First, it must provide light of an intensity that will illuminate the areas of interest in the radiograph to their best advantage, free from glare. Second, it must diffuse the light evenly over the entire viewing area. The color of the light is of no optical consequence, but most observers prefer bluish white. An illuminator incorporating several fluorescent tubes meets this requirement and is often used for viewing industrial radiographs of moderate density.
For routine viewing of high densities, one of the commercially available high-intensity illuminators should be used. These provide an adjustable light source, the maximum intensity of which allows viewing of densities of 4.0 or even higher.
Such a high-intensity illuminator is especially useful for the examination of radiographs having a wide range of densities corresponding to a wide range of thicknesses in the object. If the exposure was adequate for the greatest thickness in the specimen, the detail reproduced in other thicknesses can be visualized with illumination of sufficient intensity.
The contrast sensitivity of the human eye (that is, the ability to distinguish small brightness differences) is greatest when the surroundings are of about the same brightness as the area of interest. Thus, to see the finest detail in a radiograph, the illuminator must be masked to avoid glare from bright light at the edges of the radiograph, or transmitted by areas of low density. Subdued lighting, rather than total darkness, is preferable in the viewing room. The room illumination must be such that there are no troublesome reflections from the surface of the film under examination.
FILM PACKAGING
Industrial x-ray films are available in a number of different types of packaging, each one ideally suited to particular classes of radiography.
Sheet Films
Formerly, x-ray films were available only in individual sheets, and this form is still the most popular packaging. Each sheet of film may be enclosed in an individual paper folder (interleaved). The choice between the interleaved and non-interleaved films is a matter of the user's preference. When no-screen techniques are used, the interleaving paper can be left on the film during exposure, providing additional protection to the film against accidental fogging by light or marking by moist fingertips. In addition, many users find the interleaving folders useful in filing the finished radiographs, protecting them against scratches and dirt during handling, and providing a convenient place for notes and comments about the radiograph.
Envelope Packing
Industrial x-ray films are also available in a form in which each sheet is enclosed in a folder of interleaving paper sealed in a light tight envelope. The film can be exposed from either side without removing it from the envelope. A rip strip makes it easy to remove the film in the darkroom for processing. This form of packaging has the advantage that the time-consuming process of darkroom loading of cassettes and film holders is eliminated. The film is completely protected from finger marks and dirt until the time the film is removed from the envelope for processing.
Envelope Packing with Integral Lead Oxide Screens
The main feature of this type of packaging is that the sheet of film is an envelope is enclosed between two lead oxide screens which are in direct contact with the film. This form of packaging affords great convenience in material handling in an industrial x-ray department. As pointed out in "Lead Oxide Screens", it provides the advantage of cleanliness. This is particularly important where heavy inclusions in the specimen are significant. The use of film in this packaging prevents the images of such inclusions from being confused with artifacts caused by dust, cigarette ash, and the like being introduced between film and screen during darkroom handling. The time- consuming process of loading and unloading cassettes and film holders is avoided.
Roll Films
In the radiography of circumferential welds in cylindrical specimens, in the examination of the joints of a complete frame of an aircraft fuselage, and the like, long lengths of film permit great economies. The film is wrapped around the outside of the structure and the radiation source is positioned on the axis inside allowing the examination of the entire circumference to be made with a single exposure. Long rolls of film are also convenient for use in mechanized exposure holders for the repetitive radiography of identical specimens or for step-and-repeat devices in which radiation source and film holder move in synchronism along an extended specimen.
HANDLING OF FILM
X-ray film should always be handled carefully to avoid physical strains, such as pressure, creasing, buckling, friction, etc. The normal pressure applied in a cassette to provide good contacts is not enough to damage the film. However, whenever films are loaded in semiflexible holders and external clamping devices are used, care should be taken to be sure that this pressure is uniform. If a film holder bears against a few high spots, such as occur on an unground weld, the pressure may be great enough to produce desensitized areas in the radiograph. This precaution is particularly important when using envelope-packed films.
Marks resulting from contact with fingers that are moist or contaminated with processing chemicals, as well as crimp marks, are avoided if large films are always grasped by the edges and allowed to hang free. A convenient supply of clean towels is an incentive to dry the hands often and well. Use of envelope-packed films avoids these problems until the envelope is opened for processing. Thereafter, of course, the usual precautions must be observed.
Another important precaution is to avoid drawing film rapidly from cartons, exposure holders, or cassettes. Such care will help materially to eliminate objectionable circular or treelike black markings in the radiograph, the results of static electric discharges.
The interleaving paper should be removed before the film is loaded between either lead or fluorescent screens. When using exposure holders in direct exposure techniques, however, the paper should be left on the film for the added protection that it provides. At high voltage, direct- exposure techniques, electrons emitted by the lead backing of the cassette or exposure holder may reach the film through the intervening paper or felt and record an image of this material on the film. This effect is avoided by the use of lead or fluorescent screens. In the radiography of light metals, direct-exposure techniques are the rule, and the paper folder should be left on interleaved film when loading it in the exposure holder.
The ends of a length of roll film factory-packed in a paper sleeve should be sealed in the darkroom with black pressure-sensitive tape. The tape should extend beyond the edges of the strip 1/4 to 1/2 inch, to provide a positive light tight seal.
Identifying Radiographs
Because of their high absorption, lead numbers or letters affixed to the subject furnish a simple means of identifying radiographs. They may also be used as reference marks to determine the location of discontinuities within the specimen. Such markers can be conveniently fastened to the object with adhesive tape. A code can be devised to minimize the amount of lettering needed. Lead letters are commercially available in a variety of sizes and styles. The thickness of the letters chosen should be great enough so that their image is clearly visible on exposures with the most penetrating radiation routinely used. Under some circumstances it may be necessary to put the lead letters on a radiation-absorbing block so that their image will not be "burned out". The block should be considerably larger than the legend itself.
Shipping Of Unprocessed Films
If unprocessed film is to be shipped, the package should be carefully and conspicuously labeled, indicating the contents, so that the package may be segregated from any radioactive materials. It should further be noted that customs inspection of shipments crossing international boundaries sometimes includes fluoroscopic inspection. To avoid damage from this cause, packages, personal baggage, and the like containing unprocessed film should be plainly marked, if possible, and the attention of inspectors drawn to their contents.