# Discriminative Local Feature for Hyperspectral Hand Biometrics by Adjusting Image Acutance
## 1 Introduction  
Hand biometrics has been largely studied in the last few decades [1; 2; 3; 4; 5] because of its effectiveness in personal authentication. One of the most universal traits of a hand is the palmprint [6; 7; 8; 9]. Palmprint biometrics, with the center of the palm not touching the capture device for identification, is less likely to be copied by others and is more hygien [9]. Contrary to palmprint biometrics, hand vein biometrics, such as dorsal hand veins [10; 11; 12] and palm veins [13; 14], are studied in security with advantages in liveness detection and anti-spoofing [12]. In the meantime, hyperspectral technology known from remote sensing [15; 16; 17; 18], has been introduced in biometrics, where it is applied in high-security scenarios [18]. Therefore, hand biometrics when combined with hyperspectral technology, is a potential solution achieving better personal authentication [19; 20]. Hyperspectral hand biometrics utilizes spectral information in a hand image. The epidermal and dermal layers of the skin on a hand constitute a scattering medium that contains various combinationsof water, melanosomes, hemoglobin, bilirubin, beta-carotene, etc., which provide different absorption coefficients for an irradiated spectrum [19]. Little changes in the distribution of these layers and pigments of the skin induce significant changes in the skin's spectral reflectance, thus generating a unique response for each person that is difficult to modify and counterfeit [20]. For hyperspectral imagery in hand biometrics in particular, each spectrum penetrates a different depth of the hand, indicating the spectral property and enabling the imaging of different surface characteristics of an individual's hand. For example, near-infrared (NIR) can penetrate deeper than thermal infrared (8-12 um), making it useful for hyperspectral hand imagery [21]. Considering spectral information as a complement in hyperspectral hand biometrics, security issues can reach a higher level, which is more secure and better at anti-spoofing. Hyperspectral hand biometrics is a new trend derived from single spectral and multispectral biometrics. [PERSON] et al. [22] performed palmprint recognition on extensive single spectrum images, such as the PolyU, IITD, GPDS, and CASIA databases. [PERSON] et al. [23] first applied the combinations of different bands containing Blue, Green, Red, and NIR for multispectral palmprint verification, where its result was recently improved by [PERSON] et al. [24]. Hyperspectral palm was studied by [PERSON] et al. [25] to propose an anti-spoofing recognition system (prototype). The dorsal hand mainly concentrates on the vein structures, first discovered by [PERSON], a senior Kodak engineer when he was designing an infrared barcode system [26]. [PERSON] et al. [12] performed dorsal hand vein recognition based on an 850 nm NIR dorsal hand database, while other databases with the same band can be found in [27; 28; 29]. [PERSON] et al. [30] applied hyperspectral techniques to select the best spectrum for the improvement of dorsal hand vein recognition, which showed potential in hyperspectral hand biometrics. In hand biometrics, the local features play a crucial role for texture analysis [31; 32]. As palmprint lines are the most significant features [7], [PERSON] et al. [33] used the local line features for palmprint recognition. To explore more elaborate features, [PERSON] et al. [34] proposed a local CompCode (competitive code) method for online palmprint recognition, which was based on Gabor features. Later, this was developed into a discriminative and robust CompCode by [PERSON] et al. [35]. Most recently, a novel double-layer local direction pattern extraction method for personal authentication was proposed by [PERSON] et al. [6]. For dorsal hand vein recognition, [PERSON] et al. [36] applied local SIFT (scale-invariant feature transform) features. [PERSON] et al. [11] extracted LBP (local binary patterns) from a dorsal hand vein to build an automatic access control system.