In the manufacture of sheets of paper, and especially the manufacture of xerographic copy paper, it is desirable to test the paper for curl in order to determine that the paper is of a desired flatness. Generally, two types of curl may arise in paper manufacture as a result of moistening or drying a paper sheet. Simplex or basic curl is that which arises in one direction, such as results when a sheet of paper is rolled into a cylindrical tube. Diagonal curl is the result of a twist in a sheet of paper. For example, if the top edge of a sheet of paper is axially rotated in one direction and the bottom edge of the sheet is axially rotated in the opposite direction, the twist in the paper will produce a diagonal curl.
Current curl determination during paper production is generally performed manually and suffers from subjective variables owing to different determinations from competent testing personnel. Such things as a dominant eye, minor astigmatism, or the height of an individual tester are all variables that affect curl testing results.
The present invention relates to a unique method and apparatus for opto-electrically measuring the curl in a suspended sheet of paper to provide reliable, uniform test results.