Optical Deviation
Metrology of clear barrier coatings on packaging films relies on the consistent transmission and bending of light through the material. A refractive index drift represents the slow, unintended change in the optical density of the coating layer during processing. This variation occurs due to fluctuations in solvent evaporation, temperature, or polymer curing state.
It alters the angle and speed of light passing through the layer.
Measurement Impact
Online optical sensors calculate coating thickness by analyzing the interference patterns or reflection intensities at the material boundary. When refractive index drift is present, the sensor misinterprets the change in optical path length as a change in physical thickness, leading to incorrect dosing of the coating material. If the curing oven temperature rises, the polymer density changes and shifts the optical properties.
This shift can cause the control system to reduce the applied coating, resulting in packaging that fails to meet barrier specifications. Operators must distinguish between actual thickness changes and these optical variations to maintain quality.
Sensor Calibration
Compensating for this effect requires continuous monitoring of the web temperature and the chemical composition of the coating mix. Reference measurements are taken at regular intervals to update the calibration curves of the sensor. This adjustment ensures that the reported thickness remains accurate regardless of process conditions.
