Meaning
Optical metrology errors occur when a light beam penetrates the surface of a material and scatters internally before returning to the sensor. The phenomenon of subsurface scattering bias arises when measuring translucent or semi-crystalline polymers, as the optical scanner detects the scattered light from below the surface rather than the true surface boundary. This causes the scanner to record a height or position that is deeper than the actual surface of the part.
Injection molders of white, clear, or unpigmented plastics must account for this effect to avoid systematic measurement errors during dimensional validation.
Measurement Drift
The depth of light penetration depends on the polymer crystallinity, pigment concentration, and the wavelength of the scanner’s light source. When measuring high-density polyethylene or polyoxymethylene parts, the light can penetrate several micrometers into the material. This creates a systematic offset that makes the measured part appear smaller or thinner than its actual dimensions.
If this offset is not corrected, the metrology system might reject acceptable parts or accept out-of-tolerance components, leading to increased scrap rates and production delays. This drift can also compromise the assembly of tight-tolerance mating components.
Optical Compensation
Metrology engineers use several methods to mitigate this measurement error. Applying a thin, removable matte coating to the part surface stops the light from penetrating the polymer, ensuring that the sensor detects only the true top surface. This method is effective for laboratory calibrations but can slow down inline production testing.
Alternatively, scanner software can apply a mathematical correction based on the known optical properties of the specific polymer grade being measured.
Material Influence
Sourcing recycled or regrind resins can change the optical transmission of the polymer, thereby altering the scattering behavior. Variation in the pigment loading between different batches of raw material will also affect the measurement bias. This requires the quality control team to re-verify the sensor calibration whenever there is a shift in the material sourcing or pigment concentration.
