EGL Technical Bulletin 011. Visual Effects in Insulating Glass Units (IGUs)

Transcription

1 Visual Effects in Insulating Glass Units (IGUs)

2 Table of Contents Page No Summary Why are Reflected Images Distorted? Optical Phenomena Newtons Rings Brewsters Fringes Anisotropy Multiple Imaging Suggested Guidelines for Reducing Distortion in Reflected Images Asymmetric IGU Construction Reducing Thermal Treatments Annealed Glass 7 Annex A 9 Annex B 10 Disclaimer This Technical Bulletin published by Emirates Glass LLC is subject to change without notification and Emirates Glass LLC does not accept liability for errors and omissions contained therein September 13, Prepared By: John Reeves

3 Observing minor variations in flatness due to air space volume change should be celebrated and embraced to be acceptable, as this indicates an enduring hermetic seal of the IGU. Specifying Thermal heat treatment of Architectural float glass is equal to specifying that a variety of Optical and Visual effects will be present in the finished product. Generally, therefore in most circumstances, the Visual Effects noticed in IGUs cannot be described as defects. They are inherent in the product due to elementary physics and the nature of processing. September 13, Prepared By: John Reeves

4 Two processes on float glass have an impact on the optical flatness of the glass. First is the thermal heat treatment and second is the fabrication into a hermetically sealed Insulating Glass Unit (IGU). The variation in flatness is essentially that causes the reflected image to be distorted. See Annexes A and B. There are five variants of flatness alteration. 1. Roller Wave 2. Overall Bow 3. Local Bow 4. Edge Dip 5. Pillowing The first four result from the Thermal Heat Treatment Process and can be controlled and measured. EN 1863 and EN outline the tolerances in variation from flatness that are permitted. The last, pillowing, results from the hermetically sealed airspace changing volume due to temperature and atmospheric pressure changes. This latter has no commercially available control mechanism. For additional information on this subject, recommended reading is the Glass and Glazing Federation (GGF) data sheet 4.10 Products, Appearance and Visual Quality Specification for Insulating Glass Units and Glass Association of North America (GANA) Glazing Manual, section II, Fabricated Architectural Glass Products, Insulating Glass Units. September 13, Prepared By: John Reeves

5 Due to anticipated flatness variation it must be true that if an IGU displayed zero distortion in reflection then there would be no thermal heat treatment and the Hermetic seal would be compromised, such that pressure equalisation was constant. Zero distortion therefore, would be greater cause for concern and additionally might be cause for rejection. Four other forms of Optical Effects may manifest that are not precisely related to flatness variation. These Optical Phenomena are intriguing, but are typically not cause for rejection. These are: Newtons Rings Where the two panes of glass either touch, or almost touch due to deflection caused by the internal air volume change light may be fragmented into its constituent parts. The effect manifested is that of the colours of the rainbow appearing in roughly circular formations in the vicinity of the contact area Brewsters Fringes On very rare occasions, where perfectly flat glass panels of equal thickness, with perfectly parallel surfaces are assembled into an IGU and the individual panes are also perfectly parallel then it may be possible for the refraction of light to cause a rainbow effect to appear Anisotropy Also called iridescence and sometimes referred to as leopard spots or Quench Mark because of the way in which the visual variation appears, can appear in thermally heat treated glass when viewed at certain angles under polarised light. These zones appearing sometimes grey shading and even a slight colouration have a direct correlation ship with the quench nozzle zones produced during the Thermal treatment. Slight variations within the stress regime in the Thermally Treated glass have a bi-refringent affect with the light. September 13, Prepared By: John Reeves

6 Polarising light is dependent on the time of day, weather conditions, proximity to an ocean and, of course, observation angle Multiple Imaging Where a multi-pane IGU has glass-to-air surfaces with relatively high visible light reflective values and/or where the air space between is extended, then at certain observation angles it is possible to see reflected images either blurred at the edges or multiplied with a displaced overlap. September 13, Prepared By: John Reeves

7 4.1. Asymmetric IGU Construction As pillowing will effect equal thickness panes equally then a reduction in the visual affect would be to make the outer pane always the thicker one. A small added benefit is that asymmetric IGU also have increased sound insulation as sympathetic resonance is reduced Reducing Thermal Treatments Where possible, provided safety is not compromised and given careful consideration to mechanical load it should be possible to eliminate Toughening the glass. Heat Strengthening is known to have a reduced Roller Wave Annealed Glass Annealed float glass is very flat and has almost no optical defects. Whilst it is uncommon to use annealed glass in Buildings in the UAE one has to consider that it is widely used elsewhere. It is always possible to consider its use in the UAE and, where the inner panel is laminated, Heat treatment is often unnecessary. Thermal Breakage risk will of course need analysis. Annealed glass may be under risk of thermally induced Breakage when the temperature variation across the pane exceeds 60 o C.Before rejecting the use of annealed glass one must consider that there is approximately 5 million annealed laminated glass panes installed throughout the UAE with black painted borders. September 13, Prepared By: John Reeves

8 Those borders will absorb solar radiation at a much higher rate than the glass centre pane. These panels are in every front windshield of every car. How many do we see that are thermally cracked? Nonetheless, before opting to use annealed float glass in an IGU construction, mechanical loading and safety risk must be considered as well as thermal breakage risk. September 13, Prepared By: John Reeves

9 September 13, Prepared By: John Reeves

10 September 13, Prepared By: John Reeves

11 September 13, Prepared By: John Reeves

12 Emirates Glass L.L.C., P.O. Box: 29769, Dubai, UAE Tel: Fax: emiglass@emirates.net.ae Website: