Radiographic testing is one of the oldest and most trusted inspection methods in non-destructive testing, and for good reason. It gives inspectors a direct look inside a material or component without cutting, drilling, or dismantling anything. Whether you’re working in oil and gas, aerospace, power generation, or manufacturing, understanding how RT works and when to use it can make a real difference in your inspection program.
This guide answers the most common questions about radiographic testing in NDT, from the basic principles to the practical decisions inspectors and quality teams face every day.
How does radiographic testing actually work?
Radiographic testing works by directing radiation—either X-rays or gamma rays—through a material and capturing the transmitted energy on a detector or film placed on the opposite side. Denser areas absorb more radiation and appear lighter on the resulting image, while voids, cracks, or inclusions absorb less and appear as darker regions. This contrast reveals internal features invisible to the naked eye.
The process involves three core elements: a radiation source, the test object, and an image receptor. The source emits radiation that passes through the component being inspected. As the radiation travels through different materials and thicknesses, it attenuates at varying rates. The image receptor—whether film, a computed radiography plate, or a digital flat-panel detector—records the variation in transmitted radiation as a two-dimensional image called a radiograph.
Interpreting the radiograph requires trained eyes and certified expertise. A qualified radiographer reads the image and identifies indications that may represent defects, comparing them against acceptance criteria defined by the relevant inspection standard. The result is a permanent, documented record of the component’s internal condition at the time of inspection.
What are the main types of radiographic testing?
The three main types of radiographic testing in NDT are film radiography, computed radiography (CR), and digital radiography (DR). Film radiography is the traditional method, using photographic film as the image receptor. Computed radiography uses reusable imaging plates that are scanned and digitized after exposure. Digital radiography uses flat-panel detectors to capture images electronically in real time.
Film radiography
Film has been the standard in industrial radiography for decades. It produces high-resolution images and is well understood by inspectors worldwide. However, it requires chemical processing and controlled storage and generates waste, making it increasingly impractical for modern inspection programs.
Computed radiography (CR)
CR replaces film with flexible, reusable imaging plates that can conform to irregular shapes. After exposure, the plate is scanned by a reader unit to produce a digital image. CR is highly portable and cost-effective, making it a popular choice for field inspections and for teams transitioning away from film without fully committing to real-time digital systems.
Digital radiography (DR)
DR uses flat-panel detectors to produce images immediately after exposure, eliminating the scanning step entirely. This dramatically speeds up inspection cycles and enables real-time image review on site. DR systems deliver excellent image quality and integrate naturally with digital reporting and archiving workflows, making them the preferred choice in high-throughput and time-sensitive environments.
What’s the difference between X-ray and gamma-ray radiography?
The key difference between X-ray and gamma-ray radiography lies in the source of radiation. X-rays are generated electrically by an X-ray tube and can be switched on and off. Gamma rays are emitted continuously by a radioactive isotope, such as Iridium-192 or Cobalt-60, and cannot be turned off—only shielded.
X-ray systems offer greater control over exposure parameters, including voltage and current, which allows inspectors to fine-tune image quality for different material thicknesses. They generally produce sharper images and are well suited to workshops or fixed inspection setups. However, they require a power supply and are typically less portable than gamma-ray equipment.
Gamma-ray sources are compact, self-contained, and require no external power, which makes them highly practical for fieldwork, confined spaces, and locations where access to electricity is limited. The trade-off is that the energy of the radiation is fixed by the isotope chosen, offering less flexibility in adjusting exposure parameters. Gamma sources also require strict radiation safety protocols because they emit continuously when not adequately shielded.
The choice between the two typically comes down to the inspection environment, the material and thickness being inspected, and the level of image quality required. Many inspection programs use both, selecting the appropriate source for each specific application.
What defects can radiographic testing detect?
Radiographic testing is particularly effective at detecting volumetric defects—flaws that have a measurable depth or volume within the material. Common defects identified through RT include porosity, slag inclusions, lack of fusion, cracks, undercut, burn-through, and corrosion-related wall loss. It is one of the most reliable methods for inspecting weld quality and assessing internal material integrity.
In welds, RT can reveal a wide range of discontinuities that develop during the welding process:
- Porosity — gas pockets trapped in the weld metal, appearing as rounded dark spots
- Slag inclusions — non-metallic material trapped in the weld, visible as irregular dark areas
- Lack of fusion or penetration — areas where the weld metal has not properly bonded with the base material
- Cracks — linear discontinuities that appear as thin, dark lines on the radiograph
Beyond welds, RT is used to detect corrosion and wall loss in pipelines, pressure vessels, and structural components. It can also identify casting defects such as shrinkage, hot tears, and cold shuts in manufactured parts. One important limitation to keep in mind is that RT is less sensitive to planar defects, such as tight cracks oriented parallel to the radiation beam, where other methods like ultrasonic testing may perform better.
When should you use RT instead of other NDT methods?
Radiographic testing is the preferred method when you need a permanent visual record of internal conditions, when the component has a geometry that makes other methods difficult to apply, or when volumetric defects such as porosity or inclusions are the primary concern. RT is particularly well suited to weld inspection, casting examination, and corrosion assessment in piping and pressure equipment.
Several factors point toward choosing RT over alternatives like ultrasonic testing (UT), magnetic particle inspection (MPI), or dye penetrant testing (PT):
- You need to inspect a weld or component from only one side
- The component has complex geometry that limits probe contact for UT
- You require a documented image record for compliance, audit, or long-term archiving purposes
- The material is non-magnetic, ruling out MPI
- You are inspecting for internal volumetric flaws rather than surface-only defects
On the other hand, RT is not always the best choice. For detecting tight planar cracks, particularly those oriented perpendicular to the surface, UT often provides superior sensitivity. For surface-only defects, PT or MPI are faster and less resource-intensive. RT also involves ionizing radiation, which requires safety planning, exclusion zones, and certified personnel, adding logistical complexity compared to some alternatives.
The most effective NDT programs often combine RT with complementary methods, using each technique where it performs best rather than relying on a single approach for all inspection scenarios.
How Varex Imaging supports your radiographic testing program
We design and manufacture the imaging components and complete systems that make modern radiographic testing possible, from the X-ray source to the detector and analysis software. Whether you are running field inspections on pipelines, performing weld quality control in a manufacturing facility, or managing a large-scale asset integrity program, we offer NDT solutions built around your specific requirements.
Here is how we can support your RT program:
- Computed radiography systems for portable, flexible field inspections with reusable imaging plates
- Mobile digital radiography systems with flat-panel detectors for real-time imaging in demanding environments
- Digital weld inspection platforms, including the SmartRT system, for automated and semi-automated weld assessment
- IQ Analysis and Control Software for image processing, defect marking, measurement, and compliance reporting
- Doppler Z-MLE CUI software for quantitative wall-loss mapping without removing insulation
- A consultative approach in which we take the time to understand your inspection challenges before recommending a solution
We work with NDT engineers, quality managers, inspection service providers, and asset integrity teams across aerospace, oil and gas, power generation, and manufacturing. If you want to discuss how our radiographic testing solutions can improve your inspection program, get in touch with the Varex Imaging NDT team today.