| CPC G07D 7/121 (2013.01) [G01N 21/64 (2013.01); G01N 21/86 (2013.01); G01N 2021/6491 (2013.01); G07D 2207/00 (2013.01)] | 21 Claims |

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1. An optical sensor that detects light reflected by a transported sheet and/or light transmitted through the sheet as well as light emitted from the sheet,
the optical sensor comprising:
a light source configured to irradiate a sheet with excitation light and detection light;
a controller configured to cause the light source to emit the detection light in a plurality of light-on periods and the excitation light at least one time, and keep the light source turned off during each of a plurality of light-off periods after the emission of the excitation light; and
a light receiver configured to receive light resulting from reflection of the detection light by a sheet and/or light resulting from transmission of the detection light through the sheet in each of the plurality of light-on periods and receive phosphorescence emitted from the sheet in each of the plurality of light-off periods,
the controller configured to generate data of one pixel by summing up a plurality of output values based on a plurality of respective phosphorescence components received by the light receiver in each of the plurality of light-off periods.
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