Double glazing and BREWSTER'S FRINGES
Q. What a funny phrase, what on earth can it be about?
A. It is a rare sort of 'oily' looking effect on a double glazed sealed unit, and it does not mean that the perimeter edge seal has broken down, or that the sealed unit is faulty in any way.
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 occurrence is wavelengths of light meeting up with each other when they are exactly 180 degrees out of phase - an example of this phenomenon is 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.
In the case of insulation glass installations, Brewster's Fringes only occur when the surfaces of the glass in the double glazing sealed unit 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. What happens is that some of the incident light from the Sun meets light reflected from one of the surfaces of the insulating glass in such a way that they are 180 degrees out of phase and cancel each other out, thereby giving rise to a fringe (sort of oily looking) effect, small in area on the glass when viewed from a particular angle. Alternatively, different parts of the incident solar radiation may be refracted through the glass by different amounts and end up by being 180 degrees out of phase. This phenomenon is not a defect of the product, being dependant on the laws of physics and not the quality of the insulating glass sealed unit. In fact it arises because modern glass made by the float process is flat and, therefore, free of the distortion inherent in sheet glass.
The occurrence of Brewster's Fringes is in it's nature rather like (though very much rarer than) the fact that under certain conditions, you could see a reflection of yourself in and window or door, when it is supposed to be clear glass that you can see through, and no-one could claim that this is a defect of the glass.
With any patterned double glazed sealed units, the above phenomenon does not occur, simply because of the very nature of the diffuse qualities of an obscured glass.