Character Inspection on Beverage Bottle Caps

Light source selection is prioritized by material + reflectivity + character type. Main solutions are as follows:

I. Light Source Selection by Cap Material 

1. Metal Caps (Aluminum / Tinplate, High Reflectivity) 

First choice: Coaxial light (vertical coaxial illumination)

Vertical lighting suppresses specular reflection and provides high contrast between characters and background. Suitable for date and batch code inspection (printed or laser-engraved).

Alternative: Low-angle ring light (15°–30°) + polarizer

Side lighting enhances character edges; polarizer eliminates reflection on metal surfaces.

Color: White / Blue / Green

Use blue for black characters on red background (complementary color to suppress red and enhance black); white is preferred for general applications.

2. Plastic Caps (PP / PE, Matte / Low Reflectivity) 

First choice: Ring diffuse light (high angle 45°–60°)

Uniform illumination with minimal shadows, suitable for presence and clarity inspection of printed characters.

Alternative: Dome light (diffuse shadow-free)

More uniform lighting for curved caps, ideal for matte or textured plastic caps.

Color: White / Green

Use green for white characters on green background; white for general use.

3. Translucent / Transparent Caps (PET)

First choice: Backlight (bottom transmitted light) + top ring light

Backlight highlights character contours; top light supplements details. Suitable for inner printing or engraving on transparent caps.

II. Light Source Selection by Character Type

Printed characters (ink): Ring light / coaxial light; white or blue recommended to enhance contrast between ink and substrate.

Laser-engraved / embossed characters (raised or recessed): Low-angle ring light (15°–20°) creates shadows to highlight surface contours.

Inkjet characters (easily blurred): High-brightness ring light + coaxial light combination to eliminate fuzzy edges.

III. General Selection Rules (Quick Judgment)

High-reflective metal → Coaxial light + polarizer

Matte plastic → Ring diffuse light

Embossed / engraved → Low-angle side light

Black on red → Blue light

White on green → Green light

Uncertain → Test white ring light first

IV. Key Avoidance Tips

Polarizer must be used for metal caps to eliminate specular highlights.

Use ring light or dome light for curved caps to avoid center-dark, edge-bright unevenness.

Fine characters → Select high-brightness, high-uniformity LED light sources.