Glass color, color difference & color rendering
Glass color has a significant impact on the aesthetics of a building’s facade, and glass color uniformity is important to the cohesive and polished appearance of a facade. Glass color also affects the view through glass and the appearance of indoor objects. This article aims to explain the concepts of glass color, color difference, and color rendering, as well as the testing method of the relevant quantities.
- Glass color
- Glass color difference
- Glass color rendering
- Laboratory testing of glass color, color difference, and color rendering index (CRI)
- On-site measurement of glass color uniformity
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Glass color
The color of a glass depends on 3 factors:
- Light source: a color looks different under different light sources. For instance, a white color object appears differently when illuminated by red and blue lights.
- Observer: the sensitivity of human eyes to color varies from person to person and the color perceived is dependent on the observer.
- Reflectance and transmittance of the glass: the reflected color of a glass is dependent on its reflectance; the transmitted color of a glass is dependent on its transmittance.
For architectural glasses, the CIE standard illuminant D65 is used as the standard light source. It represents natural daylight outdoors and is also the standard light source used in glass visible light transmittance and reflectance calculations.
The CIE 1964 10° standard observer is used as the standard observer. It is more representative in architectural applications, due to the larger field of view than the 2° standard observer.
The front side reflected color of a glass is more relevant to the aesthetics of a façade and it is related to the front side reflectance of the glass.
Glass color, by default, refers to the reflected color of a glass illuminated by the standard CIE D65 light source and observed by the standard CIE 10° observer. Glass color is typically expressed in the CIELAB color space.
The CIELAB color space is three-dimensional, with 3 axes:
- 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.
At the center of the plane (a = 0 and b = 0) are the grey colors (varying from black to white with the L* value), which are considered neutral colors without a hue (red, green, yellow, or blue hue).
For more information on the calculation method of CIELAB color from glass spectral reflectance or transmittance data, please expand the block below.
Please expand the block below for the CIELAB colors of three selected example glasses:
| Reflected color (front side) | Transmitted color |
|---|---|
| L* = 35.2, a = -0.8, b = -0.7 | L* = 95.1, a = -1.8, b = 0.2 |
The reflected color of the clear glass is dark grey. The observer and natural daylight source are both on the front side of the glass, while the background on the back side of the glass is completely black, i.e., the observer can only see the reflected light, and all transmitted light is absorbed by the background. The reflected color is dark grey, due to the low reflectance of the clear glass.
The transmitted color of the clear glass is light grey (close to white and with a greenish hue). The natural daylight source is on the front side of the glass, the observer is on the back side, and there is no light source on the back side, i.e., the observer can just see the transmitted light and no reflected light. The transmitted color is light grey, due to the high transmittance of the clear glass.
The reflected color and transmitted color presented above are considerably different from a layman’s experience. In a real-world environment, an observer can see both transmitted and reflected light. There are light sources on both sides of a glass, including light reflected by surrounding objects. In the photo above, for example, the clear glass was backed by white paper. The reflected color is close to white, instead of dark grey.
| Reflected color (front side) | Transmitted color |
|---|---|
| L* = 50.0, a = -4.9, b = -19.0 | L* = 59.1, a = -0.3, b = 2.5 |
The color of the glass in the photo is different from the theoretical reflected color. Please refer to the explanations in Example 1 for some possible reasons.
For this glass, the reflected color is blue, whereas the transmitted color is mid grey.
| Reflected color (front side) | Transmitted color |
|---|---|
| L* = 30.8, a = -4.5, b = 0.8 | L* = 79.6, a = -14.0, b = 5.4 |
The color of the glass in the photo is different from the theoretical reflected color. Please refer to the explanations in Example 1 for some possible reasons.
For this glass, the transmitted color is green, whereas the reflected color is dark grey.
Glass color difference
For a uniform and consistent façade appearance, color variations between glass panels shall be negligible or minimal. Color difference is the quantitative measure of the color variation between glasses.
In the CIELAB color space, the color difference, ΔE, between a reference glass (color: L*abref) and a sample glass (color: L*absample) is calculated as:
The calculation of color difference is like the calculation of the distance between two points in a three-dimensional Cartesian coordinate system.
Please expand the block below for two examples of color differences:
| Left color tile | Right color tile |
|---|---|
| L* = 58.9, a = -0.4, b = 0.5 | L* = 59.3, a = -3.1, b = 1.4 |
| Color difference: ΔE = 2.9 | |
| Left color tile | Right color tile |
|---|---|
| L* = 55.1, a = -28.0, b = 14.9
| L* = 54.8, a = -27.7, b = 17.5
|
| Color difference: ΔE = 2.6 | |
In general, color differences with ΔE < 1 are imperceptible to human eyes. As presented in the examples above, color differences with ΔE < 3 are negligible in practice.
For glasses, ASTM C1376 requires ΔE < 4.0 with respect to a reference color. The reference color can be from two possible ways:
- A physical glass sample (e.g., a control glass sample provided by the client)
- A group of glasses identified on-site (e.g., 10 installed glasses selected by the client)
For the second option, the average color of some 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. For more information on on-site measurement of glass color uniformity, please refer to this post.
Glass color rendering
The color of an object is rendered differently in natural daylight and in daylight transmitted through a glass. For example, objects in a room with blue tinted glass window may look blueish.
Color rendering index (CRI) is the quantity measuring the ability of a light source to accurately reproduce the colors of the objects it illuminates. CRI is primarily for artificial light sources (e.g. LED and fluorescent lamps). It is rated on a scale of 0 – 100, with 100 being the highest possible score and indicating that the light source reproduces colors most accurately. A higher CRI means that colors will appear more vibrant and true-to-life under that light source. A lower CRI means that colors may appear washed out or distorted.The CRI of natural daylight is 100 (maximum value). In general, a light source with CRI > 90 is considered excellent, while with CRI < 80 is considered poor. An excellent explanation of CRI is available here.
CRI is also used for glasses, as a quantitative measure of object color change between viewed in daylight and in daylight transmitted through a glass.
The calculation method of glass CRI is available in EN 410. Essentially, the colors of 8 test colors rendered in natural daylight and rendered in daylight transmitted through a glass are virtually calculated. The color difference of a specific test color (rendered in two light sources: natural daylight vs. daylight transmitted through the glass) is used to calculate the specific CRI (in total 8 specific CRIs because of 8 test colors). The average of the 8 specific CRIs is the general CRI, which is the CRI seen in glass performance specifications.
Please expand the blocks below for the general CRI results of three selected example glasses.
General CRI = 98
The color rendering ability of the clear glass is excellent. For the colors of the glass, please click here.
General CRI = 98
The color rendering ability of this glass is excellent. Its reflected color is blue, but its transmitted color is mid grey, which is a neutral color with negligible impact on color rendering. For the colors of the glass, please click here.
General CRI = 83
The color rendering ability of this glass is good but is inferior to the two glasses presented above. CRI is correlated to transmitted color. For this glass, its transmitted color is green, which is not a neutral color. For the colors of the glass, please click here.
Laboratory testing of glass color, color difference and color rendering index (CRI)
The raw data collected in full glass optical and thermal property testing can be used to calculate the color and CRI of a glass, without additional measurements. The glass spectral transmittance and reflectance in the visible light range (380 nm – 780 nm) are used. With the glass color results, the color differences between two glasses can be easily calculated.
If a client is interested in the color related properties only, it is possible to measure the spectral transmittance or reflectance of the glass in the visible light range only, without the full glass optical & thermal property testing.
On-site measurement of glass color uniformity
For glasses installed in an existing building, it is possible to measure the reflected colors of the glasses and calculate the color differences between the measured glass colors and a reference color. The color difference results can be used to assess the glass color uniformity. As required in ASTM C1376, the color difference (ΔE) shall be less than 4.0.
For more details, please refer to this post: On-site measurement of glass color uniformity.
We’ve performed many on-site glass color uniformity measurements since 2016. Please refer to this page for more on-site measurement and monitoring services provided by us.
Acknowledgement
The online calculator by nix Color Sensor was used to convert CIELAB colors to HEX color for webpage color display in this article.
Last update: 30/01/2023
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