On a typical airport or mailroom X-ray screen, orange means organic material, green means mixed or lighter inorganic material, blue means metal, and black means the beam could not get through. The colours are not a photograph of the object — they are a false-colour map that a dual-energy scanner calculates from how each part of the bag absorbs X-rays at two energy levels.
The standard colour convention
| Colour | What it usually indicates | Everyday examples |
|---|---|---|
| Orange / brown | Organic materials — low effective atomic number (Zeff roughly below 10), mostly carbon, hydrogen, oxygen and nitrogen | Clothing, food, paper, wood, leather, most plastics and liquids — and also most explosives and drugs |
| Green | Mixtures, and light inorganic materials (Zeff roughly 10–18) | Aluminium, glass, some salts and powders, overlapping organic and metal items |
| Blue | Metals and other high-Z inorganic materials | Steel, copper, brass, coins, keys, batteries, zips, knife blades |
| Black / opaque | Too thick or dense for the beam to penetrate — no material information | Thick steel blocks, lead-lined pouches, large stacks of dense metal |
The brightness of each colour shows how much material is in the beam path: pale orange is a thin layer of organic material, deep orange a thick one. Vendors use slightly different palettes, thresholds and names, and some add extra hues (for example violet) for very high-Z materials, but the orange–green–blue scheme is the near-universal default on dual-energy cabinet scanners.
How the scanner decides the colour
In a dual-energy scanner, a fan beam from a single X-ray tube passes through the bag onto a line of detectors that measure two energy bands — usually with a thin front detector layer that picks up the low-energy part of the spectrum and a filtered rear layer that picks up the high-energy part. Two physical effects dominate:
- Photoelectric absorption grows very steeply with atomic number and is strongest at low photon energies. Metals soak up the low-energy band much more than the high-energy band.
- Compton scattering depends mainly on electron density and changes slowly with energy. Light, organic materials attenuate both bands more evenly.
Comparing the low- and high-energy signals at each pixel therefore gives an estimate of Zeff, which the software maps to a hue, while total attenuation sets the brightness. A single-energy scanner has only one measurement per pixel and can show density but not material type, which is why its images are greyscale. Most scanners also let the operator switch to greyscale, "organic only", "inorganic only", inverse and edge-enhancement views.
Why colour alone does not identify a threat
Colour is a material hint, not a verdict. Screeners are trained to combine it with shape, density and context — see image interpretation:
- Explosives look like food. Most explosives are organic and appear orange, in the same range as cheese, chocolate or soap. Automated explosive detection algorithms and CT scanners use density and Zeff more precisely, and alarms are resolved by search or trace detection.
- Overlap shifts colours. An organic item lying on top of a metal one is averaged into green. This is why dense, cluttered bags are harder to clear and why a second view (dual-view systems) or a re-scan in a different orientation helps.
- Thickness matters. Very thin metal can look green; very thick organic material can drift towards green or black because the high-energy signal saturates.
- Black areas must be resolved. An area the scanner cannot penetrate could hide anything, so standard procedure is to re-scan, separate the contents or search the bag by hand.
How CT colours differ
Checkpoint and hold-baggage CT scanners reconstruct a 3-D volume and estimate density (and, on dual-energy CT, Zeff) for each voxel, so colour maps are often more precise and can be configured differently. Many systems keep the familiar orange/green/blue palette so that screeners trained on 2-D machines can read them, and add automated threat highlighting on top.
Colours in other kinds of scanner
- High-energy container scanners using linear accelerators can also perform material discrimination with dual-energy or interlaced pulses, but at MeV energies the physics is dominated by Compton scattering and pair production, so material separation is coarser and the colour maps are vendor-specific.
- Backscatter images are normally greyscale; bright areas show organic material near the surface.
- Millimetre-wave body scanners do not use X-rays at all; current systems show a generic outline with yellow boxes over areas that need a pat-down.
Want to check what you have learned? Try the X-ray screening practice quiz.
Frequently asked questions
What does orange mean on an airport X-ray?
Orange indicates organic material with a low effective atomic number — clothing, food, paper, plastics and liquids, but also most explosives and drugs. That is why orange items with suspicious shapes or densities get a closer look.
What does blue mean on a baggage X-ray?
Blue indicates metals and other high-atomic-number inorganic materials such as steel, copper, coins, keys and batteries.
What does green mean on an X-ray scanner?
Green indicates materials with an intermediate effective atomic number, such as aluminium or glass, or a mixture of organic and inorganic materials overlapping in the beam.
Why is part of my bag black on the X-ray screen?
Black means the X-rays could not penetrate that area, so the scanner has no information about it. Screeners will usually re-scan the bag or search it by hand.
Do all scanners use the same colours?
Almost all dual-energy cabinet scanners use orange for organic, green for mixed or light inorganic and blue for metal, but exact thresholds and extra colours vary by manufacturer and settings.
Sources
- Rebuffel & Dinten (2007), Dual-energy X-ray imaging: benefits and limits, Insight 49(10)
- NIST XCOM: Photon cross sections database
- TSA: Security screening
Last reviewed on 2026-10-04.