On-site measurement of glass color uniformity

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).

The principle

There are two steps in measuring glass color uniformity:

  1. Determination a reference color
  2. Determination of color differences between the sample glasses and the reference color

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.

Inter-instrument error

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.

What is the difference between solar energy transmittance and SHGC?

As shown above, solar heat gain coefficient (SHGC) consists of two components:

  • Primary solar heat gain: the solar heat directly transmitted through a glass in its original solar radiation form.
  • Secondary solar heat gain: the solar heat absorbed by a glass and further transferred to the indoor space as heat and via all 3 heat transfer modes (conduction, connection and radiation)

The primary solar heat gain component is just the solar energy transmittance of the glass.

The secondary solar heat gain component is calculated as the solar energy absorptance of the glass multiplied by its inward flowing fraction. The solar heat absorbed by the glass causes a temperature increase of the glass. The absorbed solar heat flows to either the indoor side or the outdoor side. The fraction flowing to the indoor side is the inward flowing fraction.

For example, for a glass with 30% solar energy transmittance, 20% solar energy absorptance and 0.25 inward flowing fraction:

  • Its primary solar heat gain is 30%: 30% of the overall solar energy is directly transmitted to the indoor space
  • Its secondary solar heat gain is 20% × 0.25 = 5%: 20% of the overall solar energy is absorbed by the glass and 0.25 fraction of it is transmitted to the indoor space;
  • Its SHGC is therefore 30% + 5% = 35% or 0.35.

In summary:

SHGC = primary solar heat gain + secondary solar heat gain

Primary solar heat gain = Solar energy transmittance
Secondary solar heat gain = solar energy absorptance × inward flow fraction

Solar energy transmittance and SHGC are different. Solar energy transmittance is the primary solar heat gain component of SHGC only. The SHGC of a glass is always greater than its solar energy transmittance.

Laboratory test of thermochromic and electrochromic glass optical & thermal properties

The optical properties of a thermochromic glass change with the glass temperature. For example, the glass is tinted with increased glass temperature.

The optical properties of an electrochromic glass change with the electrical control. For example, the glass is tinted with an electrical voltage applied.

The optical and thermal properties (e.g. visible light transmittance or VLT, solar heat gain coefficient or SHGC, and shading coefficient) of a thermochromic glass or an electrochromic glass can be tested as an ordinary glass, as described in this article, except that the optical states need to be properly controlled in the lab.

Thermochromic glass optical state control

For thermochromic glasses, the glass temperature needs to be controlled. Shown below is the setup available at OTM for thermochromic glass temperature control.

A rubber pad heater with a 1-inch circular opening at the center (the amber color part) is attached to the thermochromic glass surface. The power output to the heater is regulated by a temperature controller, based on a thermocouple attached to the glass surface.

With this setup, we’ve successfully measured one thermochromic glass in the temperature range from room temperature to 65 °C, with temperature stability better than 1 °C and temperature accuracy better than 2 °C.

Thermochromic glass temperature control setup

Electrochromic glass optical state control

For electrochromic glasses, an electrochromic glass controller needs to be provided by the customer. It can be a simple device with a constant DC voltage output. Shown below is the setup.

We’ve measured a few electrochromic glasses, with customer’s controller.

Electrochromic glass electrical control setup

Online glass U-value, SHGC & shading coefficient calculator: V2.0.0

We are pleased to introduce our upgraded online glass U-value, SHGC & shading coefficient calculator. The current version is V2.0.0. Click the screenshot below to access this online calculator.

Online glass U-value, SHGC & shading coefficient calculator

Refer to this early post for the features of the V1.1.0 version.

New features added in V2.0.0

  • SHGC and shading coeffcient calculations
    • Note: limited to single glazing systems; this is due to the restriction of the mathematical model in ISO 9050/EN 410
  • More compact user interface
  • Tooltips on glass configuration schematics

Feedback and comments

We will improve this calculator regularly. If you have feebback and comments, please let us know.

Online glass U-value calculator: V1.1.0

We are pleased to introduce our online glass U-value calculator. The current version is V1.1.0. Click the screenshot below to access this online calculator.

Features:

  • No download or installation required.
    • The calculator works in all mainstream browsers with javascript enabled.
  • User friendly and responsive
    • In fact, you don’t need to click the “Calculate Glass U-value” button and the results are updated instantly after your changes.
  • Full compliance to ISO 10292 or EN 673
    • Accuracy has been validated with other independent codes.
  • Flexible and powerful
    • Major factors influencing glass U-value are supported: insulating glazing units, low-e coating, gas fills, laminated glasses.

Future developments

It is planned to include US NFRC U-value calculation in the future. SHGC and shading coefficient calculations may be included too.

Feedback and comments

We will improve this calculator regularly. If you have feebback and comments, please let us know.