Thin Wedge Prism Internal Surface Reflection for Fingerprint Imaging

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Solution Overview

Problem

Traditional fingerprint scanners using total internal reflection in wedge-shaped prisms suffer from foreshortening of the captured image, which is not effectively addressed by existing remedies such as cylindrical lenses or software processing.

Innovation Solution

A thin wedge prism design that operates on internal surface reflections instead of total internal reflection, allowing the optical path to be perpendicular to the platen surface, thereby minimizing foreshortening while maintaining sufficient contrast.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If total internal reflection is used in a wedge-shaped prism to capture fingerprint images, then the optical path can be configured to capture the image, but the captured image suffers from foreshortening

Engineering Contradiction:

Improveimage accuracyVSAvoidimage distortion

Core Design Contradiction:

Measurement precisionVSShape

Solution Approach 1:

The patent changes the optical mechanism from total internal reflection to internal surface reflection, and modifies the prism geometry parameters (thin wedge shape with specific angle relationships) to eliminate foreshortening while maintaining image capture capability

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

Instead of using total internal reflection which causes foreshortening, the patent inverts the approach by using internal surface reflection with a differently configured thin wedge prism, thereby solving the foreshortening problem while achieving the same image capture function

Inventive Principle:

Principle #13The other way round (Inversion)

2Measurement precision

If a standard wedge-shaped prism is used with total internal reflection, then image capture is achieved, but contrast is insufficient

Engineering Contradiction:

Improveimage contrastVSAvoidimage quality

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The patent changes the reflection mechanism from total internal reflection to internal surface reflection and adjusts the prism geometry parameters to optimize both contrast and image quality simultaneously

Inventive Principle:

Principle #35Parameter changes

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 significantly reduces image foreshortening and enhances contrast, providing a more accurate and cost-effective method for capturing fingerprint images without the need for additional hardware or complex software processing.

Implementation Method 1

the apparatus operates on the principle of internal surface reflections instead of total internal reflection (TIR)

Methodology Applied

Scientific EffectInternal surface reflection: Reflection

Data Source

PatentUS7747046B2Apparatus and method for obtaining images using a prism

Publication Date: 2010.06.29 HID GLOBAL CORP

AI summary

An improved apparatus and method for obtaining images through a prism are provided. In an embodiment, a thin fingerprint prism wedge, with a geometry designed to minimize foreshortening and maximize contrast, is provided in an optical path between an optical sensor and an object to be imaged. In some embodiments, the apparatus operates on the principle of internal surface reflections instead of total internal reflection (TIR).