Radiography in Modern Industry - Kodak

photographic step. The useful scale of photographic papers is much less than that ofmicroradiographic films or plates. Hence, if a microradiograph containing a wide range ofdensities is projected directly onto a photographic paper, details may be lost in either the high orlow density areas or in both. Thus, direct printing is only useful for microradiographs of relativelylow contrast. A density difference of about 1.2 to 1.4 between the highest and lowest densities ofinterest in the microradiograph is about the maximum density scale that can be accommodatedby the lowest contrast photographic enlarging paper (Grade 1). Microradiographs with lowerdensity scale can be printed on papers of higher contrast as shown in the table below.Paper Grades for Reproduction of MicroradiographsEnlarging Paper GradeNumberDensity Scale of Microradiograph or Intermediate1 1.2 to 1.42 1.0 to 1.23 0.8 to 1.04 0.6 to 0.85 below 0.6The densities of very small areas on microradiographs may be difficult or even impossible tomeasure with the result that trial exposures on several grades of paper may be necessary. Thecriterion, whether the grade of paper is chosen from the table or by trial and error, is that thedensities in the area of interest in the microradiograph just "fill up" the exposure scale of thepaper.Often, the density scale of interest in the microradiograph is greater than 1.4, or a duplicate-tonereproduction is required, or many copies of the enlarged microradiograph are needed. It is thennecessary to make an intermediate copy on film at the desired magnification, and to contact-printthis intermediate on a suitable grade of paper.Because of the wide range of densities in most microradiographs, a low-contrast intermediate isusually necessary. A fine grained sheet film of the type used for portraiture, developed to agamma of about 0.4, is generally satisfactory.By contact printing this intermediate, copies of the microradiograph can be conveniently producedin any desired quantity. The grade of paper to use with any intermediate can be chosen fromTable VII or by trials with several grades of paper. Grade 0 is available as a contact-printingpaper, and will satisfactorily reproduce intermediates where the density range is 1.4 to 1.6. Lossof significant detail can result if some areas on the paper print are overexposed to a uniform blackand/or others so underexposed that they present a uniform white. Fitting the paper grade to thearea of interest in the intermediate insures that significant details appear with the maximumcontrast, but without loss of "highlight" or "shadow" detail.Electron RadiographyIn electron radiography, electrons emitted by lead foil irradiated by x-rays pass through a thinspecimen of low atomic number. They are differentially absorbed in their passage and record thestructure specimen on a film (See Figure 106).Radiography in Modern Industry 168