Optical Fingerprint Sensor Layout With Thick Protective Glass
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Solution Overview
Problem
Fingerprint sensors with optical image capture face challenges in integrating light sources and photo-detectors due to the need for a short distance between the finger and the sensor, limiting the use of protective elements and requiring optical guidance means, which are technologically and economically restrictive.
Innovation Solution
An optical imaging device with a matrix of point light sources and interlaced photo-detectors, using a thick optical medium for light propagation and protection, allowing light to be emitted isotropically and reflected to the photo-detectors, enabling image capture without near-field proximity and dedicated magnification optics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thick protective element is interposed between the sensor and the finger, then protection of the sensor's electronics is improved, but the near-field configuration requirement cannot be met
Solution Approach 1:
The patent transitions from a planar near-field configuration to a three-dimensional far-field configuration by introducing a thick optical medium (glass plate) that enables light propagation in the vertical dimension. This allows the photodetector array to be positioned on the rear face of the glass plate while the finger contacts the front face, creating sufficient physical separation for protection while maintaining optical functionality through the glass medium.
Solution Approach 2:
The glass plate serves as an intermediary optical medium that transmits light between the finger and the photodetector array while providing mechanical protection. The patent specifically mentions using a glass plate with refractive index n=1.5 and thickness greater than 50 μm (preferably >100 μm) as the protective element that enables far-field operation by mediating the optical interaction while physically separating the finger from the sensor electronics.
2Reliability
If optical guiding means are used to guide light between the finger and sensor, then the protective element requirement is met, but technological and economic complexity increases
Solution Approach 1:
The patent extracts and eliminates the need for complex optical guiding means (such as optical fibers or specialized lenses) by using the glass plate's inherent optical properties. Instead of adding dedicated light guidance components, the solution relies on the glass plate's ability to transmit and refract light, thereby removing unnecessary technological complexity while maintaining protection functionality.
Solution Approach 2:
The glass plate performs multiple functions simultaneously: it serves as the protective cover for the sensor electronics, provides the optical medium for light propagation, and enables the far-field configuration. This multi-functionality eliminates the need for separate optical guiding components, reducing both technological and economic complexity.
3Measurement precision
If the photodetector pitch is reduced to achieve higher resolution, then image resolution is improved, but the optical medium thickness requirement becomes more restrictive
Solution Approach 1:
The patent changes the optical parameters by introducing a glass medium with refractive index n=1.5, which modifies the light propagation characteristics. This parameter change allows the system to achieve high resolution with larger photodetector pitches because the refractive medium enhances light collection efficiency and maintains optical resolution despite the increased physical thickness of the protective glass plate.
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 achieves higher image resolution, such as 2000 ppi or greater, with improved protection and no loss of precision, using standard protective glass and reducing the need for thinning, while providing high contrast and compatibility with CMOS or OLED technology.
Implementation Method 1
an angle of incidence with a value between, or equal to, that of a first critical angle of refraction defined by the optical index of the optical medium and the optical index of the atmosphere
Implementation Method 2
light rays from the switched-on light source and having undergone at least one reflection against the capture surface
Implementation Method 3
an array of photodetectors fabricated on the substrate and interleaved with the array of light sources
Data Source
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AI summary
Optical imaging device (100) comprising: - an array of substantially point-like light sources (104), - an array of photo-detectors (106) interlaced with the array of light sources, - an optical medium of greater thickness than the pitch of the photo-detectors and forming capture and detection surfaces, - means for successively turning on and off each of a part of the light sources and reading a part of the photo-detectors receiving rays from the lit source and having undergone reflection against the detection surface by forming an angle of incidence estimated as a function of the optical medium and the element to be imaged.