-affected harmful factors
If a narrow beam light source is used to reduce reflections, then stray light is reduced, but the device complexity increases
Solution Approach 1:
Instead of trying to eliminate reflections through complex optical elements, the invention uses the reflective property to benefit by creating a light trap with highly reflective internal surfaces. The light trap converts stray light reflections into a useful function by systematically directing reflected light away from the detection path through controlled reflections off the trap's internal surfaces.
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 configuration ensures efficient smoke detection with reduced stray light reflections and prolonged functionality in dusty environments by minimizing dust-induced false alarms and maintaining signal integrity.
Implementation Method 1
The light trap causes any light incident on its inner surface to be reflected at least twice, preferably at least three times, more preferably at least four times, most preferably at least five times before the light can leave the light trap
Implementation Method 2
When the aerosol contains sufficient concentrations of smoke particles, the light emitted by the source is scattered by the smoke, a small portion of which will be incident on the photo detector
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
A smoke detector comprising a light source for generating a beam of light and arranged to transmit the beam along an optical axis; a sensor for receiving light scattered from the beam off the optical axis by smoke; and a light trap comprising one or more walls that define a volume for receiving light that passes unscattered from the light source past the sensor along the optical axis, where said volume comprises a first end closest on the optical axis to the light source; and a second end furthest on the optical axis from the light source; wherein said volume is open at said first end but is otherwise closed, and wherein at least one of the walls is sloped from the first end to the second end, such that the light trap progressively narrows from the first end axially towards the second end.