Short answer
Most recycling sorters identify the material with near-infrared (NIR) spectroscopy, which reads chemical bonds rather than visible colour, and use a separate visible-light camera to sort by colour. Carbon black absorbs across the near infrared as well as the visible, so a carbon-black item returns no usable spectrum and is often not identified; whether a dark item sorts depends on the pigment, the machine and its tuning.
An NIR sorter shines broadband light on objects on a fast belt and reads the reflected spectrum between roughly 780 and 2,500 nanometres. In that range, polymers show overtone and combination bands of C–H, O–H and similar bonds; a Fraunhofer paper describes identification resting mainly on C–H stretching overtones near 1.1–1.25 and 1.65–1.7 micrometres. Each polymer has a fingerprint, so PET, HDPE, PP and PS can be told apart regardless of their visible colour. The sorter then fires an air jet to eject each identified item into the right fraction. A red PP tub and a white PP tub look identical to NIR; both are simply PP.
Because NIR does not see visible colour, sorting clear from coloured PET, natural from coloured HDPE or flint from green glass uses visible-light cameras or spectrometers. Modern machines often combine both channels: TOMRA's flake sorter pairs an NIR spectrometer with a full-colour camera, and Pellenc ST describes a single spectrometer covering both NIR and visible ranges. Colour sorting also happens after grinding, when flakes are sorted to remove off-colour pieces before they are melted. Textile sorters work the same way, pairing NIR for fibre with an RGB camera for shade.
Carbon black is used because it absorbs almost all visible light cheaply and permanently. It also absorbs across the near infrared, so even a small amount leaves the sensor with no reflected spectrum to read — the Fraunhofer paper calls it all light absorbed and no detectable signal. The item is not identified and typically goes to residue. WRAP's work on black plastic packaging reached the same conclusion for UK recycling and promoted NIR-detectable black colourants: pigment combinations without carbon black that still look black to the eye but let the polymer's NIR fingerprint through. North American recycler guidance treats any dark packaging below set lightness and NIR-reflectance thresholds as needing a sortation test.
Sorters read the outside surface. A full-body shrink sleeve or a large label in a different polymer can make the machine identify the sleeve instead of the bottle, and a printed sleeve can make a clear bottle look coloured to the camera. Metallised labels can trigger metal separators. Very small items fall through screens before reaching the sensors at all. Dirty, wet or crushed items give weaker spectra. None of these is a colour problem in the design sense, but all of them are decisions made when choosing a pack's colour and decoration, which is why design-for-recycling guides discuss them together.
| Sensor | Reads | Blind to | Typical colour-related failure |
|---|---|---|---|
| NIR spectrometer | Polymer or fibre chemistry | Visible colour | Carbon-black items give no signal |
| Visible (RGB) camera | Colour and transparency of the outer surface | Polymer type | Printed sleeve makes clear bottle look coloured |
| Combined NIR/VIS spectrometer | Both from one spectrum | What lies under an opaque sleeve | Sleeve read instead of bottle |
| Metal / eddy-current sensor | Conductive material | Plastic colour | Metallised label ejects a plastic bottle |
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
NIR sorting of plastics identifies polymers from overtone and combination bands of C–H and related bonds in the 780–2,500 nm range, and carbon black absorbs so strongly across this range that no usable signal is returned.
FactModerate evidence· United Kingdom
Black plastic packaging coloured with carbon black is not sorted by the NIR equipment widely used in recycling and commonly ends up as residue; NIR-detectable black colourants avoid carbon black while still appearing black.
Source: Recyclability of black plastic packaging; Black plastic packaging hub
FactStrong evidence
Commercial flake and packaging sorters combine an NIR channel for polymer identification with a visible-light camera or spectrometer for colour.
ConventionModerate evidence· North America
Recycler design guidance asks for sortation testing of coloured HDPE whose lightness is below a set L value or whose NIR reflectance is low, because such items have a high probability of not being recognised.
Source: APR Design Guide for Plastics Recyclability: HDPE Rigid – Colored
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.