An image acquisition device comprising: a light source configured to emit light to a sheet; a light receiving unit configured: to receive a first transmitted light generated by transmission of a first emitted light having a first quantity of light through the sheet to output a first image signal as data of one line constituting a transmission image, the first emitted light being emitted from the light source; and to receive a second transmitted light generated by transmission of a second emitted light having a second quantity of light through the sheet to output a second image signal as data of one line constituting a transmission image, the second emitted light being emitted from the light source; and an image generating unit configured: to generate a first transmission image from the first image signal; and to generate a second transmission image from the second image signal, wherein the light source is configured to emit infrared light or visible light to the sheet, the first emitted light and the second emitted light are within the same wavelength range, the first quantity of light and the second quantity of light are each a quantity of light per cycle, the second quantity of light is set to be smaller than the first quantity of light, the sheet is a banknote, a gift voucher, or a check having a transparent portion, the image generating unit is configured to generate a transmission image including an image of the transparent portion, the first quantity of light is set to a quantity of light at which the first transmission image includes a saturated region corresponding to the transparent portion, and the second quantity of light is set to a quantity of light at which a region in the second transmission image corresponding to the saturated region is unsaturated, and a control unit is configured to control an operation of the image acquisition device and a recognition unit to determine a feature of ink in an opaque portion of the sheet from the first transmission image and determine at least one selected from an external shape of the sheet, the presence or absence of a defect in the transparent portion, and a shading pattern in the transparent portion from the second transmission image. a light source configured to emit light to a sheet; a light receiving unit configured: to receive a first transmitted light generated by transmission of a first emitted light having a first quantity of light through the sheet to output a first image signal as data of one line constituting a transmission image, the first emitted light being emitted from the light source; and to receive a second transmitted light generated by transmission of a second emitted light having a second quantity of light through the sheet to output a second image signal as data of one line constituting a transmission image, the second emitted light being emitted from the light source; and to receive a first transmitted light generated by transmission of a first emitted light having a first quantity of light through the sheet to output a first image signal as data of one line constituting a transmission image, the first emitted light being emitted from the light source; and to receive a second transmitted light generated by transmission of a second emitted light having a second quantity of light through the sheet to output a second image signal as data of one line constituting a transmission image, the second emitted light being emitted from the light source; and an image generating unit configured: to generate a first transmission image from the first image signal; and to generate a second transmission image from the second image signal, to generate a first transmission image from the first image signal; and to generate a second transmission image from the second image signal, wherein the light source is configured to emit infrared light or visible light to the sheet, the first emitted light and the second emitted light are within the same wavelength range, the first quantity of light and the second quantity of light are each a quantity of light per cycle, the second quantity of light is set to be smaller than the first quantity of light, the sheet is a banknote, a gift voucher, or a check having a transparent portion, the image generating unit is configured to generate a transmission image including an image of the transparent portion, the first quantity of light is set to a quantity of light at which the first transmission image includes a saturated region corresponding to the transparent portion, and the second quantity of light is set to a quantity of light at which a region in the second transmission image corresponding to the saturated region is unsaturated, and a control unit is configured to control an operation of the image acquisition device and a recognition unit to determine a feature of ink in an opaque portion of the sheet from the first transmission image and determine at least one selected from an external shape of the sheet, the presence or absence of a defect in the transparent portion, and a shading pattern in the transparent portion from the second transmission image.