In brief
- Same physics as film — what changes is the detector
- CR uses reusable, flexible imaging plates; DR/DDA uses a rigid digital panel
- Lower dose and fewer retakes — a genuine ALARA measure, not just convenience
- Image quality is measured, not assumed: basic spatial resolution and SNR
- Still ionising radiation — a controlled area is still required
Digital radiography replaces radiographic film with an electronic detector. The radiation source, the physics of absorption and the interpretation of the resulting image are unchanged from film radiography — what changes is how the image is captured, how quickly it is available, and what can be done with it afterwards.
Two techniques dominate industrially, and they are not interchangeable.
Computed radiography (CR)
A flexible photostimulable phosphor imaging plate is exposed exactly as a film cassette would be. Radiation excites electrons within the phosphor layer, trapping them in a latent image. The plate is then read in a laser scanner, which releases the trapped energy as light, digitises it, and erases the plate for reuse.
Advantages:
- Flexible. The plate wraps around pipe, conforms to curved castings, fits into restricted geometry. This is CR’s decisive advantage over rigid panels
- Wide exposure latitude — considerably wider than film, so a single exposure copes with substantial thickness variation and retakes drop sharply
- Reusable — thousands of cycles per plate
- Low capital cost relative to detector arrays
- Familiar workflow — same cassettes, same positioning, no darkroom and no chemistry
Limitations: requires a scanning step, so the image is not instantaneous; spatial resolution is generally below a good detector array; plates degrade with handling and must be inspected for artefacts.
Digital detector arrays (DR / DDA)
A rigid panel of detector elements converts radiation directly into an electronic signal, displayed within seconds.
Advantages:
- Immediate image — no scanning, no processing
- Best signal-to-noise ratio and dynamic range
- Lowest dose for equivalent image quality, which shortens exposure and shrinks the controlled area occupancy
- Highest throughput — suited to production lines and repetitive inspection
Limitations: highest capital cost; rigid panels restrict access geometry and cannot conform to curvature; panels are damage-sensitive; and they require periodic characterisation to demonstrate performance.
CR or DR — choosing
| CR | DR / DDA |
|---|
| Detector | Flexible imaging plate | Rigid panel |
| Result available | After scanning | Seconds |
| Geometry | Conforms to pipe and curvature | Restricted by panel rigidity |
| Spatial resolution | Good | Best |
| Signal-to-noise | Good | Best |
| Dose | Lower than film | Lowest |
| Capital cost | Moderate | High |
| Best suited to | Field work, circumferential welds, CUI, varied geometry | Production lines, repetitive work, fixed setups |
In practice CR is the field technique and DR the production technique — but the deciding factor is usually geometry, not preference. A panel that cannot reach the joint is not an option regardless of its specification.
Image quality is measured, not assumed
This is the part most often glossed over in marketing material about digital radiography, and it is where the technique either delivers or does not.
Basic spatial resolution (SRb) — the smallest detail the system can resolve, determined with a duplex wire image quality indicator under ISO 19232-5. It is a property of the whole system: detector, geometry, focal spot and processing. A high pixel count does not by itself give high spatial resolution.
Signal-to-noise ratio (SNR) — normalised for basic spatial resolution, this governs whether low-contrast indications are visible above noise. Standards specify minimum values.
Contrast sensitivity — still demonstrated with a conventional IQI under ISO 19232-1, exactly as with film. If the required wire is not legible, the radiograph is invalid regardless of how good the image looks on screen.
Digital enhancement does not create information. Contrast adjustment, filtering and zoom make existing information easier to see. They cannot recover detail the system never captured — and aggressive processing can introduce artefacts that mimic indications. Raw data is retained for exactly this reason.
Advantages over film that actually matter
Speed to decision. Minutes instead of hours. When a welder is standing beside the joint, the difference between a decision now and a decision tomorrow is real money.
Lower dose, fewer retakes. Wider latitude means fewer exposures repeated for density. Combined with shorter exposure times, this is a genuine ALARA measure — less cumulative dose to the team, and less time with a controlled area in force.
Archiving and transfer. Radiographs stored digitally can be shared with the client and with inspection authorities immediately, re-analysed years later without degradation, and kept in DICONDE format — the industrial equivalent of medical DICOM, designed so images and their metadata stay together and remain readable across systems.
Remote review. A Level 3 or a third-party inspector can review the radiograph without travelling to site.
No chemistry. No darkroom, no processing chemicals, no chemical waste stream.
What does not change
It is still ionising radiation. A controlled area, cordoning, dose monitoring and an authorised operator are still required. Digital techniques reduce dose; they do not remove the hazard.
Access is still needed on both sides of the wall. Source one side, detector the other. For pipe this can be achieved from outside using a double-wall technique.
Planar defects are still frequently missed. Better images do not overcome the physics: a tight crack perpendicular to the beam produces almost no thickness difference and therefore almost no contrast. Where planar defects govern, use PAUT or TOFD.
Thickness limits still apply, set by source energy.
Our capability. Gamma radiography is available for projects in Poland, where we hold the required authorisation for radioactive sources. Outside Poland we operate X-ray equipment up to 250 kV. Beyond the practical range of that equipment we inspect by PAUT or TOFD.
Standards
| Scope | Standard |
|---|
| RT of welds — digital techniques (CR and DDA) | ISO 17636-2 |
| Computed radiography — general principles | ISO 16371-1 |
| Computed radiography — classification of systems | ISO 16371-2 |
| Image quality indicators — wire type | ISO 19232-1 |
| Duplex wire IQI — basic spatial resolution | ISO 19232-5 |
| Acceptance levels — steel, nickel, titanium | ISO 10675-1 |
| Weld quality levels | ISO 5817 |
| Personnel certification | ISO 9712 |
| US — computed radiography practice | ASTM E2033 |
| US — DDA practice | ASTM E2698 |
| US — DDA performance characterisation | ASTM E2597 |
| US practice — radiographic examination | ASME BPVC Section V, Article 2 |
Frequently asked questions
What is the difference between CR and DR?
CR uses a flexible imaging plate that is exposed, then read in a scanner and reused. DR uses a rigid digital panel that produces the image within seconds. CR is more adaptable to geometry; DR is faster and offers better resolution and signal-to-noise.
Is digital radiography more sensitive than film?
Detector arrays generally achieve better signal-to-noise and require lower dose, while good film can still offer higher spatial resolution. Sensitivity is demonstrated on each radiograph through IQI legibility, not assumed from the technique.
Does digital radiography still require a controlled area?
Yes. It is still ionising radiation. Dose per exposure is lower and exposures are shorter, which reduces total dose, but the safety regime is unchanged.
In what format are digital radiographs supplied?
Commonly JPG or PDF with the interpretation report, and raw files in DICONDE or TIFF on request. DICONDE keeps image and metadata together and remains readable across systems.
Can digital radiography detect cracks?
Only when favourably oriented, exactly as with film. Tight planar defects perpendicular to the beam are frequently missed regardless of detector type. Use ultrasonic techniques where cracks govern.
What is basic spatial resolution?
The smallest detail the imaging system can resolve, measured with a duplex wire IQI under ISO 19232-5. It is a property of the whole system, not of pixel count alone.
Need digital radiography to ISO 17636-2?
We operate both CR and DR systems and hold ISO 9712 Level 3 in RT — we write and approve the procedure, not only take the exposures. Gamma radiography in Poland, X-ray across Europe.
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