Apparatus and method for detecting foreign substance based on a difference in reading level of reflected light in a first wavelength range and a second wavelength range

A detecting method, comprising:

• emitting, from a light source, at least light in a first wavelength range and light in a second wavelength range that is longer than the first wavelength range, to a background member having reflectances different in the first wavelength range and the second wavelength range;
• receiving, with an imaging device, reflected light from the background member irradiated with the light in the first wavelength range and generating first read information;
• receiving, with the imaging device, reflected light from the background member irradiated with the light in the second wavelength range and generating second read information;
• detecting presence or absence of a black foreign substance based on a first difference in reading level of the first read information. the first difference being negative;
• and
• detecting presence or absence of a white foreign substance based on a second difference in reading level of the second read information, the second difference being positive.

A non-transitory recording medium storing a plurality of program codes which, when executed by one or more processors, causes the processors to perform a detecting method comprising:

• emitting, from a light source, at least light in a first wavelength range and light in a second wavelength range that is longer than the first wavelength range, to a background member having reflectances different in the first wavelength range and the second wavelength range;
• receiving, with an imaging device, reflected light from the background member irradiated with the light in the first wavelength range and generating first read information;
• receiving, with the imaging device, reflected light from the background member irradiated with the light in the second wavelength range and generating second read information;
• detecting presence or absence of a black foreign substance based on a first difference in reading level of the first read information. the first difference being negative;
• and
• detecting presence or absence of a white foreign substance based on a second difference in reading level of the second read information, the second difference being positive.