Last update: 06.09.2019
Standards & norms

Guidelines for assessing the visual quality of glass in buildings

The appearance of coated glass and defect detection is described in the EN 1096-1 standard. The defects affecting appearance are specific to the glass substrate (e.g. float glass acc. to EN 572-2) or specific to the coating. If a defect specific to the glass substrate is more visible because of the coating, it will be treated as a coating defect.

Visual quality of coated glass

The appearance of coated glass and defect detection is described in the EN 1096-1 standard. The defects affecting appearance are specific to the glass substrate (e.g. float glass acc. to EN 572-2) or specific to the coating. If a defect specific to the glass substrate is more visible because of the coating, it will be treated as a coating defect.

Detection of defects

The defects are detected visually by an observation of the coated glass in transmission and/or reflection. An artificial sky or daylight may be used as the source of illumination.
The artificial sky is a plane emitting diffuse light with a uniform brightness and a general colouring index Ra higher than 70 (see CIE 013.3-1995).

It is obtained by using a light source whose correlated colour temperature is in the range between 4,000 K and 6,000 K. In front of the arrangement of light sources is a light scattering panel, without spectral selectivity. The illuminance level on the glass surface shall be between 400 lx and 20,000 lx.
Daylight illumination is a uniform overcast sky, without direct sunlight.

Conditions of examination


Coated glass may be examined in stock size plates or in finished sizes ready for installation. The examination may be undertaken in the factory or on site when glazed.

The pane of coated glass being examined is viewed from a minimum distance of 3 m. The actual distance will depend on the defect being considered and the illumination source being used. The examination of the coated glass in reflection is performed by the observer looking at the side that will be the outside of the glazing. The examination of the coated glass in transmission is performed by the observer looking at the side that will be the inside of the glazing. During the examination, the angle between the normal to the surface of the coated glass and the light beam proceeding to the eyes of the observer after reflection or transmission by the coated glass, shall not exceed 30° (see Figures below).

Examination procedures for coated glass

For panes of coated glass in finished sizes ready to be installed, both the main area and an edge area of the pane shall be examined (see Figure below).

Areas to be examined on finished sizes

Each examination shall take no more than 20 s. 


Uniformity defects and stains

Under the conditions of examination, given in 8.3, note any coating variations either within one pane or between neighbouring panes that are visually disturbing.


Punctual defects

Under the conditions of examination, given in 8.3, note any spots, pinholes and/or scratches that are visually disturbing.

For spots/pinholes, measure the size and note the number relative to the size of the pane. If there are any clusters found their position relative to the through vision area shall be determined.
For scratches, determine whether or not they are in the main or edge area. Measure the length of any scratches noted. For scratches > 75 mm long, determine the distance between adjacent scratches. For scratches ≤ 75 mm long, note any areas where their density produces visual disturbance.

Acceptance criteria for coated glass defects

Acceptance criteria for coated glass defects

Visual quality of insulating glass units (IGU)

The criteria of the visual glass quality of IGU made of base glass components are described in the EN 1279-1 standard (normative Annex F).

The optical and visual quality requirements for glass components shall be taken from the appropriate European Standards. 

The information provided by the tables below give the maximum allowable fault per insulating glass unit, as well as the faults that are specific to the assembly. This information shall not be used for insulating glass units with at least one component made of patterned glass, wired glass, wired patterned glass, drawn sheet glass, or fire-resistant laminated glass.

Observation conditions

The panes shall be examined in transmission and not in reflection. 

The discrepancies shall not be marked on the pane. 

The insulating glass units shall be observed from a distance of not less than 3 m from the inside to the outside and at a viewing angle as perpendicular as possible to the glass surface for up to one minute per m2. The assessment is carried out under diffuse daylight conditions (e.g. overcast sky), without direct sunlight or artificial lighting. 

Insulating glass units assessed from the outside shall be examined in installed condition, taking into consideration the usual viewing distance with a minimum of 3 m. The viewing angle shall be as perpendicular as possible to the glass surface. 

Observation zones

IGU made of two panes of monolithic glass

Spot faults

Spot faults



Maximum allowable number of residue spots and stains



Linear/extended defects

Hairlines scratches are allowed, provided that they do not form a cluster.

The maximum number of linear / extended fault    

Maximum number of linear fault


IGU other than those made of two panes of monolithic glass panes

The allowable number of discrepancies defined in "IGU made of two panes of monolithic glass" Chapter is increased by 25 % per additional glass component (in multiple glazing or in a laminated glass component). The number of allowable defects is always rounded up. 

•    Triple glazed unit made of 3 monolithic glass panes: the number of allowable faults of F.3 is multiplied by 1,25. 
•    Double glazed unit made of two laminated glass panes with 2 glass components each: the number of allowable faults of "IGU made of two panes of monolithic glass" Chapter is multiplied by 1,5.


IGU containing a heat-treated glass

The visual quality of thermally toughened safety glass, with or without heat soaking and of heat strengthened glass, when assembled in an insulating glass unit or in a laminated glass which is a component of an insulating glass unit, shall fulfill the requirements of their respective product standard. 

In addition to these requirements, for heat treated float glass, the overall bow relative to the total glass edge length may not be greater than 3 mm per 1,000 mm glass edge length. Greater overall bow may occur for square or near-square formats (up to 1:1.5) and for single panes with a nominal thickness < 6 mm.


Edge defects


Allowable edge defects are given in the relevant standard for each glass pane component. 
External shallow damage to the edge or conchoidal fractures that do not affect the glass strength, and which do not project beyond the width of the edge seal, are acceptable. 

Internal conchoidal fractures without loose shards, which are filled by the sealant, are acceptable.


Tolerance on spacer straightness

For double glazing the tolerance on the spacer straightness is 4 mm up to a length of 3,5 m, and 6 mm for longer lengths. The permissible deviation of the spacer(s) in relation to the parallel straight glass edge or to other spacers (e.g. in triple glazing) is 3 mm up to an edge length of 2,5 m. For longer edge lengths, the permissible deviation is 6 mm. 

Examples of spacer deviation

Examples of spacer deviation


Curved IGU

The visual quality of curved insulating glass units and their glass components shall fulfill the requirements of ISO 11485-1 and ISO 11485-2.


Other visual aspects of insulating glass units

Some physical effects can occur that are visible on the glass surface and shall not be taken into account when assessing the visual quality. These are not considered as defects (informative Annex G of EN 1279-1).


Inherent colour

Variations in the colour impression are possible due to the iron oxide content of the glass, the coating process, the coating itself, variation in the glass thickness and the unit construction and cannot be avoided.


Difference in insulating glass unit colour

Façades made of IGUs incorporating coated glass can present different shades of the same colour, an effect that can be amplified when observed at an angle. Possible causes of differences in colour include slight variations in the colour of the substrate onto which the coating is applied and slight variations in thickness of the coating itself. 
An objective assessment of the differences in colour can be done using ISO 11479-2.


Interference effect

In insulating glass units made of float glass, interference effects may cause spectral colours to appear. Optical interference is due to superposition of two or more light waves at a single point. 
The effects are seen as variation in intensity of the coloured zones, which change when pressure is applied to the glass. This physical effect is reinforced by the parallelism of the surfaces of the glass. Interference effects occur at random and cannot be avoided.


Specific effect due to barometric conditions

An insulating glass unit includes a volume of air or other gas, hermetically sealed by the edge seal. The state of the gas is essentially determined by the altitude, the barometric pressure and the air temperature, at the time and place of manufacture. If the insulating glass unit is installed at another altitude, or when the temperature or barometric pressure changes (higher or lower pressure), the panes will deflect inwards or outwards, resulting in optical distortion.


Multiple reflections


Multiple reflections can occur in varying intensity at the surfaces of glass units. These reflections can be seen particularly clearly if the background viewed through the glazing is dark. This effect is a physical property of all insulating glass units.


Anisotropy (iridescence)


Insulating glass units that contain a heat-treated glass component may show visual phenomena known as anisotropy, see EN 12150-1, EN 1863-1. See Anisotropies (strain pattern) Chapter


Condensation on the external surface of the IGU


Condensation can occur on the external glass surfaces when the glass surface is colder than the adjacent air. 
The extent of condensation on the external surfaces of a glass pane is determined by the U-value, the air humidity, air movement and the indoor and outdoor temperatures. 
When the ambient relative humidity is high and when the surface temperature of the pane falls below the ambient temperature, condensation at the glass surface occurs.
→ see chapter for information on how to minimise condensation on external surfaces.


Wetting of glass surfaces


The appearance of the glass surfaces can differ due to the effect of rollers, finger-prints, labels, vacuum suction holders, sealant residues, silicone compounds, smoothing agents, lubricants, environmental influences, etc. This can become evident when the glass surfaces are wet by condensation, rain or cleaning water.

Assessment of the visible section of the edge seal of the insulating glass unit


Features on the glass and spacer resulting from production processes can be recognised in insulating glass units in the visible section of the edge seal. By definition, this section is not included in the area between the sight lines that is subject to assessment. If the edge seal of the insulating glass unit is exposed on one or more sides due to design requirements, features resulting from production processes may be visible in the area of the edge seal.
If the edge seal on the insulating glass unit is exposed due to design requirements, typical features of the edge seal may become visible that are not covered by this guideline. In such cases, individual arrangements should be agreed.


IGU with internal muntins


Due to climatic influences (e.g. insulated glass effect) shocks or manually generated vibration, clapping noise may occur in the muntins. The production process results in visible saw cuts and the slight removal of paint near the saw cuts.

In assessing deviations from right angles and misalignment within the glazing zones, the manufacturing and installation tolerances and the overall impression must be taken into account.

Effects due to temperature-related changes in the lengths of muntins in the gas-filled cavity are basically unavoidable. Misalignment of muntins caused by production cannot be ruled out.