Sensor apparatus and image forming apparatus incorporating same

A sensor apparatus comprising:

• an irradiation system provided with a light source and configured to emit linearly polarized light of a first polarization direction onto a sheet-like object, in a direction oblique to a direction orthogonal to a surface of the object;
• a first photodetector arranged on an optical path of light that is emitted from the irradiation system and then is reflected at the object by regular reflection;
• a first optical element, arranged on an optical path of light reflected by diffuse reflection from an incidence plane of the object, configured to transmit linearly polarized light of a second polarization direction that is orthogonal to the first polarization direction;
• a second photodetector configured to receive the linearly polarized light transmitted through the first optical element;
• and
• a detection unit configured to determine at least one of basis weight and thickness of the object, on a basis of the linearly polarized light of the second polarization direction that is orthogonal to the first polarization direction of the light emitted by the irradiation system.