Plastic and polymer lines sort material by transmission, fluorescence and colour under multi-band lighting, and the coated colour filters in the camera head select the bands that separate one resin from another. Sourcing has moved toward narrower bands, higher transmission and more durable coatings that survive conveyor wash-down. This note covers the trends, the documentation that matters, and the trade-offs that actually move cost and lead time.
Coated Color Filters in Plastic and Polymer Inspection
In polymer inspection the filter sits in front of the camera and selects the band the line is lit with, so a clear plastic and a coloured or fluorescent one separate cleanly. Because many resins look alike in white light, the discrimination comes from a narrow band matched to the resin's own transmission or fluorescence signature. The filter is therefore the discriminator, and its band edge and transmission decide how reliably the sorter rejects contaminated or wrong-material product.
How coated color filter glass works in practice
A coated colour filter is a dielectric stack that selects a band by interference and rejects the rest. Its transmission peak and band edges are set by the layer design, and both shift with angle of incidence and with temperature, so a filter lit at a fixed angle behaves as designed while one in a moving inspection head needs margin. The practical result is a high, stable transmission at the resin band and strong rejection of the ambient that would otherwise blur the sort.
Design rules that reduce cost
Two rules remove most avoidable cost. The first is to choose a standard band set shared across the product line rather than a custom filter per resin, which keeps volume on a few catalogue items. The second is to relax the angle tolerance where the illumination is fixed and on-axis, because a tighter angle spec forces more layers for no field benefit. A durable hard coat and fewer stock-keeping units both cut recurring cost without hurting the sort.
Standards, documentation and traceability
Surface and flatness tolerances for the filter substrate are generally stated in ISO 10110, and a spectral transmission certificate per lot lets the line be re-qualified after a filter change. RoHS and REACH status should be stated by the supplier against the current official text, and for food-contact-adjacent sorting the relevant framework documentation supports the audit. Keeping the spectral curve with the part means a replacement reproduces the original sort performance.
What drives cost and lead time
Cost and lead time are driven by layer count, bandwidth and substrate. A narrower band or a steeper edge needs more layers and a longer, more careful deposition; a custom centre wavelength costs more than a catalogue one; and a small order of a special glass extends lead time well beyond the coating step. Volume on a standard item and a realistic edge slope are the levers that actually shorten the queue.
Inspection and measurement
The acceptance checks are a spectrophotometer trace of transmission across the band, an angle-dependent test where the head moves, and an environmental check for the wash-down and temperature the line sees. The useful measurement is the full transmission curve, not a single on-band point, because a filter that leaks at the edge of the resin band will mis-sort exactly the difficult cases. A first-article curve gives the reference the field filter is judged against.
Requirements specific to Plastic and Polymer Inspection
Plastic and polymer lines add conveyor wash-down, fixed or moving illumination and the need to separate look-alike resins to the usual colour-filter choice. Match the band to the resin's own transmission or fluorescence signature, require strong ambient rejection so the sort is not blurred, and specify a durable hard coat for the washing the line sees. Keep a spectral curve per lot so a replacement filter reproduces the original discrimination.
- Band matched to resin transmission or fluorescence
- Strong ambient rejection for clean separation
- Durable hard coat for conveyor wash-down
- Standard band set to protect cost and lead time
- Per-lot spectral curve for re-qualification
- Angle margin where the inspection head moves
Framework references: surface and flatness tolerances for the filter substrate are generally stated in ISO 10110; spectral acceptance should be defined by the full transmission curve rather than a single on-band point. RoHS and REACH declarations are the supplier's to state against the current official text, not to be assumed.
Selection data at a glance
| Parameter | Typical value | Notes |
|---|---|---|
| Band | Matched to resin | Transmission or fluoresence |
| Transmission | High at band | Sort contrast |
| Rejection | Strong ambient | No blur |
| Coating | Durable hard | Wash-down |
| Angle | Margin if moving head | Shift budget |
| Traceability | Per-lot spectral curve | Re-qualify |
Frequently asked questions
Why match the filter band to the resin instead of using white light?
Because many resins look alike under white light and only separate by their own transmission or fluorescence signature. A narrow band matched to that signature turns a colour-similar pair into a clean sort, while white light leaves the difficult cases ambiguous.
Does a moving inspection head change the filter choice?
It adds an angle-shift margin. The band centre moves with angle of incidence, so a head that swings or a line with wide field of view needs the filter designed with margin at the extreme angles, or the edges of the frame drift off-band and mis-sort.
Why measure the full transmission curve, not one point?
Because a filter that leaks at the edge of the resin band will mis-sort exactly the hard cases while looking fine at the centre. The full curve is what defines whether the sort stays clean under real ambient, so acceptance is judged on the curve against a first-article reference.
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