Abstract: Some problems were analyzed and discussed, which include the meaning of SNR in DR and the purpose of normalization of signal-to-noise ratio (SNR) and the relationship between SNR and compensation principles (CP). Contrast in the testing result of film radiography and DR is both the product of a certain factor and the main subject contrast, and the factor in DR using flat panel detector is SNR, while that in film radiography is related with gradient-to-noise ratio. Film system in radiography testing can be classified according to the SNR parameters (gradient-to-noise ratio), and computed radiography (CR) system can be classified by SNR parameter likewise. With the help of normalized SNR, the image quality in DR and film radiography can be measured and evaluated equivalently. DR technique suffers from inherent detector unsharpness at present, but the unsharpness can be compensated by increased SNR, which is called CP II. This compensation principle seems to have relation with requirement of normalized SNR, but we should know that basic spatial resolution of a digital detector and the measured unsharpness in the testing is different. In the practical testing, we should know the properties of the detector used and the requirement of detection of certain flaws thus ensuring that applicable process is adopted consequently.