Thermochromic Layer for Optical Fingerprint Sensor Noise Reduction
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Solution Overview
Problem
Existing flat optical fingerprint sensors face challenges in illuminating the finger without directly illuminating the sensor pixels, leading to luminous background noise that degrades detection performance due to the proximity of the finger to the sensor.
Innovation Solution
A device with a layer of material whose color varies based on physical parameters, such as temperature, is used on the sensor surface, allowing the finger to be placed in contact or quasi-contact to create a colored pattern that persists long enough for image acquisition, enabling illumination through this layer to reach the sensor pixels without direct illumination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the finger is placed in contact or quasi-contact with the sensor during acquisition, then the fingerprint pattern can be captured, but the finger directly illuminates the sensor pixels causing luminous background noise that degrades detection performance
Solution Approach 1:
The patent introduces an intermediary layer between the finger and sensor pixels consisting of light-guiding elements (prisms or microlenses) that redirect incident light. This intermediary structure guides light from the finger toward the sensor pixels through total internal reflection or focal concentration, while preventing direct illumination of the pixels by the finger, thereby reducing luminous background noise while maintaining measurement precision
Solution Approach 2:
The patent adds a vertical dimension to light guidance by incorporating light-guiding elements with specific geometric structures (prisms with angled surfaces or microlenses with focal points) that redirect light paths in three-dimensional space. This dimensional transformation allows light to reach the sensor pixels from the finger contact area without direct line-of-sight illumination, resolving the contradiction between contact-based pattern capture and noise reduction
2Illumination intensity
If a light source is placed next to the finger to illuminate it, then the fingerprint can be visualized, but the substrate must be non-transparent requiring additional optical components like plane waveguides
Solution Approach 1:
The patent makes the substrate serve multiple functions: it acts as both the structural support for the sensor pixels and as a light-guiding element through its transparent properties. The transparent substrate guides light from the illumination source through total internal reflection to the finger contact area, eliminating the need for separate non-transparent substrates and additional waveguide components, thereby reducing device complexity while maintaining illumination intensity
Solution Approach 2:
The patent merges the substrate function with the light-guiding function by making the substrate transparent and designing it to utilize total internal reflection for light guidance. This combination integrates the structural support role and optical guidance role into a single component, eliminating the need for separate plane waveguides and reducing overall optical system complexity
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 approach simplifies sensor illumination management, reduces background noise, and enhances detection performance by converting the finger's patterns into a colored representation that can be accurately captured by the sensor.
Implementation Method 1
a layer of a material whose color varies according to a physical parameter characteristic of the body, coating one face of the sensor
Implementation Method 2
an image sensor comprising a plurality of photosensitive cells
Data Source
Figure 1~2C
Figure 3~5
AI summary
The invention relates to a device for acquiring a characteristic image of a body (201), comprising: an image sensor (101) having a plurality of photosensitive cells (103); and a layer (107) of a material whose color varies according to a physical parameter characteristic of said body (201), covering a face of the sensor (101).