THz to GHz: Terahertz Imaging and Inspection Bands

Reading a terahertz inspection spec in gigahertz

Terahertz inspection sits awkwardly between two vocabularies. The imaging system, its emitter and its usable bandwidth are quoted in THz; the electronics that drive it — the synthesizer, the multiplier chain, the lock-in reference, the antenna feed — are all specified in GHz. If you are commissioning a paint-thickness gauge or setting up a scan of a composite panel, one of those two numbers usually has to be rewritten before it can be compared with anything. Drop the THz figure in below and the gigahertz equivalent appears while you type.

Conversion factor: GHz = THz × 1 000. A source specified at 0.35 THz for standoff screening is 350 GHz, and a 4 THz time-domain bandwidth reaches 4 000 GHz.

What the frequency choice buys you

Depth against detail

Lower frequencies travel further into foam, ceramic and packaging; higher ones resolve thinner layers. A multi-coat paint stack is read with the whole band, then the individual clearcoat, basecoat and primer echoes are separated in time.

Photon energies in millielectronvolts

Across 0.1–10 THz a photon carries roughly 0.41–41 meV. That is thousands of times below the energy needed to strip an electron, which is the whole reason personnel screening with these wavelengths is treated differently from X-ray.

Wavelengths you can picture

1 THz is a wavelength of about 0.30 mm; 0.3 THz is close to 1 mm. Converting to GHz first makes the arithmetic against a millimetre-scale defect size much easier to sanity-check.

Setting up a scan from the source datasheet

Start with the figure on the emitter sheet

Enter the centre frequency or the band limit as printed — 0.1, 0.35, 1.2, 4. Values below 0.000001 THz would switch the display to scientific notation, but nothing in an inspection spec goes anywhere near that floor.

Match it against the driving electronics

The gigahertz figure is the one that lines up with a multiplier output, a mixer LO or a horn antenna’s rated band. Editing the left field re-runs the comparison on every keystroke, which is quicker than stepping through a spreadsheet.

Copy it into the acquisition settings

The copy button on each field puts the plain number on the clipboard — no unit, no thousands spaces — so it pastes cleanly into a scanner configuration file. Ctrl+C with the cursor in a field does the same.

Flip it when the number arrives in GHz

Antenna and amplifier catalogues talk in gigahertz, so press the swap button (↔) — or simply type into the right-hand field — to get back to the THz value your imaging software expects. Either dropdown can be searched if you need a different frequency unit.

Terahertz sources and the inspection jobs they do

Each row pairs the frequency coverage a supplier prints in THz with the same span written in gigahertz. The right-hand column is what the coverage is normally used for in a non-destructive inspection lab.

Source or inspection task	Terahertz	Gigahertz	What it is used on
Photoconductive-antenna TDS	0.1–4 THz	100–4 000 GHz	The workhorse pulsed system; usable dynamic range usually thins out near the top
Air-plasma TDS	0.3–10 THz	300–10 000 GHz	Extended bandwidth for material characterisation and ellipsometry
Frequency-multiplied CW source	0.1–1.2 THz	100–1 200 GHz	Continuous-wave imaging and single-frequency transmission checks
Quantum cascade laser	2–5 THz	2 000–5 000 GHz	High-power narrowband work well above the electronic sources
Standoff personnel screening	0.1–0.35 THz	100–350 GHz	Sees through clothing; the low end keeps atmospheric loss manageable
Automotive paint gauging	0.1–1 THz	100–1 000 GHz	Separating clearcoat, basecoat, primer and e-coat on a wet or dry body
Foam and CFRP inspection	0.2–1 THz	200–1 000 GHz	Disbonds, voids and water ingress in aerospace panels and insulation
Tablet coating thickness	0.1–3 THz	100–3 000 GHz	Pharmaceutical film coatings measured without cutting the tablet

Bandwidth limits converted end to end

Enter the low limit, copy it, then the high limit: a 0.2–1 THz inspection window comes out as 200–1 000 GHz with both ends in the same notation as the RF chain.

Fine steps for a narrowband source

A CW source tuned to 0.567 THz reads as 567 GHz, and up to eight decimals are kept, so a small tuning offset does not vanish into a rounded figure.

Other frequency units on the same page

Both selectors search the full frequency list, so a lock-in reference in kHz or a laser repetition rate in MHz can be checked against the imaging band without leaving the page.

Terahertz inspection questions

Why is terahertz radiation described as non-ionising?

Photon energy scales with frequency, and at these frequencies it is tiny. A 1 THz photon carries about 4.1 meV; across the whole 0.1–10 THz range the figure runs from roughly 0.41 to 41 meV. Knocking an electron off a molecule takes on the order of 10 eV, some two thousand times more at 1 THz. The beam can heat a sample if you pour enough power in, but it cannot break chemical bonds the way X-rays do — which is why screening systems using these wavelengths are treated as a different class of instrument.

How deep into a part does the beam actually get?

It depends on both the material and where in the band you look, and the trend is consistent: penetration drops as frequency rises. Low-density foam and dry packaging can be sounded through several centimetres near 0.1–0.2 THz (100–200 GHz), while a filled polymer or a thick paint stack may only give useful echoes for a millimetre or two. Scattering makes it worse — a coarse composite weave or a pigment-loaded coat sends energy sideways long before absorption alone would stop it. In practice you choose the lowest frequency that still resolves the layer you care about.

What bandwidth does a time-domain system really cover?

A standard photoconductive emitter and receiver pair spans roughly 0.1–4 THz, that is 100–4 000 GHz, but the honest working range is narrower: dynamic range falls away steadily above about 3 THz, so the top of the sheet is not the top of the useful data. Air-plasma generation pushes coverage towards 10 THz at the cost of a much bigger optical setup. Time resolution matters as much as bandwidth — the spectral resolution you get is set by how long a delay-line trace you record, not by the emitter.

What sets the spatial resolution of a terahertz image?

Diffraction, in the ordinary way: the focused spot cannot be much smaller than the wavelength. At 0.3 THz (300 GHz) that is close to 1 mm, at 1 THz about 0.30 mm and at 3 THz roughly 0.10 mm, with the optics’ numerical aperture deciding how near you get. Depth resolution is a separate matter — it comes from the pulse length in time, which is why a broadband system can separate coating layers far thinner than its lateral spot size. Near-field probes beat the diffraction limit but only right at the surface.

Which materials are opaque to the beam and which are see-through?

Metal reflects essentially all of it, which is useful: a metal substrate under a coating gives a clean back-wall echo to time against. Liquid water absorbs enormously, so damp cardboard, fresh adhesive or a hydrated tissue sample blocks the beam within a fraction of a millimetre — also useful, because it makes moisture ingress easy to spot. Reasonably transparent are dry paper and card, most plastics and polymer foams, ceramics, textiles and many pharmaceutical excipients — the split that makes this band suited to non-metallic parts and to objects hidden under clothing.