Effect of exposure and scattering on signal-to-noise ratio for improved defect sensitivity in X-ray radiography

The effect of radiographic exposure and scattering on signal-to-noise ratio (SNR), image quality indicator (IQI) sensitivity and defect sensitivity has been investigated in this paper. The effect of scattering on SNR and IQI sensitivity is examined for three different specimens, viz. a SS-Ni alloy plate, a SS compact tension specimen and a SS weld plate, using different object-to-film distances (OFD or air gap) without lead intensifying screens and in contact radiography with lead intensifying screens. It is observed that an optimal exposure improved the SNR. By optimising the exposure conditions, defect sensitivity is improved from 1.86% to 1.36% in a stainless steel (SS) plate of 8.13 mm thickness. Use of a lead screen enhanced the SNR and IQI sensitivity due to the improved contrast sensitivity of the increased exposure by intensification action and by absorption of secondary scattered photons. Up to an optimal OFD, the IQI sensitivity achieved is close to that achieved by contact radiography with lead screens. It is observed that at an increased air gap, the achievable IQI sensitivity is limited by the geometrical unsharpness factor.