Quality of Vision – A GGF Guide

15A common sense approach needs to be taken and an understanding that modern processed glass is not going to be flawless, as glass is for looking through not looking at.

Sealed Units

Sealed units provide a high standard of vision.  The following is a guide to the quality that can be expected.

Glass used in the manufacture of sealed units is similar to that used traditionally for single glass and will, therefore, have a similar level of visual quality.



How to do a Professional Inspection

Stand in the room no less than 2 metres away from the sealed unit and look directly through it.

  • For toughened, laminated or coated glasses, stand no less than 3 metres away.
  • Where it is not possible to stand at the right distance then stand as far away as you can from the sealed unit.
  • Do so in natural daylight, but not directly towards the sun and with no visible moisture on the surface of the glass.
  • Exclude 50mm wide band around edge of the glass from the check.
  • Glass must be viewed at an angle of 900.

What to Expect when Carrying Out an Inspection

Flat transparent glass, including laminated, toughened or coated glass is acceptable if the following are neither obtrusive nor bunched:

  • Bubbles or blisters
  • Fine scratches not more than 25mm long
  • Minute particles

The obtrusiveness of blemishes is judged by looking through the glass, not at it, under natural light.  It must be understood that the glass used in sealed units is a processed glass, and as a consequence, blemishes are to be expected.


Special Glasses

Toughened glass may show visual distortions which are accentuated by reflections in sealed units.  This is a natural phenomenon and not a fault.

Laminated glass may have a few more blemishes due to it being made of several layers.

Some low emissivity coatings may produce transient visual effects.

In some lighting conditions the coating may look like a transparent film or produce a haze, i.e. a cloudy look to the surface.

When light coloured objects such as net curtains are placed close to the glazing they may look slightly darker.


Whilst sealed units will generally reduce the incidence of condensation, it may still occur under certain conditions.

Multiple Reflections

This occurs in certain light conditions and is caused by multiple surface reflections within sealed units which may vary from pane to pane. 

Brewster’s Fringes – the Rainbow Effect

Small transitory rainbow effects are sometimes produced by the glass refraction of light.  Their appearance is due to high quality flat glass sheets being placed parallel to each other.

 Patterned Glass

This document does not apply to patterned glass as its manufacturing process is different.

As members of the Glass and Glazing Federation (GGF) we pride ourselves on the level of service and quality of our aluminium bi-folding doors, sliding patio doors, casement windows, curtain walling and facades and our range of Alu-Clad Timber doors and windows.

We are always happy to help and will endeavour to answer any questions you may have.  Please telephone us on 0113 3869207, or use the Contact Us page.

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