NDIR Sensor Optical Cavity Using High-Reflectivity End Mirrors
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Solution Overview
Problem
Conventional non-dispersive infrared (NDIR) sensors face challenges in achieving compactness and low power consumption while maintaining optical efficiency, particularly in reducing the thickness of the optical cavity without compromising light source size or increasing electrical consumption.
Innovation Solution
The design incorporates reflective elements with a high reflection coefficient (>75%) at the ends of the optical cavity, allowing the light source to be positioned between these elements, and using a waveguide to guide infrared light along the thickness of the cavity, eliminating the need for internal mirrors and optimizing light source placement for improved efficiency and compactness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the thickness of the optical cavity is reduced to achieve compactness, then the overall sensor size decreases, but the light source size must be reduced which leads to loss of optical efficiency
Solution Approach 1:
The patent guides infrared light along the thickness direction (Z-axis) of the optical cavity using reflective elements, transforming the light propagation path from a planar configuration to a three-dimensional one. This allows the light to traverse the cavity thickness multiple times, increasing the interaction length with gas molecules without increasing the planar footprint, thereby achieving compactness while maintaining optical efficiency
Solution Approach 2:
The patent employs reflective elements with curved surfaces (parabolic or hyperbolic shapes) to guide and focus infrared light along the cavity thickness. The curved geometry enables efficient light redirection and multiple passes through the gas sample, maximizing optical path length within a compact volume without requiring a larger light source
2Area of stationary object
If conventional mirrors are used to guide light in the optical cavity, then the light path can be folded to reduce planar size, but the thickness of the optical cavity remains large due to the light source dimensions
Solution Approach 1:
Instead of folding the light path in the planar direction (X-Y plane) as in conventional designs, the patent utilizes the thickness direction (Z-axis) of the cavity by positioning reflective elements at the ends of the cavity. This three-dimensional light guiding approach allows the cavity to be thin in the planar direction while maintaining sufficient optical path length through multiple reflections along the thickness direction
3Loss of energy
If the power of the light source is increased to compensate for reduced source size, then optical efficiency can be maintained, but electrical consumption increases
Solution Approach 1:
The patent implements multiple light passes through the gas sample by positioning reflective elements to guide infrared light back and forth along the cavity thickness. This continuous recycling of light photons through multiple interactions with the gas maximizes the useful optical action, allowing a low-power light source to achieve the same detection sensitivity that would otherwise require a high-power source
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 enhances optical efficiency and reduces the thickness of the NDIR sensor, achieving compactness without increasing power consumption, while maintaining effective gas detection capabilities.
Implementation Method 1
first and second reflective elements extending respectively to the first and second ends of the optical cavity, each of the first and second elements having a reflective surface, and having a reflection coefficient of infrared light greater than or equal to 75% for any angle of incidence
Implementation Method 2
a light source arranged to emit infrared light into the optical cavity
Implementation Method 3
the gas inside the optical cavity absorbs infrared light at a wavelength (on a spectral band) specific to the gas to be detected
Data Source
Figure 1~2
Figure 3~4
Figure 5~6
AI summary
This sensor comprises an optical cavity adapted to receive the gas, and delimited by first and second opposite ends and a connecting part (6a) linking said ends; a light source (2) arranged to emit infrared light into the optical cavity; at least one infrared detector (3a, 3b) arranged to detect the infrared light; at least one mirror (4b) arranged in the optical cavity to guide the infrared light towards said at least one infrared detector (3a, 3b); the sensor being notable in that it comprises first and second reflective elements (5a, 5b) extending respectively to the first and second ends of the optical cavity, and having a coefficient of infrared light reflection greater than or equal to 75% for any angle of incidence.