An imaging device includes: a sensor to detect a first target spectrum, the first target spectrum corresponding to a thermal imaging region of an infrared (IR) spectrum; and an optical device to transmit external light to the sensor, the optical device including: a substrate; and a plurality of nanostructures on the substrate, and to collimate at least the first target spectrum in the external light on the sensor. The plurality of nanostructures are spaced apart from each other, and at least one of the plurality of nanostructures has a different geometric size from that of another.