A rectangular transparent sheet 15 is printed with black dotted density patterns 21 and 22. The proposed method and system is characterized by the use of protocol-oriented rule rearrangement, the probable bit vector based on the aggregated bit vectors and folded bit vectors, an ABV-FBV index table dataset whose data structure is based on a featured split full-tree schema, and a DCBV dataset for packet classification. Using the image data in the spectral domain, one or more one-dimensional spectral signatures can be generated and features of the signatures can be extracted and compared to features of one or more known objects. Furthermore, an isolation effect of a leakage signal of a transmitting unit to a receiving unit can be maximized while adaptively changing an attenuation value of a variable attenuator. The optical system includes a first imaging optical system comprising a concave reflecting mirror and a dioptric optical member arranged along a first optical axis. This prevents foreign material from being trapped between the transfer belt such as an intermediate transfer belt or transfer material conveyance belt and a sheet member and thereby reduces density irregularities in a longitudinal direction of the belt.