Formula method and double labeling method are two positioning methods on depth of defects. Relative
to the formula method,the double labeling method solves the problems brought by the measurement error of focal
length and focus shift by method of double labeling. This is a method of greater practical significance. Meanwhile,
this method is not wildly used in the field of ray inspection. The first reason is the principles are complicated, and targeted
researches are less; the second reason is, the measurement accuracy cannot be quantified, the credibility is dubious.
Formula method is used in this essay. Through the analysis on change and linearization of principle formula,
the essay defines the position principle on defect depth of double labeling method, derives the position formulation of
defect depth. Using the reduced model, the essay do further analysis and calculation on position accuracy of double
labeling method. Finally, according to the specific article to be detected, the test verified the calculation formula and
the measurement accuracy, certified the reliability on method and accuracy from the point of theory and practice.