In aircraft radiographs, penetrameters must always be removed from the specimen
Detail sensitivity of the radiographic image is revealed by the capability of visualizing the penetrameter holes. When
the 2 percent penetrameter is used on the test object, it is usually required that the 2T penetrameter hole be visible. If
the 2T hole can be seen, the image is said to have 2 percent radiographic sensitivity. The film reader can then assume
the capability of seeing any discontinuity that represents a 2 percent dimensional change of the object total thickness.
The 1T hole does not represent 1- percent image sensitivity because the thickness of the penetrameter has not been
reduced to 1 percent of the test object thickness. Calculations reveal that visualization of the 1T hole in a 2 percent
penetrameter actually reveals 1.4- percent image sensitivity. Resolution of the holes in the penetrameter is a combined
measure of image sharpness and contrast and is thus a measure of the image quality, but note that the regular and
expected outline of the holes is more readily seen than a crack line. The penetrameter should not be placed over an
area of interest, since the penetrameter or the lead identification numbers, may hide discontinuities. In some cases, the
penetrameter cannot be placed on the actual test specimen. In these instances, it is acceptable to place the penetrameter
on a separate block of the same material and of the same thickness as the specimen. In penetrameter placement, it
should be remembered that the purpose of the penetrameter is to reveal the image quality to the film reader, it should
therefore be placed in the least advantageous position. However, the density should not vary more than +30, or -15,
percent from the area of interest. The plaque penetrameters suffers from a number of disadvantages, the most serious
of which is the minimum thickness of 0.005 inches. MIL-STD-00453 provides additional information on the use of
penetrameters. The preceding actions have shown that effective radiographic inspection requires techniques that have
optimum geometry, film choice, contrast and density. Subsequent paragraphs explain how characteristic curves and
technique charts can provide quantitative data to permit precise adjustments.
The exposure factor is a quantity that combines milliamperage (X-ray) or source strength (gamma rays), time and
distance. Radiographic techniques are sometimes given in terms of kilovoltages and exposure factor, or radioactive
isotope and exposure factor. In such a case, it is merely necessary to multiply the exposure factor by the square of the
distance to be used in order to find; for example, the milliampere-minutes or millicurie hours required.
Contrast in a radiograph is the difference in the resultant density that is produced for a given change of X-ray or
gamma ray absorption. It is affected by many factors, some of which must be a compromise. Thus, operator judgment
again becomes important. The choice of X-ray equipment is one of the most important of these considerations. The
shorter the effective wavelength of the X-rays the greater the penetrating power. Also, the higher the kilovoltage on the
tube, the shorter the effective wave length of the generated radiation. As a result, the higher the x-ray tube voltage, the
greater the penetrating power of generated X-rays. This is true for steel with X-rays generated below 8 to 10 MeV, for
aluminum up to 20 to 22 MeV, and for lead up to only 2 to 3 MeV. See Table 6-15.