A detector includes a light source to irradiate an object with light, a sensor to image a first pattern and a second pattern formed on the object with the light irradiated by the light source to generate image data, the first pattern and the second pattern imaged by the sensor at different times, and a circuit to control the light source to adjust a light quantity of the light according to a type of the object and irradiate the object with the light quantity adjusted according to the type of the object, and calculate a relative position of the object between the first pattern and the second pattern.