Most common form of blem is known as spots or blackspots. Blackspots can occur either during manufacturing of the IIT or due to incorrect use (for example laser damage).
Spots that occur during manufacturing can vary in size and their location determine the price and classification of the ITT. For example spots may determine if a tube falls under Photonis commercial or ECHO category. Location of spots in ITTs are divided between 3 zones. Different IIT classifications have standards on how many or big spots may occur in every zone.
Note: ECHO tube that has a blackspot due to incorrect use, is still in ECHO classification. It’s just damaged.