Conditions for sustained interference: The interference pattern in which the positions of maximum and minimum light intensity remain fixed all along the screen, is called a sustained or permanent interference pattern.

For producing sustained interference, the following conditions must be satisfied:

1) The two sources must be coherent. It means that the two sources vibrate either in the same phase or have a constant phase difference between them.

2) The sources should emit light of the same wavelength and amplitude. 

3) The sources should be narrow and close to each other to observe distinct fringes.

4) At minima, the path difference between the waves should be an odd multiple of λ/2, and at maxima, an even multiple of λ/2.

5) The two light waves must be in the same state of polarization.