830 



HANDBOOK OF PHOTOGRAPHY 



ruining tlie radiograph. In most instances shielding with lead blocks or formed lead 

 sheets will be sufficient, but on oddly shaped objects it is often necessarj^ to resort to a 

 more plastic type of shielding material. There are a number of these on the market, 

 none of which is entirely satisfactory. Were it not for its tendency to amalgamate 

 with some metals and penetrate the crystalline interstices of others, mercury would 

 be an ideal shield where a liquid is needed of high atomic weight. Even so, it has been 

 used on occasion and gives a fine radiograph free from all scattering fog. Various 

 lead salts in solution have been used as has the finelj^ powdered red oxide of lead made 

 into a paste with different oils. 



These various shielding materials may also be used as filler where on the same film 

 it is necessary to penetrate two different thicknesses. In some cases, correct exposure 

 for one part will result in a high overexposure of a thinner part. The thinner part is 

 then built up to the same density as the thicker portion. 



Underexposed j 



Uniform 

 exposure 



Fig. 3. 



F/7m 



-For best results in examining curved objects, the film should be symmetrically 

 placed with respect to the source of radiation as shown at the right. 



-F,'/m Lead 



Scoff ereo/ rays 



Less scortfering 



Overexposure and fog 



Fig. 4. — Shielding by means of lead blocks will assist in reducing scattered radiation. 



With small, easily penetrated objects fluorescent screens are not needed. How- 

 ever, beginning with 3^ in. of steel or its equivalent, exposure times are so prolonged 

 or such a high voltage is necessary that too much time is used for each picture or the 

 final film loses detail due to excessive penetration. At this point it is advisable to 

 resort to use of fluorescent screens, one on each side of the film in a lighttight holder. 

 Care must be taken not to bend the screens for their fluorescent coating is easih' 

 cracked, thus introducing errors into the radiograph. A slight curvature is permissi- 

 ble, but where more than this is required to place the screen against the area to be 

 inspected, it is wisest to keep the film flat even if the resulting picture loses something 

 in quality. Where the film holder can be placed against the weld or casting, many 

 simple methods will come to mind for holding it in place and for keeping good contact 

 between screens and film. For example, while examining welding in a large tank, the 

 film may be held against the inner surface by use of a flexible wooden pole slightly 

 longer than the internal diameter of the tank. 



