AGP Guide to visual quality of Commercial glass & Insulating Glass Units (IGUs)
V1.0 12/03/2025
It is important to understand that although our Glass & Insulating Glass Units (IGUs) are particularly good, they will not be everyone’s idea of perfect. There are national & international standards about quality for the manufacture & processing of both glass & IGUs.
The intention of this guide is to explain the key points for:
Allowable visual characteristics
Brewster’s Fringes:
A rainbow effect that can sometimes become visible in certain light conditions in IGUs made with identical glass thickness to each pane. Faint coloured bands of irregular shapes. Multiple light reflections interfering and overlapping. Allowable.
Defects & the relevant standards.
Normal visual characteristics (not considered defects):
Deflection, reflection, Brewster’s Fringes, Iridescence, preferential wetting, ATS properties, compliance markings, inherent colour, external dew, Low-E & reflective coating properties, laminated glass properties, & enamelled or ceramic digital printed glass properties.
Deflection and reflection:
An IGU is a flexible pressure vessel that will expand and contract depending on the climate load on site at any given time compared to our factory at the time of manufacture.
The IGU panes will also flex under wind load which will make the reflection move. Allowable.
Iridescence (Anisotropy):
Often referred to as strain patterns, stress patterns or Leopard Spots. Optical light interference, non-uniform light refractions passing through the glass. Created by a variance in the stress through the thickness of the heat strengthened or toughened glass. A striped pattern sometimes visible in certain light conditions. More noticeable in polarized light. Iridescence is a visible effect of the glass processing. Allowable.
Preferential wetting:
When the glass is wet, patterns become visible on the damp surface. Showing outlines of the processing and installation equipment that has been in contact with the surface. Although the equipment has not left a residue on the glass surface it has changed the surface tension. Allowable.
Compliance marking:
Safety glass, heat strengthened glass and IGUs must be marked to show compliance with the New Zealand Building Code. Glass marking is a small roundel laser etched or sand blasted on the glass near a corner. IGU spacer marking is typically yellow printing on the spacer near the spacer join. Allowable.
ATS - Architectural Thermal Spacer® properties:
ATS position at linear edge of glass, specified position ± 2.0mm. Allowable.
ATS corner radius ≤ 8mm. allowable.
ATS join. Architectural Thermal Spacer has an automated factory seam where the slightly curved shape of the spacer is flattened down to close the join. Allowable.
Inherent colour:
Variance in colour impression are possible due to the normal variance in float glass & normal variance in the coating process for Low-E & reflective coatings, the use of various glass thicknesses, difference of viewing angle, & difference in lighting condition. Variance in colour impression is unavoidable. Specifically ordering all IGUs at a common thickness in one batch, will minimise any variance. Allowable.
External dew:
Solux-E®, Solux Ultra™ & other high performance Low-E IGUs are so efficient, dew can sometimes form on the outer surface. This is not a defect, rather a by-product of the low emissivity coating & argon gas retaining more heat inside the building. This results in a lower external surface temperature. It is transient & weather dependant. Allowable.
Lamination haze:
Visible due to the presence of laminated glass in single glazing or an IGU make-up. The plastic interlayer partially reflects the light creating the look of a milky sheen in low light conditions. Inherent property of this glass type. Allowable.
Low-E and reflective coated glass properties:
Haze, distortion, edge deletion, & spots.
Haze is a characteristic that is a common consequence of the structure of the coating. Some of the light that enters glass is absorbed, and some scattered by components within the glass. If sufficient light is scattered, then it will appear as a haze in the glass. Allowable.
Digital printing:
Client is responsible for ensuring they have the rights to use any image or pattern supplied to AGP for digital printing.
Digital printing will have barely visible striping in the printing direction, print edges are straight in the print direction and slightly serrated transversely. these are limitations of the technology. Allowable.
Printing viewed perpendicular through the glass with defused light such as an overcast sky, without direct sunlight or artificial lighting.
Puncture spot in enamelled or printed glass: refer to table F.1. Allowable.
Distortion of reflected image can be caused by the framing method, IGU cavity & the glass heat treatment. Allowable.
Edge deletion where Low-E coating is removed at the perimeter coated glass.
Coloured frame line where a portion of the coating is on the primary seal on a linear edge. Allowable.
Coloured frame triangle where a portion of the coating is on the primary seal in a corner. Allowable.
White line where a portion of edge deletion is visible inside the primary seal, inside the vision area. Allowable.
Edge deletion specified position ± 2.0mm. Allowable.
Puncture spot in coated glass: refer to table F.1. Allowable.
Spandrel glass, enamelled Lacobel T, or ceramic digital print full coverage:
Spandrel glass shall not be backlit (neither naturally nor artificially), Spandrel glass is designed to only be viewed from surface #1 & in external reflection. Area behind the spandrel glass must be ventilated and remain dry. Building materials shall not be bonded to nor placed against the back of the spandrel glass.
Hairline scratches & baked in foreign objects ≤ 10mm in length. Allowable.
Paint overhang at edges in framed panels & at holes. Allowable.
Unprinted glass boarders ≤ 2mm. Allowable.
Print position & size tolerance ± 2mm. Allowable..
Variance in colour impression are possible due to the normal variance in float glass & normal variance in the ceramic digital print process, these are unavoidable. Specifically ordering all printed glass at a common thickness in one batch, will minimise any variance. Allowable.
Laminated glass used in structural glazing or silicone butt glazing:
An interlayer edge blushing over time is allowable if less than 6mm from edge (excluding halo). Allowable.
Custom laminates displacement tolerance: 2.0mm displacement allowable.
Structurally glazed & silicone butt joined glazing:
If IGU includes Low-E, that pane will have automated belt arris edge work, that may be visible. Allowable.
HAZE ON GLASS
Defects
Viewing defects:
Before inspecting for defects the glass shall be cleaned in accordance with AGP’s Cleaning and Maintenance Guide. Glass & IGUs shall be viewed from 3m and perpendicular to the glass face, inspected in transmission not reflection with a daylight background unless stated otherwise.
The glass or IGU shall not be in direct sunlight. The viewing period shall not exceed 60 seconds/m2. Defects shall not be marked on the glass. There are two visible zones on an IGU, the edge zone measured as 50mm from the inside edge of the spacer or the inside edge of the frame & the main zone the remainder in the centre of the IGU. Any defect exceeding the limits list here shall be deemed unacceptable.
Newton’s rings:
Where the panes of an IGU come close to touching due to climate loads. Can sometimes be visible as a basketball sized round area of condensation in the middle of the IGU. Suggestive of the incorrect spacer size for the application. Defect.
Reference standards for visual quality:
AS/NZS 4667:2000 Quality requirements for cut to size and processed glass.
BS EN 1279.1 Annex F and Annex G.
AS/NZS2208:1996 including amendment 1 1999. Section 2.
For items not directly covered in these standards the limitations published here are to be used.
For any discrepancies between these standards the above list is the correct hierarchy.
Any quality demands additional to these, or stricter quality requirements must be communicated to the AGP Account Manager at the time of quotation request or before order placement and will only count as having been accepted if they are confirmed by AGP’s Account Manager in writing.
Roller wave:
Roller wave is only considered a defect when it exceeds our published limitations. Also known as Tempered glass distortion. It is an inherent property of this type of glass processing.
Roller wave limitation: ≤ 0.15mm allowable. Measured at least 300mm from the leading or trailing edge using a 300mm ruler & feeler gauge or a roller wave gauge.
Edge kink limitation: ≤ 0.30mm allowable. Measured at the edge using a 300mm ruler & feeler gauge or edge kink gauge.
Viewing Areas Diagram
Key
R Zone of 15mm usually covered by the frame, or corresponding to the edge seal in case of unframed edge
E Zone at the edge of the visible area, with a width of 50mm
M Main zone
Spot faults in glass:
Table F.1 - Allowable number of spot faults
Zone
Size of fault (excluding halo) (Ø in mm)
the pane
(m2)
R All sizes No limitation
Spot faults interfere with the visual transparency when viewed through the glass. It may involve opaque dots (tin marks), holes in the Low-E or reflective coating, holes in the enamelled or ceramic digital printed finish, bubbles, foreign objects or point inclusions in laminated glass.
E Ø ≤ 1 Accepted if less than 3 in each area of Ø ≤ 20cm
1 < Ø ≤ 3 4 1 per metre of perimeter Ø > 3 Not Allowed
Glass impurities:
M Ø ≤ 1 Accepted if less than 3 in each area of Ø ≤ 20cm 1 < Ø ≤ 2 2 3 5 5 + 2/m² Ø > 2 Not Allowed
Glass impurities refer to any material that remains on the glass surface in a shape of a spot or a stain.
Maximum allowable number of residue spots & stains is defined in table F.2.
Zone
Table F.2 - Allowable number of residue spot and stains
Dimension and type (Ø in mm)
Size of the pane S (m2) S ≤ 1 1 < S All No limitation
R Spots Ø ≤ 1 No limitation
E Spots 1mm < Ø ≤ 3 4 1 per m of perimeter Stain Ø ≤ 17 1
Ø > 3 and stain Ø > 17 maximum 1
M Spots Ø ≤ 1
Spots 1 < Ø ≤ 3
Maximum 3 in each area of Ø ≤ 20cm
Maximum 2 in each area of Ø ≤ 20cm
Spot Ø > 3 and stain Ø > 17 Not Accepted
Linear / elongated faults:
Linear or elongated faults may have the form of deposits, stains & scratches and occupy a certain length and area. The allowable number of linear or elongated faults is defined in table F.3.
Table F.3 - Allowable number of linear / extended faults
The above tables are for double glazed IGUs made with monolithic glass panes. The number of allowable faults increases by 25% per additional glass component. i.e. double glazing with one pane of laminated glass or a triple glazing with three panes of monolithic glass the above fault limits are multiplied by 1.25. The number of allowable faults are rounded up to the next whole number.
Spacer deviation diagram:
1 Spacer
2 Theoretical shape of the spacer
3 Theoretical position of the spacer
4 Deviation
Figure F.2 - Examples of spacer deviation
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The allowable tolerance for straightness & deviation from parallel is 3mm for lengths <3.5m or 6mm for lengths ≥3.5m. allowable.
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