Thin Sheet Papillary Print Sensor Ghost Image Elimination
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Solution Overview
Problem
Conventional papillary print sensors face bulkiness and manufacturing challenges due to the need for thick sheets to prevent ghost images, which limits their thinness and bulkiness and complicates their production.
Innovation Solution
A papillary print sensor with a sheet of reduced thickness, where light rays are propagated without reflection on the acquisition surface for a first part and reflected towards it for a second part, allowing separate acquisition times for each to prevent ghost images, and an imager acquires images during these times to combine them into a final image.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the sheet thickness is increased to prevent ghost images, then the reliability of fingerprint acquisition is improved, but the sensor becomes bulkier and more difficult to manufacture
Solution Approach 1:
The patent applies periodic action by using time-gated acquisition to separate ghost images from real fingerprint images. The system acquires images at specific time intervals after light emission, exploiting the time delay difference between direct light paths and reflected ghost paths. This temporal separation allows the sensor to eliminate ghost images while maintaining thin sheet dimensions.
Solution Approach 2:
The patent implements preliminary action by pre-calculating and pre-defining acquisition time windows that correspond to expected light arrival times. The system prepares timing gates in advance based on the sheet thickness and optical path characteristics, allowing it to selectively capture only the desired light signals before ghost images can interfere with the acquisition.
2Volume of moving object
If the sheet thickness is reduced to make the sensor thinner, then the sensor compactness is improved, but ghost images appear in the fingerprint acquisition
Solution Approach 1:
The patent uses periodic time-gated acquisition to distinguish between light rays that have undergone reflection (ghost images) and those that have not. By acquiring images at specifically timed intervals corresponding to the expected arrival times of direct light paths, the system can ignore or exclude the later-arriving ghost images, thus eliminating this harmful effect while maintaining thin sheet design.
Solution Approach 2:
The patent replaces the mechanical solution of increasing sheet thickness to prevent ghost images with an optical-temporal solution. Instead of relying on physical thickness to block reflected light paths, the system uses time-resolved detection to selectively capture only the desired light signals, substituting a mechanical prevention approach with a temporal discrimination approach.
3Reliability
If the sheet thickness is increased to prevent light ray reflection issues, then the optical propagation reliability is improved, but the manufacturing complexity increases
Solution Approach 1:
The patent changes the critical parameter from sheet thickness to acquisition timing. Instead of controlling manufacturing parameters (thickness) to ensure proper optical propagation, the system controls operational parameters (acquisition time gates) to achieve the same reliability. This shifts the control from the manufacturing domain to the operational domain, simplifying manufacturing requirements.
Solution Approach 2:
The patent implements preliminary action by pre-defining acquisition time windows during the design phase based on expected optical path characteristics. This allows the system to accommodate variations in sheet thickness within a broader manufacturing tolerance range, as the timing gates can be adjusted or designed to work with a range of thickness values, thereby easing manufacturing constraints.
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
The solution enables a thinner, less bulky sensor that effectively acquires fingerprint images without ghost images, simplifying manufacturing and improving sensor design.
Implementation Method 1
the sheet being adapted for the propagation of light rays from the acquisition surface to the outlet by reflection on the first face and the second face
Data Source
AI summary
The invention relates to a papillary print sensor comprising an acquisition surface (14) extending over a length L, a light source (11) configured to emit a light pulse, a sheet (12) adapted to propagate light rays by reflection on the first face (12a) and the second face (12b), which defines a critical angle (θc), and an imager (13), wherein the thickness (e) of the sheet is less than a thickness emax=L/2×tan(θc), such that a first part of the light rays (20b, 20c) is propagated without reflection on the acquisition surface (14) while a second part of the light rays (20a, 20d) is reflected towards said acquisition surface (14) after reflection on the second face (12), and the imager (13) is configured to acquire a first image during the reception of the first part of the light rays and a second image during the reception of the second part of the light rays.