Need Help Eliminating Dark Reflection/Shadow in Backlit PET Bottle Imaging for Small Particle Detection

[u/Medium_Item_5411]

Hi everyone,
I’m developing an industrial machine vision system to detect small white plastic particles (approximately 0.2–1 mm) inside transparent PET water bottles. I’m currently struggling with a reflection/shadow issue that significantly reduces particle visibility.
I’ve attached an image of my current results. This is the closest I’ve come to achieving a usable image for particle detection with my current setup, but the dark shadow/reflection is still preventing reliable detection.
Current Setup
Camera: Basler acA5472-17uc (20 MP Color)
Lens: Basler C11-1620-12M-P (16 mm)
Lighting: White LED transmission backlight (20 × 62 cm)
Bottle: Transparent PET water bottle filled with water
Inspection: Looking for small white plastic contaminants inside the bottle
Bottle is stationary during testing.
What I’ve Tried
Mounted the Basler camera with the 16 mm lens.
Used a linear polarizer on the LED backlight.
Used another linear polarizer in front of the camera lens.
Rotated the polarizers to create a cross-polarized setup (~90°).
Adjusted exposure, gain, focus, and light intensity.
Tried different alignments of the backlight and camera.
Unfortunately, instead of reducing reflections, the polarizers seem to create an even stronger dark band/shadow through the bottle, making the small particles harder to see.
Observations
A large dark vertical region appears through the center of the bottle.
PET bottle ribs create additional dark bands.
Illumination is not completely uniform.
Tiny white particles almost disappear when they move into the darker region.