Fingerprint Sensing Exposure Correction for Faster Image Capture
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Solution Overview
Problem
Existing fingerprint sensing technologies face challenges in quickly and accurately determining the appropriate exposure time for capturing high-quality fingerprint images, especially under varying ambient light conditions, as the auto-exposure process is time-consuming and requires multiple frames for stabilization.
Innovation Solution
A fingerprint sensing device and method that include an analog-front-end (AFE) circuit, analog-to-digital converter (ADC), and correction circuit, which perform multiple fingerprint sensing operations in short preset exposure time periods to generate output digital codes, allowing for quick calculation of optimal exposure time and correction values for subsequent operations, thereby improving image quality and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If auto exposure (AE) process is used to set exposure time for capturing fingerprint image, then the exposure time can be appropriately determined, but the process becomes time-consuming since it requires multiple frames to stabilize the convergence
Solution Approach 1:
The system performs preliminary fingerprint sensing operations with multiple different exposure time periods before the actual fingerprint recognition. These preliminary operations generate reference digital codes that are used to calculate the optimal exposure time and correction values in advance, avoiding the need for time-consuming auto-exposure stabilization during actual recognition.
Solution Approach 2:
The system performs multiple fingerprint sensing operations with exposure time periods that may be longer or shorter than the optimal exposure time. By intentionally using excessive or partial exposure times in the preliminary operations, the system gathers sufficient data to calculate the precise optimal exposure time and correction values, ensuring high-quality fingerprint capture in subsequent operations.
2Measurement precision
If multiple frames are used to stabilize convergence in AE process, then the exposure time determination becomes accurate, but the fingerprint recognition speed decreases
Solution Approach 1:
The system performs multiple fingerprint sensing operations with different exposure time periods as preliminary actions before actual recognition. These operations establish the convergence relationship between exposure time and digital code values, allowing the system to quickly determine optimal parameters without requiring multiple frames during the actual recognition process.
Solution Approach 2:
The system uses the digital codes obtained from preliminary fingerprint sensing operations to automatically calculate its own optimal exposure time and correction values. This self-service approach eliminates the need for external auto-exposure control processes and enables rapid parameter determination for subsequent high-speed fingerprint recognition.
3Reliability
If correction values are calculated based on multiple preliminary sensing operations, then the fingerprint sensing quality improves under varying light conditions, but the initial processing time increases
Solution Approach 1:
The system performs preliminary fingerprint sensing operations with multiple different exposure time periods to gather reference data. Based on these preliminary results, the system calculates correction values that compensate for variations in ambient light conditions. Although this initial processing takes time, it enables stable and reliable fingerprint sensing in subsequent operations under varying light conditions.
Solution Approach 2:
The system changes the exposure time period parameter in preliminary operations to collect data across different exposure conditions. By analyzing how digital code values change with different exposure times, the system calculates correction values that adapt to ambient light conditions, improving the reliability of fingerprint sensing without requiring continuous parameter adjustments during actual recognition.
Data Source
AI summary
A fingerprint sensing device that includes an analog-front-end (AFE) circuit, an analog-to-digital converter (ADC) and a correction circuit is introduced. The AFE circuit generates an image signal, and the ADC converts the image signal to an output digital code. The correction circuit receives a plurality of first output digital codes that are generated by performing a plurality of first fingerprint sensing operations in a plurality of first exposure time periods. The correction circuit is further configured to calculate a second exposure time period for a second fingerprint sensing operation according to the first output digital codes and the first exposure time periods, wherein the fingerprint sensing device performs the second fingerprint operation in the second exposure time period to generate a second output digital code.