Image reading device and image reading method

An image reading device comprising:

• an illuminator to irradiate a subject with visible light and invisible light;
• an imager to receive the visible light and the invisible light reflected by the subject to capture a visible image and an invisible image;
• a background portion in an image capturing range of the imager,
• the background portion having:
• a reflectance at a specific wavelength of the visible light equal to or higher than a lower limit of visible light reflectance, the lower limit of visible light reflectance specifying a lower limit of reflectance of the visible light;
• and
• a reflectance at a specific wavelength of the invisible light equal to or lower than an upper limit of invisible light reflectance, the upper limit of invisible light reflectance specifying an upper limit of reflectance of the invisible light;
• and
• an image processor configured to detect a feature of one of the subject and the background portion from at least one of the visible image and the invisible image,
• wherein:
• the background portion has a difference between a maximum reflectance value and a minimum reflectance value at a plurality of specific wavelengths of the visible light equal to or less than an upper limit of difference in visible light reflectance, the upper limit of difference in visible light reflectance specifying an upper limit of the difference,
• the image processor is configured to detect saturation of the subject as the feature of the visible image, and
• the upper limit of difference in visible light reflectance of the background portion is equal to or lower than 44% when the plurality of specific wavelengths are 450 nm, 550 nm, and 600 nm.