Medium carrying device and image forming apparatus

An image forming apparatus, comprising:

• a medium carrying device that carries a print medium comprising a first medium and multiple second media disposed with a specified interval each other on the first medium along a carrying path, the first medium being a continuous sheet,
• an image forming part that performs image formation to the print medium carried up by the medium carrying device, wherein
• the medium carrying device further comprises
• a detection part that includes a light emitting part and a light receiving part arranged sandwiching the carrying path, and that detects the print medium carried on the carrying path according to a light amount of a light that is received by the light receiving part, the light being emitted from the light emitting part and coming through the carrying path, and
• a control part that controls operations of the carrying part and the detection part,
• the print medium comprises a first region positioned between the multiple second media and a second region that is a region where the second media are disposed on the first medium,
• the detection part performs light amount measurements, the light amount measurements being processes using the light emitting part and the light receiving part in which a light emitting amount by the light emitting part is determined so that the light receiving part receives a predetermined light receiving amount from the light emitting part,
• the control part
• using a second emitted light amount (VDA2) of the emitted light of the light emitting part, determines that a trailing edge position of the second region has reached based on a first threshold light amount (VL4) by performing the light amount measurement in a state where the print medium is carried to a third position that is a position where the second region is facing with the detection part and is located in the front side of a first position where the first region of the print medium is facing with the detection part,
• based on a second threshold light amount (VL5) determined based on the first threshold light amount, sets a first emitted light amount (VDA1) of the emitted light of the light emitting part, the first emitted light amount being less than the second emitted light amount, the second threshold light amount being used in image formation to the print medium by performing another light amount measurement in a state where the print medium is carried to the first position so as to set the first emitted light amount as a predetermined emitted light amount of the light emitted by the light emitting part for carrying the print medium and received by the light receiving part, and
• using the first emitted light amount, sets a third threshold light amount (Vlabel) that is used in detecting a boundary position between the first region and the second region by the detection part based on the receiving light amount in a state where the print medium is carried from the first position to a second position that is a position where the second region is facing with the detection part.