Glass color uniformity is important to building facade aesthetics. It is possible to instrumentally measure the glass color uniformity on-site with a handheld spectrophotometer (a type of instrument for color measurement).
There are two steps in measuring glass color uniformity:
- Determination a reference color
- Determination of color differences between the sample glasses and the reference color
The reference color cannot be defined numerically (refer to the inter-instrument error section for the reasons) and it must be defined physically. There are two possible ways:
- A phyical glass sample (i.e. a control glass provided by the client)
- A group of glasses identified on-site (e.g. 10 installed glass selected by the client)
For the second option, the average color of the selected glasses is used as the reference color. According to ASTM C1376, a minimum of 10 glasses are required. The second option is often used in practice, as it is more convenient to use installed glasses as the reference.
Color difference between a sample glass and the reference color
In the CIELAB color space, a color is expressed as 3 values: L*, a and b:
- L*: represents the lightness of a color, with the range 0 – 100 (0: black; 100: white)
- a: represents the position of a color between red and green (a > 0: redish; a < 0: greenish), with the typical range -128 – 127
- b: represents the position of a color between yellow and blue (b > 0: yellowish; b < 0: bluish), with the typical range -128 – 127
The color difference, ΔE, between the reference color, L*abref, and a specific sample color, L*absample, is calculated as:
If the color difference (ΔE) is smaller than the criteria, the sample glass color is close to the reference color, i.e. in good color uniformity; otherwise, the color uniformity is poor. The criteria defined in ASTM C1376 is ΔE < 4.0.
The glass color measurement instrument (typically a handheld spectrophotometer) measures the color in CIELAB color space.
Due to the wide range of CIELAB color space (typical ranges: L*: 0 – 100; a: -128 – 127; b: -128 – 127), ordinary color measurement instruments cannot measure colors with sufficient accuracy.
In contrast, the range of color difference (ΔE) is small (typical range: ΔE < 10, as larger color differences can be easily perceived by human eyes). Most color measurements can measure color differences with sufficient accuracy (e.g. better than ±0.2).
In order to achieve satisfactory measurement accuracy of color difference (ΔE), the same instrument shall be used in both reference color and sample color measurement. If the reference color and sample color are measured by different instruments, the inter-instrument error makes the color difference (ΔE) results very unreliable. This is the reason that, in the reference color section, physical colors need to be used as the reference color.