Optical Finger Navigation Speckle Path Extension
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Solution Overview
Problem
Conventional optical finger navigation (OFN) sensors face challenges in maintaining accuracy and size constraints due to the use of incoherent or coherent illumination, which affects speckle pattern contrast and pixel size compatibility, especially in thin form factors where the speckle size is too small relative to pixel pitch, leading to reduced contrast and increased device thickness.
Innovation Solution
An optical finger navigation system that includes an image sensor, a tracking surface, and an optical element with a redirection surface to extend the optical path, allowing for a sufficient speckle size to be produced at the image sensor, independent of the device thickness, thereby enabling thinner form factors and maintaining high accuracy without additional optical elements or increased thickness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If coherent illumination is used to produce speckle patterns for tracking finger movement, then speckle pattern tracking capability is improved, but the average speckle size becomes too small relative to pixel pitch in thin sensors, reducing contrast and tracking accuracy
Solution Approach 1:
The patent introduces a lateral optical path extension using a light guide structure that redirects light laterally instead of vertically. This transforms the optical path from a single-dimensional vertical distance to a multi-dimensional path that travels laterally through the light guide, effectively increasing the optical distance without increasing the vertical thickness of the sensor assembly.
Solution Approach 2:
The patent introduces a light guide as an intermediary component between the light source and the detector. This light guide mediates the optical path by redirecting coherent light laterally to traverse a longer distance before reaching the detector, thereby enabling sufficient speckle size generation while maintaining a thin overall sensor profile.
2Measurement precision
If the optical distance between tracking surface and detector is increased to produce larger speckles, then speckle size increases above pixel pitch threshold, but device thickness increases
Solution Approach 1:
The patent redirects the optical path from a vertical dimension to a lateral dimension using the light guide structure. The coherent light travels laterally through the light guide, accumulating optical path length without increasing the vertical thickness of the device. This dimensional transformation allows sufficient speckle size generation while maintaining a thin device profile.
3Measurement precision
If additional optical elements (lenses, apertures) are added to work with incoherent light sources, then imaging capability is improved, but overall sensor size increases
Solution Approach 1:
The patent extracts and eliminates the need for additional optical elements by using coherent illumination directly. The coherent light source inherently produces speckle patterns that can be detected without requiring extra lenses or apertures, thereby simplifying the optical system and reducing sensor size while maintaining imaging capability.
Solution Approach 2:
The patent makes the coherent light source serve multiple functions: it provides illumination and simultaneously generates the speckle pattern needed for tracking. This multi-functionality eliminates the need for separate optical elements that would be required with incoherent light sources, reducing device 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
The system achieves a thinner form factor while maintaining high accuracy by lengthening the optical path through the optical element, allowing for adequate speckle size detection with standard pixel dimensions, reducing sensitivity to component tolerance and alignment, and simplifying fabrication and implementation.
Implementation Method 1
The optical element includes at least one optical redirection surface to redirect light along an optical path of a second distance between the tracking surface and the image sensor
Implementation Method 2
The propagation of the coherent beam from different locations of a diffusing surface such as a tracking surface produces the speckle pattern on the detector
Data Source
AI summary
An optical finger navigation device includes an image sensor, a tracking surface, and an optical element. The image sensor generates an image representative of light detected at a pixel array that has a pixel pitch which at least partially defines a surface area of a pixel. The tracking surface is approximately parallel to the image sensor. A first distance between and substantially orthogonal to planes of the tracking surface and the image sensor is insufficient under a set of illumination constraints to produce at the image sensor a speckle size above a threshold. The threshold is approximately one order of magnitude less than the pixel pitch. The optical element is interposed between the tracking surface and the image sensor. The optical element includes at least one optical redirection surface to redirect light along an optical path of a second distance between the tracking surface and the image sensor.