Colour is a powerful quality signal in plastics and packaging – from PET bottles and caps to films, trays, and masterbatch pellets. Yet many teams see perfectly good parts “fail” colour targets for a simple, avoidable reason: edge loss.
What is edge loss?
Edge loss is a measurement artefact that happens when light entering a translucent or semi‑transparent sample spreads beneath the surface and escapes at the boundaries of the illuminated area (the “edge”) instead of returning to the detector. Because some light leaks away, the instrument records lower reflectance (darker colour) than it should. In transmission, the opposite can happen: light bypasses the detector path and you see inconsistent transmittance/haze values.
Where it shows up most:
- Thin or semi‑transparent mouldings (PP/PE tubs, translucent caps)
- Textured or slightly opalescent plastics (filled PP, masterbatch plaques)
- Films and laminates (BOPP, PET, barrier films)
- Small parts measured with too small an aperture or without a proper mask
- Liquids, pellets, or powders measured in undersized cells
Why it matters for packaging
- False rejects/accepts: Parts look OK by eye but fail instrumentally (or vice‑versa).
- Colour drift over time: Different fixtures, apertures, or backings create different amounts of edge loss, so results don’t track across suppliers or shifts.
- Poor correlation to the customer’s instrument: If your customer uses different geometry, gloss mode, or aperture, your readings won’t line up. purchase specs, so results correlate across the supply chain
Practical ways to reduce edge loss
- Use a d/8° integrating‑sphere geometry and measure in SCI (specular‑included) to minimise surface texture effects when appropriate. Switch to SCE (specular‑excluded) to capture appearance differences like gloss but keep it consistent with your standard.
- Choose the largest suitable aperture (LAV/MLAV) for the part. Larger measurement areas reduce the proportion of light escaping at the illuminated edge.
- Mask and fixture correctly: For plaques and films, use tight‑fitting masks and centre the sample to stop light from leaking out of the measurement port.
- Control sample thickness and backing: Stack thin films to reach your standard thickness and use a consistent black or white backing as your method requires.
Konica Minolta tools that help
CM‑26dG Portable Spectrophotometer (d:8° with integrated 60° gloss)
- Why: On‑line/near‑line checks with the same geometry and gloss mode as the lab minimise method drift and edge‑loss variability between stations.
CM‑5 Benchtop with Oversized Tube Cells (for liquids, pellets, powders)
- Why: Larger‑diameter optical cells reduce edge loss when measuring slurries, masterbatch pellets, powders, and resins that scatter light strongly.
CM‑36dG / CM‑36dGV Benchtop Spectrophotometers (d:8° with built‑in 60° gloss)
Why: Sphere geometry with simultaneous colour + gloss helps you separate true colour from appearance. Transmission capability lets you measure films/laminates properly (Total or Regular) and avoid reflectance edge‑loss artefacts on thin materials.
Key takeaways:
- Edge loss is not a material fault; it’s a measurement setup issue.
- Using the right geometry, aperture, and fixtures removes most of the problem.
- Konica Minolta’s CM‑36dG/CM‑36dGV, CM‑26dG, and CM‑5 with large‑diameter cells provide the sphere geometry, gloss control, transmission capability, and accessories you need to measure plastics and packaging consistently across sites and suppliers.
