What are the Condition for Sustained Interference?

Condition for sustained interference:

The interference pattern in which the positions of maximum and minimum intensity of light remain fixed with time, is called sustained or permanent interference pattern. The conditions for the formation of sustained interference may be stated as :

(i) The two sources should be coherent

(ii) Two sources should be very narrow

(iii) The sources should lie very close to each other to form distinct and broad fringes.