From hand-swipe payments to smart access control, palmprint scanners are reshaping the landscape of identity authentication with their high precision and anti-counterfeiting capabilities. The realization of this technology is essentially a sophisticated optical imaging process, its core lying in how to clearly capture the feature information of the palm’s surface and subcutaneous tissue through optical lenses and optical path design.
I. Technical Classification and Optical Requirements of Palmprint Scanners
1.1 Contact and Non-Contact: Two Mainstream Technical Paths
Palmprint scanners are mainly divided into two categories based on their acquisition methods: contact and non-contact.
Contact scanners are typically based on a prism structure, using the pressure of the palm to disrupt optical total internal reflection for imaging. Their advantages include stable images and minimal ambient light interference, but due to physical contact, there are hygiene concerns, and the prism surface is prone to wear over long-term use, placing high demands on the durability of the optical coating.
Non-contact scanners, on the other hand, use active light sources and camera modules to complete acquisition from a distance of several centimeters to tens of centimeters. These devices prioritize user experience and hygiene, but must overcome the impact of ambient light interference and hand posture changes on image quality. Therefore, they place stricter design requirements on light source wavelength selection, optical distortion control, and depth-of-field management.
1.2 Optical Challenges Under the Trend of Multimodal Fusion
To enhance anti-counterfeiting capabilities, modern high-end palmprint scanners generally adopt multimodal recognition, simultaneously acquiring palm prints (surface texture) and palm veins (subcutaneous blood vessel distribution). This means the optical system must be compatible with two different imaging bands (visible light and near-infrared) within the same device, ensuring accurate image registration. This places higher standards on lens coating processes, transmittance consistency, and stray light suppression capabilities.
II. Optical Imaging Principles: In-depth Analysis of Two Core Technologies
2.1 Contact Imaging: The Optical Mechanism of Suppressed Total Internal Reflection (FTIR)
The core optical element of a contact palmprint scanner is a prism, and its working principle is based on suppressed total internal reflection.
When light enters the interface between the prism and air at a specific angle, the condition for total internal reflection is met, and the photosensitive element receives a uniform bright field. When a palm presses against the prism, the “ridges” (protrusions) of the skin’s texture come into close contact with the prism surface, disrupting total internal reflection and causing light to scatter and darken in those areas; while the “valleys” (recesses) of the texture remain in a state of total internal reflection, appearing as bright areas. Thus, the alternating bright and dark palmprint image is captured by the sensor.
The key to this technology lies in the prism’s manufacturing precision, coating uniformity, and precise control of the optical path angle. MOK Optics has accumulated extensive experience in optical prism manufacturing, enabling it to provide prism components with high surface accuracy, low stress, and anti-reflection coatings for specific wavelengths, ensuring the clarity and stability of FTIR imaging.
2.2 Non-contact Imaging: Composite Acquisition with Multispectral Illumination
Non-contact devices cannot rely on physical pressure; therefore, they employ an active light source and a high-resolution imaging module working in tandem. The imaging mechanism consists of two levels:
Surface palmprint imaging: Typically uses visible light (such as green light) as the illumination source. Green light, with its short wavelength (approximately 520-550nm), has high reflectivity on the skin surface and effectively suppresses interference from subcutaneous vascular signals, thus forming a high-contrast textured image.
Subcutaneous palm vein imaging: Utilizes near-infrared light (typical wavelength 760nm-940nm) to penetrate skin and muscle tissue. Deoxyhemoglobin in the blood strongly absorbs this wavelength, causing veins to appear as dark stripes in the image, forming a unique “palm vein pattern.” This feature is located subcutaneously and is extremely difficult to forge with prostheses.
In multimodal devices, the optical system needs to achieve coaxial acquisition of dual-band images on the same sensor target surface by rapidly switching light sources or using a beam splitter. This places stringent requirements on the lens’s chromatic aberration correction capability, infrared and visible light confocal design, and the anti-reflective performance of the coating.
III. The Core Role and Manufacturing Key Points of Optical Lenses
Whether it’s a contact prism or a non-contact lens assembly, the imaging quality of a palmprint scanner highly depends on the precision manufacturing of the optical lenses.
High transmittance and coating technology: For the visible and near-infrared bands, the lens needs to have an anti-reflection coating layer for specific bands to minimize light energy loss and ensure imaging brightness and signal-to-noise ratio in low-light environments.
Distortion control and resolution: In non-contact scanners, the lens needs to cover the palm area within a short object distance (usually greater than 150mm) while controlling optical distortion (usually required to be <1%) to ensure the geometric accuracy of feature point extraction.
Strray light suppression: In multispectral imaging, different light sources may produce multiple reflections on the lens surface, forming ghosting or flare, interfering with image recognition. Stray light interference can be effectively suppressed by optimizing the design of the exfoliating threads on the inner wall of the lens barrel and the blackening process at the lens edges.
IV. Image processing workflow and optical system integration
The acquired raw image is only the first step; subsequent image processing steps are required to complete recognition:
Image preprocessing: This includes noise reduction filtering, contrast stretching, and brightness uniformity correction to compensate for imaging differences caused by uneven lighting or hand posture.
Geometric Correction: Non-contact acquisition may cause perspective distortion due to hand tilt, requiring calibration algorithms to restore the true proportions.
Feature Extraction and Comparison: The endpoints and bifurcation points of palm print ridges, or the intersections of palm veins, are encoded to generate feature templates for matching and verification.
Throughout the process, the image quality acquired by the optical system directly determines the difficulty of subsequent processing and the upper limit of recognition accuracy. This is why MOK Optics consistently focuses on manufacturing precision optical lenses—only by achieving excellence in optics can reliable raw data be provided for the entire recognition system.
Conclusion
From suppressed total internal reflection in contact prisms to non-contact multispectral fusion, the technological evolution of palmprint scanners has always revolved around optimizing optical imaging mechanisms. As an optical lens manufacturer specializing in biometrics, MOK Optics is committed to providing the industry with high-precision, high-transmittance, and low-distortion optical solutions, contributing to the accuracy and security of every identity authentication. The next time you raise your hand to pass through a gate, that seemingly ordinary lens is the crucial bridge connecting light and shadow with digital identity.