Optical sensor for noncontact measurement of lignin content in high-speed moving paper surfaces

An optical sensor for real-time measurement of lignin content in moving paper sheets is described in this paper. The sensor is useful for high-speed automated sorting of papers according to lignin content in a mixed-waste stream for efficient recycling. The sensor measures lignin fluorescence when excited in the visible region. Results indicate that when paper surfaces are excited with a laser source (532 nm), the lignin fluorescence spectrum shows a peak near 650 nm. Measurement of this peak intensity from paper samples is a measure of lignin content and correlates linearly with the standard chemical method for lignin determination (Klasson lignin content). Newsprint samples, typically containing high lignin, produce high intensity, while ledger printing and writing grades with low lignin content produce low-fluorescence intensity. The proposed sensor configuration is discussed and results from a systematic study of variables, namely, text printed on surface, homogeneous dyed in color, printed color patches, paper thickness, sensor configuration parameters such as excitation source intensity, and distance from sample surface are reported. In dynamic tests on a simulated conveyor operating at realistic speeds, the sensor is able to reliably measure fluorescence intensities on unprinted samples without ambiguity and identify high and low lignin-containing samples.