The quality of glass used in the production of double glazed sealed units has improved significantly over recent years. However due to the manufacturing process of both the glass and the sealed unit it goes into, there may be instances in certain lighting conditions, of small scratches, blemishes or imperfections being apparent.

The information detailed below is designed to help our customers understand the tolerances allowed under the standards we and the rest of our industry work to.

1. Transparent glass used in the manufacture of insulating glass units is identical to that used traditionally for single glass and will, therefore, have a similar level of quality.

2. Both panes of the sealed unit shall be viewed at right angles to the glass from the room side standing at a distance of not less than 2 metres but for toughened, laminated or coated glasses (not less than a distance of 3 metres) in natural daylight and not in direct sunlight with no visible moisture on the surface of the glass. The area to be viewed is the normal vision area, with the exception of a 50mm wide band around the perimeter of the unit.

3. Flat transparent glass, including laminated or toughened (tempered) glass, shall be deemed acceptable if the following phenomena are neither obtrusive nor bunched : totallly enclosed seeds, bubbles or blisters; hairlines or blobs; fine scratches not more than 25mm long, minute embedded particles. Obtrusiveness of blemishes shall be judged by looking through the glass, not at it, under lighting conditions as described paragraph 2.

4. When thermally toughened glass is viewed by refection, the effect of the toughening process may be seen under certain lighting conditions. The visibility of surface colouration or patterns does not indicate deterioration in the physical performance of the toughened glass. Because of the nature of the toughening process, distortion will be accentuated when the glass is viewed in reflection or incorporated in insulating glass units.

5. Visible double reflection can occur under certain lighting aspect conditions, especially when viewed from an angle. This is an optical phnomenom arising from multiple surface reflections in selaed units.

6. The manufacture of flat laminated glass does not usually affect the visual quality of the glass incorporated in insulating glass units. However, the faults generally accepted in paragraph 3, may be increased in number if glasses and interlayers are used in the production of laminated glass. When viewed under certian light conditions, insulating glass units incorporating clear or tinted flat laminated glass may show a distortion effect caused by reflection on the multiple surfaces of the components of the laminated glass.surfaces

7. Brewster’s Finges . The apperance of the optical phenomenon known as Brewster’s Fringes is not the defect of the glass, and can only 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 180 degrees 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 them self in any window or door – and no-one could claim that this was a defect of glass.

Note: Patterned Glass - the above criteria does not apply to patterned glass, as due to the method of manufacture, imperfections such as seeds and bubbles are deemed to be acceptable.