In diagnosing the carious lesions, the tooth surface is examined visually. Which of the following is not considered as visual examination?
The caries can be detected by 

Visual methods (Cavitation, Chalky appearance of teeth)
Tectile methods (Checking carefully by using an explorer)
Cavitations
Brownish black discoloration of pits and fissures
Opacification
Softness of the tooth surface