Local grammage distribution and formation of paper by light transmission image analysis

A method based on light transmission coupled with computer-based image analysis has been developed for determining the distribution of local grammage and hence the formation of paper. This technique is fast, inexpensive, and accurate. The method, tested over the range of 20 to 75 g/m 2 basis weight, for various finishes and formations, has a resolution of at least 0.1 mm, with an accuracy better than 0.5 g/m 2 . A data processing procedure was developed to convert local opacity to local basis weight. A frequential analysis based upon a Fast Fourier transform treatment of the data can extract the contour map of the flocs in the paper, providing a visual representation of flocs not previously available. Further processing of the power spectrum data gives a single number as a new floc formation index (FFI) describing paper formation. The FFI criterion discriminates much more sensitively between different qualities of formation than does the standard formation number, while not having the disadvantage of the multiple indices of second-order statistics.