*2.1. Image Acquisition of Dorsal Hand Vein*

The dorsal hand vein exists under subcutaneous tissues. Using a general camera is difficult to capture clear images of dorsal hand vein in the condition of visible light source, so an infrared light source was adopted in our devices [11]. The appearance of acquisition equipment is shown in Figure 1a,b is the internal structure. Figure 2 is the captured images, Figure 2a,b are both dorsal hand vein images for the same person but captured by two different devices.

**Figure 1.** Image acquisition device. (**a**) Appearance; (**b**) internal structure.

**Figure 2.** Dorsal hand vein images captured by two different devices. (**a**) Captured by No. 1 device; (**b**) captured by No. 2 device.

As we can see in Figure 2, the vein texture is clear and the details are rich. In order to research the image recognition of the dorsal hand vein under weak constraints, we need to create a database with diversity. Dorsal hand vein images of the same person under different devices are quite different, including changes in rotation angle, size, brightness and noise. This is mainly due to differences in parameters such as contrast, brightness, focal length and lens optical performance of different collection devices, as well as in the state of the collector's hand. Therefore, it is more difficult for cross-device dorsal hand vein recognition.
