Side-Illuminated Fingerprint Sensor Using Total Internal Reflection
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Solution Overview
Problem
Optical fingerprint identifying technologies in liquid crystal display devices face challenges due to low light energy transmission through liquid crystal layers and interference from ambient light, making it difficult to accurately differentiate between fingerprint troughs and peaks.
Innovation Solution
A fingerprint identifying device with a transparent glass cover plate having an inclined side surface and a detection light source, where the light is totally reflected when the cover plate is in contact with air, and the photoelectric sensor receives the reflected light, ensuring a greater intensity difference between light from troughs and peaks, thereby improving accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If optical fingerprint identifying technology is implemented through liquid crystal display devices, then the identifying range is relatively large and cost is relatively low, but light energy transmission is low and ambient light interference occurs making it difficult to accurately differentiate fingerprint troughs and peaks
Solution Approach 1:
The patent introduces a new spatial dimension by placing the light source and sensor at the side surface of the transparent cover plate rather than transmitting light through the liquid crystal layer. This side-illumination approach eliminates the need for light to pass through multiple liquid crystal layers, thereby solving the low light energy transmission problem while maintaining measurement precision through total internal reflection at the cover plate-finger interface.
Solution Approach 2:
The patent extracts the fingerprint detection function from the liquid crystal display structure by using the transparent cover plate as an independent optical element. The cover plate with its specific refractive index (1.55) and inclined side surface (45 degrees) serves as the primary detection component, separating the detection mechanism from the display layers and eliminating ambient light interference issues.
2Measurement precision
If a transparent cover plate with inclined side surface is used for total internal reflection, then light intensity difference between troughs and peaks increases improving accuracy, but device structure becomes more complex
Solution Approach 1:
The transparent cover plate serves multiple functions: it acts as both the display device's protective cover and the fingerprint sensor's optical element. The inclined side surface (45 degrees) enables total internal reflection for fingerprint detection while the top surface maintains transparency for display purposes. This multi-functionality increases measurement precision without significantly adding device complexity.
Solution Approach 2:
The patent specifies precise parameters for the cover plate: refractive index of 1.55 and side surface inclination angle of 45 degrees. These parameter changes optimize total internal reflection efficiency, creating sufficient light intensity difference between fingerprint troughs and peaks. The standardized parameters balance measurement precision improvement with manufacturing feasibility.
3Use of energy by moving object
If the detection light source is positioned on the side surface of the transparent cover plate, then light energy transmission is improved and ambient light interference is reduced, but the device structure and light path design become more complex
Solution Approach 1:
The light source is repositioned from the traditional backlight position to the side surface of the transparent cover plate. This dimensional change in light source placement enables direct side-illumination of the finger, improving light energy transmission to the detection point. The light path travels through air and the cover plate side surface, avoiding multiple passes through liquid crystal layers, thereby reducing ambient light interference.
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 design enhances the accuracy of fingerprint identification by increasing the light intensity received by the sensor and reducing interference, leading to improved recognition rates.
Implementation Method 1
the light beam is incident to the upper surface of the transparent cover plate through the side surface of the transparent cover plate, and is totally reflected on the upper surface of the transparent cover plate to reach the photoelectric sensor
Implementation Method 2
a refractive index of the transparent cover plate is less than a refractive index of a skin of a human fingerprint part so that the light beam is reflected and refracted in a case that the upper surface of the transparent cover plate is in contact with a ridge of a finger
Data Source
Figure 1~2a
Figure 2b~2c
Figure 2d~3
AI summary
A fingerprint identifying device and a fingerprint identifying system are provided. The fingerprint identifying device including: a substrate; a transparent cover plate, located right above the substrate; a detection light source, located on a first side of the transparent cover plate; and a photoelectric sensor, located on a side of a lower surface of the transparent cover plate, wherein, a refractive index of the transparent cover plate is less than a refractive index of a skin of a human fingerprint part, the transparent cover plate has a side surface on the first side, and an angle α between the side surface and an upper surface of the transparent cover plate is an acute angle, the detection light source is configured to provide a light beam towards the side surface of the transparent cover plate, so that the light beam is incident to the upper surface of the transparent cover plate through the side surface of the transparent cover plate, and the light beam is totally reflected on the upper surface of the transparent cover plate in a case that the upper surface of the transparent cover plate is in contact with air, the photoelectric sensor is configured to receive a light beam reflected from the upper surface of the transparent cover plate. The fingerprint identifying device is capable of improving the identifying accuracy thereof.