Minimal dentinal destruction to get evident on the radiograph:
Minimal dentinal destruction of 40 microns is required to be seen on a radiograph.
Lesions confined to the enamel may not be evident until approximately 30% to 40% demineralization has occurred. For this reason, the actual depth of penetration of a carious lesion is often deeper than seen in the image.
 
Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 288
30 microns
40 microns
20 microns
200 microns