The nuances of glazing colour

Colour differences and tolerances

Based on the unique colour target for each coated glass product, ASTM C1376 requirements for colour uniformity can be applied. For instance, if there are concerns colour uniformity requirements have not been met on the project site it is appropriate to compare the colour data for the glazing in question, measured with a spectrophotometer, to the colour data of glazing with an acceptable appearance. This colour measurement can be done pre- or post-installation on the glazing exterior by anyone trained in the use of a spectrophotometer made for such field measurement applications.

The colour difference equation can then be used to calculate the differences between two colour measurements for comparative purposes per ASTM D2244. This equation is referred to as DE*ab and calculated using the following formula:

This formula can be used to compare the reflected colours of two low-e-coated glass samples, as in
this example.

The first sample’s reflected colour is:

L* = 33.0

a* = 4.0

b* = 2.0

The second sample’s reflected colour is:

L* = 33.5

a* = 5.0

b* = 4.0

The colour difference between them is calculated as follows:

ΔE*ab = SQRT((33.5-33.0)²+(5.0-4.0)²+(4.0-2.0)²) = 2.3

The calculation between a colour target and a colour measurement taken on a piece of glass can be done in a similar fashion. ASTM C1376 describes how the colour target can be obtained and used to determine colour uniformity. This allows glass colours to be compared objectively without the effect of ambient light, viewing angle, or an individual viewer’s subjective perception.

Colour uniformity considerations in glazing design

The early stages of glazing design and material selection are the optimal time to decide the appropriate level of specified glazing colour uniformity. The following factors related to the perception of colour need to be considered.

1. Viewing glass samples against a white background emphasizes transmitted colour, while a very dark background emphasizes reflected colour.
   As mentioned, glass installed on buildings includes components of both colour types blended together.
2. Glass samples should be evaluated in natural daylight, since artificial light may emit wavelengths of light that can skew the visual perception of glass colour.
3. Perceived glass colour can be influenced by sample size. The colour of a 305 x 305-mm (12 x 12-in.) sample may not appear the same as a 1 x 3-m (4 x 10-ft) glazed unit of the identical glass. This is known as field-size metameric failure, which occurs because the relative proportions of the three cone types in the human eye (i.e. red, green, and blue) vary from the centre of the visual field to the periphery.
4. Use of tinted glass on the outer lite—especially darker tints—masks much, if not all, of the colour differences in the low-e coating behind it. When low-e coatings are placed in front of darker tints, the opposite effect occurs and the coating colour differences are enhanced. Care must be taken when using dark tints to ensure consistent tin/air-side orientation in IGU or laminate assemblies.
5. In some cases, low-e coatings developed and optimized for the second surface of an IGU can be placed on the third surface, but might have more colour. For solar control applications, a dark-tinted outer lite can lessen perceived colour variations when third-surface coatings are desired. An IGU with designated second-surface coating that is manufactured or installed accidentally backwards will have a noticeable colour difference, because the coating as installed is now on the third surface. The energy performance of such a “backwards-installed” glazing will also be affected.
6. Coatings on multiple surfaces will have combined colour effects. For example, the colours of a low-e coating on the second and fourth surfaces are additive. Another example is low-e coatings on multiple surfaces of triple IGUs.
7. Surrounding conditions—such as overhangs, shadows, and reflections from trees or other buildings—affect perceived colour uniformity.
8. Building orientation, different elevations, and viewing conditions can affect colour perception. How direct and indirect sunlight shines on the glazing is determined, in part, by whether the building faces north-south or east-west. For example, north elevations tend to receive less direct sunlight and, therefore, more shadows; this can affect perceived colour.
9. Glazing with exposed edges appears different to the eye because of edge lighting effects. With an exposed glass edge, light can enter the body of the glass through the edge and illuminate it.
10. Interior shading devices and shadow boxes can affect perceived colour (Shadow boxes, an alternate design to typical spandrel units, generally consist of transparent glazing, a cavity behind the glazing, and an insulated back panel or tray, which can have colour and texture. They are typically used in nonvision areas of a building’s glass curtain wall design to obscure mechanical elements, between-floor voids, and other items designers may want to hide within a building façade. For more information on spandrels, click here. ).
11. Glazing constructed of different glass types—such as spandrels, laminated glass, and glass with dot/line patterns—can affect how the colour is perceived.
12. Provided heat strengthening or tempering is done properly, the resultant colour is not affected.