Radiographic testing (RT): film, CR and DR

Radiographic testing (RT) is a volumetric non-destructive testing (NDT) method. Radiation — X-rays or gamma rays — passes through the material, and a detector on the far side records an image of what lies inside. On weld seams and castings alike, RT shows discontinuities such as porosity, slag inclusions and lack of penetration as a direct, permanent image. Its greatest strength is that image: a discontinuity appears as a real picture, and that record can be archived. Its greatest responsibility is the ionizing radiation it works with.

1. How it works

As radiation passes through material, it is absorbed at different rates depending on thickness and density. A void such as porosity, or any low-density discontinuity, absorbs less radiation at that spot. More radiation therefore reaches the detector, and the image shows a darker trace on film — or a different gray value on a digital system. How visible that discontinuity is (contrast) depends mainly on the thickness difference, the absorption behavior of the material and the radiation energy, plus the contrast response of the film or detector.

2. Radiation sources: X-rays and gamma rays

X-ray tube: Generated electrically. The energy (kV) is adjustable and the unit can be switched off, so the source is live only during the exposure. It generally gives better contrast and image quality, which makes it the first choice on thin to medium sections — but it needs electrical power and heavier equipment.
Gamma source (isotope): Needs no power, is portable and reaches tight areas — but the source is permanently active (it cannot be switched off) and its energy is fixed. Common isotopes and approximate steel thickness ranges:
- Selenium-75 (Se-75): low energy, thin sections (~5–40 mm), good contrast.
- Iridium-192 (Ir-192): medium energy, the most widely used (~20–90 mm).
- Cobalt-60 (Co-60): high energy, thick sections (~40–200 mm), low contrast.

Select the source according to thickness, access, the required quality level and the safety conditions on site.

3. Imaging media: film, CR and DR

Three generations of radiography record the same physics with different detectors:

Film radiography: Silver-halide film that requires chemical processing (development). It gives high resolution and a physical archive, but it is slow and produces chemical waste.
CR (computed radiography): A reusable phosphor imaging plate stores the latent image, and a scanner converts it to a digital image. CR is faster than film and offers a digital archive and generally a lower dose. Its spatial resolution usually sits slightly below film, but the wide dynamic range compensates for that.
DR (digital/direct radiography): A digital flat panel detector delivers the image instantly. It is the fastest method and normally the lowest in dose, and it allows real-time evaluation. Equipment cost is high.

Digital methods (CR and DR) have clearly improved productivity and traceability in the field, and ASME and ISO both cover digital detectors in separate provisions.

4. Exposure arrangements on welds

Weld radiography rests on the geometry between the source, the seam and the film. The standards (ISO 17636, ASME Section V) define these main arrangements:

SWSI (single wall, single image): The beam passes through one wall only. On accessible plate welds and large-diameter pipe, place the film inside or outside the component.
DWSI (double wall, single image): The beam passes through two walls, but only the weld in the wall nearest the film is evaluated. Used on medium and small-diameter pipe.
DWDI (double wall, double image): On small-diameter pipe, both sides of the seam appear on a single film. Take this exposure elliptically (offset) so that the two wall images do not overlap.

Choose the arrangement according to pipe diameter, access and the area to be evaluated.

5. Image quality: IQI, density and unsharpness

Three elements prove that a radiograph is reliable.

Sensitivity — the IQI (penetrameter). Place an image quality indicator beside the part being examined: wire type (ISO 19232-1) or step/hole type (ISO 19232-2; ASME uses the hole-type plaque). The thinnest wire or the smallest hole visible on the radiograph shows what level of detail the system can resolve at that thickness. If the required wire or hole is not visible, the radiograph is rejected — because the discontinuity you are looking for may have stayed invisible as well.

Optical density (film). Density in the area of interest must fall within the code limits — typically about 1.8–4.0. ASME sets a lower limit of 1.8 for X-rays and 2.0 for gamma; ISO 17636-1 requires roughly 2.0 and above depending on the class. On digital systems the equivalent requirements are signal-to-noise ratio and gray value.

Unsharpness — geometric unsharpness (Ug). The focal spot is not a true point, so the edges of a discontinuity spread slightly. Geometric unsharpness is given by Ug = f · b / d, where f is the effective focal or source size, b the discontinuity-to-film distance and d the source-to-discontinuity distance. The smaller the source and the larger the source-to-film distance (SFD), the lower the unsharpness. That is why the standards set a minimum source-to-film distance and a maximum Ug. Measure total image unsharpness with the duplex wire indicator (ISO 19232-5).

6. Radiation safety

The one factor that sets RT apart from every other NDT method is ionizing radiation. Protection rests on the ALARA principle — as low as reasonably achievable — and stands on three pillars:

Time: Keep exposure time short.
Distance: Dose falls with the square of the distance — stay away from the source.
Shielding: Put lead, concrete or steel between you and the source.

Cordon off the exposure area as a controlled area with barriers and warning signs. Give personnel dosimeters (film, TLD or electronic) and monitor their accumulated dose.

7. Related standards

ASME BPVC Section V, Article 2: Radiographic examination (film and digital detector provisions).
ISO 17636-1 / ISO 17636-2: Radiographic testing of welds — Part 1 film, Part 2 digital detectors (CR/DR).
ISO 19232: Image quality of radiographs — indicators (Part 1 wire type, Part 2 step/hole type, Part 5 duplex wire/unsharpness).
ISO 10675-1: Acceptance levels for radiographic testing of welds — Part 1: steel, nickel, titanium and their alloys.

Apply the acceptance criteria of the fabrication or in-service code that governs the examination.

8. When RT, when UT?

RT and UT complement each other, and the dividing line is largely the orientation of the discontinuity. RT is superior on volumetric discontinuities (porosity, slag inclusions) and on planar discontinuities that happen to be favorably oriented to the beam. Lack of root penetration is a good example: the root gap runs parallel to the beam and produces a sharp line on the film. RT can just as easily miss planar discontinuities lying perpendicular or at an angle to the beam — lack of fusion (LOF) and tight cracks. So take an additional angled exposure whenever you suspect LOF. UT is the stronger method on planar discontinuities, and it gives depth information. On thick and critical welds, automated UT, PAUT and TOFD are steadily taking over from RT. Even so, for access, a permanent record and the evaluation of volumetric discontinuities, RT remains indispensable in many applications.

Türkiye-specific note

In Türkiye (Turkey), acquiring, using, transporting and storing ionizing radiation sources requires a license from the Nuclear Regulatory Authority (NDK), Türkiye's nuclear regulator, under Nuclear Regulation Law No. 7381 (Türkiye, 2022). Authorization for industrial radiography runs through the e-NDK system.

From the field

The most misleading thing about RT is that a good-looking image gets mistaken for a good examination. It isn't one. Without an IQI proving the required sensitivity, without correct geometry (source-to-film distance, beam angle, low geometric unsharpness) and without correct exposure, that film can hide a discontinuity without any trouble at all. And don't forget: RT is a radiation job — safety comes before the image. The best radiograph is the one that was exposed correctly and read correctly. The equipment does not decide the result; technique and discipline do.

Take it to the field: To keep these NDT methods, standard references and field steps in your pocket — completely offline and free — take a look at the Doawise NDT Guide app.

At DoaWise we carry out non-destructive testing services, radiographic testing included, to international standards — from weld and casting examination to digital radiography — producing recordable, auditable results.