Active vs. passive imaging
Passive scanners detect the natural millimetre-wave radiation your body emits — like thermal imaging, but at shorter wavelengths. Active scanners transmit their own signal and measure the reflection, giving much higher resolution and the ability to see into cavities. All modern airport portals are active.
How it resolves objects
At 30 GHz the wavelength is 10 mm. A pistol or knife under clothing creates a shadow and reflection pattern different from skin. The scanner uses holographic reconstruction or synthetic aperture techniques to build a depth image. Materials matter: metals reflect strongly, plastics and ceramics weakly, but all are detectable against the skin background.
Privacy and algorithmic masking
Early scanners produced recognisable human images, sparking outrage. Modern systems use automated threat detection (ATD) software that analyses the raw data and displays only a generic stick figure with a yellow box over the anomaly. No human sees the body image unless the machine flags something. The EU mandates this; the US TSA adopted it after trials.
Beyond airports
Millimetre-wave scanners are being tested for border checkpoints, stadium entry, and even walk-through fever screening. The same radar physics, repackaged for different threats. The technology will become as invisible as metal detectors are today — and just as ubiquitous.