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Finger vein imaging using near infrared methods have high sensitivity towards illumination and often suffer from bad image sharpness and loss of information. An embedded image acquisition subsystem with adaptive illumination control based on image quality assessment is introduced in this paper in order to acquire finger vein image with good quality for high accuracy authentication. The quality of the acquired finger vein image is assessed using two-dimensional (2D) entropy and the infrared illumination is adaptively adjusted based on the assessment result. 2D entropy for finger vein image quality assessment that is suitable for hardware implementation is proposed. Mathematically, the proposed 2D entropy significantly improves the performance as well as resource utilization of the quality assessment module compared to previous work. Besides that, buck converter is designed as the Light-Emitting Diode (LED) driver circuit to control the brightness level of the high power infrared LED array efficiently for finger vein image capture. The proposed subsystem is deployed in the FPGA-based finger vein biometric system operates on Nios2-Linux Real Time Operating System (RTOS). Experimental results show that finger vein images acquired through the proposed image acquisition subsystem contain more information as well as better image sharpness compared to finger vein images captured under fixed illumination.