Glass:
Brewsters Fringes

Double glazing and BREWSTER'S FRINGES

Question: What a funny phrase, what on earth can it be about?
Answer: 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 worldwide by modern methods of glass manufacture. Brewster's Fringes appear when wavelengths of light meeting up with each other exactly 180 degrees out of phase - this phenomenon is known to physicists as the 'interference' of light. The effect is similar to, although usually much smaller than, the interference fringes that can sometimes be seen on toughened glass windscreens.

In the case of insulation glass installations, Brewster's Fringes occur only 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 an 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 dependent 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 its nature rather like (though very much rarer than) the fact that, under certain conditions, you might see a reflection of yourself in a window or door, even though it is supposed to be completely transparent clear glass, and no one could claim that this is a defect of the glass.

Special note:
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.

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