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