Devices that acquire image data by capturing a paper sheet to perform recognition of a kind and an authentication of the paper sheet, management of the paper sheet, and the like, are known in the art. Patent Document 1, for example, discloses a banknote processing machine that acquires image data of a banknote by capturing the banknote, and recognizes a kind and an authentication of the banknote based on characteristic features obtained from the image data. Specifically, in the banknote processing machine disclosed in Patent Document 1, a light guiding plate is arranged above the banknote, light is emitted on the banknote from a light source arranged on a side of the light guiding plate, a reflection image of the banknote is acquired by receiving light reflected from the banknote in a first sensor. The first sensor is arranged above the banknote, similarly to the light guiding plate. Additionally, a transmission image of the banknote is acquired by receiving light that has passed through the banknote in a second sensor that is arranged below the banknote. In this banknote processing machine, by sequentially turning on light sources of a plurality of wavelengths, an image corresponding to each of the wavelengths can be captured.
The banknote processing machine disclosed in Patent Document 1 can acquire a transmission image and a reflection image of one side of the banknote; however, to recognize a kind and an authentication of the banknote, it is desirable to read reflection images of the both sides of the banknotes to obtain characteristic features of each side of the banknotes. Devices that acquire a transmission image and reflection images of both sides of banknotes are known in the art. In the banknote image detection devices disclosed in Patent Document 2 and Patent Document 3, for example, one detecting unit is arranged above and another detecting unit is arranged below the banknote, each detecting unit includes a light source and an image detecting sensor, thereby acquiring a transmission image and reflection images of both sides of the banknote. In the device of Patent Document 2 and the device of Patent Document 3, the positional arrangement of the light source and the image detecting sensor in the detecting unit is different.