1. An acquisition system comprising: a camera designed to acquire an image of a finger when the finger is in front of the camera, a lighting device designed to illuminate said finger; a control unit comprising a processor for controlling an intensity of illumination of said lighting device; and at least one system for measuring a transmission power of said finger upstream of the camera with respect to the passage of said finger towards said camera, the control unit being designed to control the illumination intensity of said lighting device according to the transmission power measured by said measuring system, wherein the system is adapted to: (1) check an exposure of the image acquired during the acquisition by the camera and adjust the illumination intensity of the lighting device if the exposure of said image is judged to be incorrect to: an illumination intensity lower than previous illumination intensity if the exposure of said image is judged to be overexposed, and an illumination intensity higher than the previous illumination intensity if the exposure of said image is judged to be underexposed, and/or, (2) check an exposure of a particular channel of the image acquired during the acquisition by the camera, and select: either said particular channel of the acquired image if the exposure of said image is judged to be correct, or another channel of the acquired image overexposed compared with said particular channel if the exposure of said image is judged to be underexposed, or another channel of the acquired image underexposed compared with said particular channel if the exposure of said image is judged to be overexposed, and/or, (3) measure the transmission power of the finger, the image of which is to be acquired, the acquisition system comprising a photoemitter and a photodetector, the acquisition system being adapted to control the light intensity of the photoemitter to: a light intensity lower than the light intensity of the measure if the light intensity measured by the photodetector is higher than a threshold intensity, a light intensity higher than the light intensity of the measure if the light intensity measured by the photodetector is higher than a threshold intensity, and perform a new measure.