InGaAs Sensor Selection for Foreign Matter Detection
Select standard InGaAs for cost-effective detection up to 1700 nm, or cooled extended InGaAs to capture 2200 nm polypropylene combination bands cleanly.
Normalized Difference Polypropylene Index
Algorithmic Ratio
Mathematical comparisons of reflectance values at two specific infrared wavelengths identify synthetic contaminants in natural fiber streams. The normalized difference polypropylene index uses the difference between a reference wavelength and a polypropylene absorption peak, divided by their sum. This calculation creates a value between negative one and positive one that highlights plastic fragments.
Using a ratio rather than a single intensity value makes the system less sensitive to changes in lighting or tuft density.
Spectral Contrast
Polypropylene has a distinct chemical signature in the short wave infrared range that differs from cotton. While cotton reflects heavily at certain points, the normalized difference polypropylene index exploits the fact that synthetic twine absorbs energy at approximately 1735 nanometers. This contrast allows the imaging software to map out the exact shape of a contaminant.
These maps are used to coordinate the timing of the rejection system.
False Positive
Natural variations in cotton maturity or the presence of specific seed materials can occasionally mimic the synthetic signal. Refinement of the normalized difference polypropylene index involves choosing narrow band filters that minimize interference from organic debris. The goal is to isolate the carbon-hydrogen stretching signal unique to the polymer.
Inspection Throughput
Processing these ratios in real time requires dedicated high speed hardware. Every pixel in a line scan camera undergoes the normalized difference polypropylene index calculation before a decision is made to trigger the pneumatic valves. High throughput ensures that the mill can maintain full production speeds without sacrificing cleanliness.