Apparatus for and method of inspecting sheet body

An apparatus for inspecting a light-transmissive sheet body to detect a stripe defect therein, comprising:

• a light-emitting unit for applying light to the sheet body;
• a light-detecting unit for detecting light having passed through the sheet body;
• a slit member disposed in at least one of said light-emitting unit and said light-detecting unit, for shaping said light into a line of light through a slit defined in said slit member and guiding said line of light to said light-detecting unit;
• a processor for processing signal data of the light detected by said light-detecting unit;
• an extremal value calculator for determining an extremal value of said signal data for each reference length of said sheet body;
• and
• a defect disappearing position identifier for determining a first function established by a plurality of extremal values obtained from a range including said defect in said sheet, and a second function established by a plurality of extremal values obtained from a range not including said defect in said sheet, and identifying a position on said sheet body where said first function changes to said second function, as a defect disappearing position where said defect disappears.

A method of inspecting a light-transmissive sheet body to detect a stripe defect therein, comprising the steps of:

• detecting light emitted from a light-emitting unit having passed through the sheet body with a light-detecting unit as a line of light oriented in the direction of the stripe defect,
• wherein said light-emitting unit and said light-detecting unit are provided in a plurality of pairs spaced at equal intervals along the width of said sheet body;
• processing signal data obtained from said line of light with a processor to detect the stripe defect in said sheet body;
• determining an extremal value of said signal data for each reference length of said sheet body;
• and
• determining a first function established by a plurality of extremal values obtained from a range including said defect in said sheet, and a second function established by a plurality of extremal values obtained from a range not including said defect in said sheet, and identifying a position on said sheet body where said first function changes to said second function, as a defect disappearing position where said defect disappears.