The appearance of the optical phenomenon known as Brewster’s Fringes is not a defect of the glass, and can occur with any glass of high optical and surface quality.

This phenomenon is a result of the high quality now being achieved world wide by modern methods of glass manufacture.

Brewster’s Fringes occur if wavelengths of light meet up with each other when they are exactly 180º out of phase – an example of the phenomenon known to physicists as the interference of light.

The effect is similar to, although usually much smaller than, the interference fringes which can sometimes be seen on toughened glass windscreens.

Brewster's Fringes occur when the surfaces of the glass are flat and the two panes of glass are parallel to each other, i.e. when the light transmission properties of the installation are of a very high order.

This phenomenon is not a defect of the product, being dependent on the laws of physics and not on the quality of the insulating glass. In fact it arises because modern glass made by the float process is flat, therefore, free of the distortion inherent in sheet glass.

The occurrence of Brewster’s Fringes is in its nature rather like (though very much more rare than) the fact that under certain conditions, the observer will see a reflection of himself in any window or door – and no-one could claim that this was a defect of glass. 

The above has been quoted from the Glass & Glazing Federation's "Visual quality standard for installed insulating glass units constructed from flat transparent glass" - dated Jan 1998.