Computed Radiography

Although film radiography is still popularly used in many NDT applications, Computed Radiography (CR) and Digital Radiography (DR) have replaced film in many areas. Film radiography has been discussed in previous sections. This section briefly compares CR and DR. Comparing to film radiography, both of them provide digital X-ray images. 

CR generates digital images indirectly. It uses an Imaging Plate (IP), which can be considered as reusable films contain photostimulable phosphor, to detect the X-ray radiation. After exposure
to X-ray, the IP is then read by the CR scanner to convert the captured information into a digital image. After the IP is read, exposing the IP to room-intensity white light will erase the left information and get the IP ready for X-ray exposure again. 

Comparing to film radiography, CR eliminates the need of chemical development process and dark room, generates digital images instead of film images, and still keeps the flexibility to bend around the object. However, CR still relies on a scanner for readout. DR offers instantaneous X-ray digital images with high signal to noise ratio, but its field applications may be limited in some scenarios, since the detectors are rigid, fragile and expensive electronics.

Before beginning the evaluation of a radiograph, the viewing equipment and area should be considered. The area should be clean and free of distracting materials. Magnifying aids, masking aids, and film markers should be close at hand. Thin cotton gloves should be available and worn to prevent fingerprints on the radiograph. Ambient light levels should be low. Ambient light levels of less than 2 fc are often recommended, but subdued lighting (rather than total darkness) is preferable in the viewing room. The brightness of the surroundings should be about the same as the area of interest in the radiograph. Room illumination must be arranged so that there are no reflections from the surface of the film under examination.

Film viewers should be clean and in good working condition. There are four groups of film viewers. These include strip viewers, area viewers, spot viewers, and a combination of spot and area viewers. Film viewers should provide a source of defused, adjustable, and relativity cool light as heat from viewers can cause distortion of the radiograph. A film having a measured density of 2.0 will allow only 1% of the incident light to pass. A film containing a density of 4.0 will allow only 0.01% of the incident light to pass. With such low levels of light passing through the radiograph, the delivery of a good light source is important.

The radiographic process should be performed in accordance with a written procedure or code, or as required by contractual documents. The required documents should be available in the viewing area and referenced as necessary when evaluating components. Radiographic film quality and acceptability, as required by the procedure, should first be determined. It should be verified that the radiograph was produced to the correct density on the required film type, and that it contains the correct identification information. It should also be verified that the proper image quality indicator was used and that the required sensitivity level was met. Next, the radiograph should be checked to ensure that it does not contain processing and handling artifacts that could mask discontinuities or other details of interest. The technician should develop a standard process for evaluating the radiographs so that details are not overlooked.

Once a radiograph passes these initial checks, it is ready for interpretation. Radiographic film interpretation is an acquired skill combining visual acuity with knowledge of materials, manufacturing processes, and their associated discontinuities. If the component is inspected while in service, an understanding of applied loads and history of the component is helpful. A process for viewing radiographs (e.g. left to right, top to bottom, etc.) is helpful and will prevent overlooking an area on the radiograph. This process is often developed over time and individualized. One part of the interpretation process, sometimes overlooked, is rest. The mind as well as the eyes need to occasionally rest when interpreting radiographs.

When viewing a particular region of interest, techniques such as using a small light source

and moving the radiograph over the small light source, or changing the intensity of the light source will help the radiographeridentify relevant indications. Magnifying tools should also be used when appropriate to help identify and evaluate indications. Viewing the actual component being inspected is very often helpful in developing an understanding of the details seen in a radiograph.

Interpretation of radiographs is an acquired skill that is perfected over time. By using the proper equipment and developing consistent evaluation processes, the interpreter will increase his or her probability of detecting defects.