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.

Dirt collection index testing of architectural coatings

Dirt pickup resistance (DPUR)

Dirt pickup resistance (DPUR) represents the ability of an architectural coating (e.g. paint) to resist dirt in exposure to natural environments.

Though it is named “dirt pickup”, it is not defined in terms of the amount of dirt accumulated on a surface, but in terms of the color change of a surface before and after a period of exposure. Due to this definition, surface color changes due to other factors, such as exposure to UV radiation, also impact DPUR.

A detailed description of the dirt pickup mechanism is available in this article: Towards a comprehensive understanding of dirt pickup resistance.

In simple words, DPUR is the color change of an architectural coating due to exposure to the natural environment.

Dirt collection index?

Dirt collection index, Dc, is a property defined in ASTM D3719 for DPUR characterization:

Dc = L*exposed / L*unexposed


  • Dc: dirt collection index
  • L*exposed: L* value of the exposed surface
  • L*unexposed: L* value of the unexposed surface

L* is the lightness of a color in the CIELAB color space. L* = 0 for a perfect black surface and L* = 100 for a perfect white surface.

There are other similar properties characterizing DPUR. To the author’s knowledge, dirt collection index is the only one defined in an international standard. Unfortunately, ASTM D3719 was withdrawn in 2019 and there is no replacement standard at the moment of writing (please leave your comment at the bottom of this page, if you are aware of some alternative international standards on DPUR characterization).

Laboratory testing of dirt collection index

For laboratory testing of dirt collection index, a test sample needs to be measured two times, in the unexposed state and in the exposed state, with the following 3 steps:

  1. Unexposed sample measurement: a new sample is measured before weathering
  2. Weathering: the sample is weathered outdoors for a certain period
  3. Exposed sample measurement: the exposed sample is measured after weathering

For the weathering part, the customers may perform the weathering by themselves according to their preferred conditions.

The colors can be measured with either a handheld spectrophotometer or our UV/VIS/NIR spectrophotometer.

Most customers perform the dirt collection index together with the solar reflectance index (SRI) testing (also in the unexposed and exposed states). In that case, the spectral reflectance data collected in the SRI testing can be re-used to calculate the dirt collection index.

Proficiency testing on insulation material thermal conductivity test

OTM participated in a proficiency testing (PT) program recently, with satisfactory results. A PT program is for test quality evaluation of laboratories. Typically, samples with known results are distributed by the PT organizer to a group of participating laboratories. The results tested by the laboratories are compared by the PT organizer to evaluate the test quality of each laboratory.

Below is a summary of the PT program on thermal conductivity testing and the performance of OTM in this program:

  • Number of participating laboratories: 84
  • Test type: insulation material thermal conductivity test
  • Sample: polystyrene board, at 25 °C mean temperature
  • Average results from all laboratories: 0.0344 W/(mK)
  • Result tested by OTM: 0.0344 W/(mK), with Z-score = 0

OTM is SAC-SINGLAS (ISO 17025) accredited for insulation material thermal conductivity testing. We perform both internal and external quality assurance exercises regularly for consistent and reliable measurement accuracy.

Solar absorptance and thermal emittance of transparent/translucent materials

We had a post on the sample requirements of translucent membrane products for SRI testing before: SRI testing: Can membrane products be tested without substrate? Essentially, if a transparent/translucent material is to be laid on top of an opaque substrate, it is required to test the transparent/translucent material together with the substrate.

There are also scenarios that transparent/translucent materials are with standalone installations and without substrates, .e.g. canopies made of fabric materials. In such cases, it is sometimes necessary to determine the solar absorptance and thermal emittance of transparent/translucent materials.

Solar absorptance of transparent/translucent materials

According to ASTM E903, the solar absorptance of a transparent/translucent material can be calculated as:

Solar absorptance = 1 – solar transmittance – solar reflectance

The solar transmittance and solar reflectance can be measured with a UV/VIS/NIR spectrophotometer.

Thermal emittance of transparent/translucent materials

It is out of the scope of ASTM C1371 to measure the thermal emittance of a transparent/translucent material. A manufacturer’s technical note is available to measure the IR transmittance and thermal emittance of transparent/translucent materials, with a portable emissometer.

SRI of transparent/translucent materials

It is out of the scope of ASTM E1980 to determine the solar reflectance index (SRI) of transparent/translucent materials. As suggested by its title, ASTM E1980 is for “horizontal and low-slope opaque surfaces”.

Do you test daylight reflectance at different inclination angles?

According to BCA’s requirements on daylight reflectance, there are different requirements for the following installation locations:

  • Facade
  • Low-sloped roof, with less than 20 degrees of inclination angle from the horizontal plane
  • High-sloped roof, with more than 20 degrees of inclination angle from the horizontal pane

We were asked by many customers if we test the daylight reflectance at different inclination angles, in order to meet BCA’s requirements.

The short answer is: no, we do not test the daylight reflectance at different inclination angles.

Daylight reflectance, as a material property, is independent of its inclination angle. The angle in BCA’s requirement is the installation angle, it does not mean that daylight reflectance needs to be tested at various inclination angles.

More explanations

Daylight reflectance is a material property and it does not change with its inclination angle.

For example, for a surface with 10% of total daylight reflectance, its total daylight reflectance remains 10%, when the surface is tilted.

The test angle in the laboratory is dependent on the test instrument design and it is different from the installation angle in a building project.

Import user IGDB format file into LBNL Optics

On customers’ requests, we can provide the IGDB format files of the glass samples tested by us. Shown below is a screenshot of an IGDB format file:

The IGDB format file can be imported into LBNL Optics and WINDOW for further calculations.

For more detailed operations of the two software tools, please visit the respective help documents. The steps of importing user IGDB format file into LBNL Optics is described below.

Step 1. Create a new Optics user database

Use “Datebase” -> “Create new user database” to create a new user database file *.mdb.

When asked “Do you want to switch to this new (empty) database?”, click “Yes”.

Step 2. Import the IGDB format file

Use “File” -> “Import Text File(s)…” to import the IGDB format file.

Online luminance contrast calculator V1.0.0

We are pleased to introduce our online luminance contrast calculator (V1.0.0, first version). Click the screenshot below to access this online calculator.

The calculation principle of this calculator is similar to the online total daylight calculator. The luminous reflectances of the two sRGB colors selected are calculated with color space conversion. The luminance contrast is then calculated from the two luminous reflectance results.

Online total daylight reflectance calculator: V1.0.0

We are pleased to introduce our online total daylight reflectance calculator (V1.0.0, first version). Click the screenshot below to access this online calculator.

Calculation principle

The calculator simply converts an sRGB color (common in screen displays and websites) to a CIEXYZ color, whose Y component is equivalent to the total daylight reflectance of the color (refer to this Wikipedia article for more details).

This calculator calculates total daylight reflectance only and it cannot calculate diffuse and specular daylight reflectances, as the latter two components are dependent on surface finishing, but not on surface color.

The conversion is a theoretical conversion and does not introduce conversion errors. In practice, one needs to manually match a physical color with screen displayed color. This manual process introduces some errors. Nevertheless, this online tools is still useful in estimating the total daylight reflectance of color samples.

20% total daylight reflectance

BCA requires that, for roof surfaces with greater than 20° inclination angle, the total daylight reflectance shall be less than 20% (What is daylight reflectance?). The table below lists 4 colors (grey, red, green, and blue) with close to 20% total daylight reflectance.

If your color is brighter than the 4 colors (except blue color, read the explanation below the table), it is possible that it cannot meet the 20% requirement.

Color typeColor valueTotal daylight reflectanceColor display
GreyRGB (123, 123, 123)
Hex (#7B7B7B)
0.198 (19.8%)
RedRGB (248, 0, 0)
Hex (#F80000)
0.200 (19.8%)
GreenRGB (0, 144, 0)
Hex (#009000)
0.199 (19.9%)
BlueRGB (0, 0, 255)
Hex (#0000FF)
0.072 (7.2%)

Human eyes are less sensitive to blue and red colors, but more sensitive to green and grey colors. In the table above, the total daylight reflectance of the most saturated blue color in the sRGB color space is only with 7.2% of total daylight reflectance.

On-site glass optical & thermal property monitoring system

A measurement system was integrated by OTM for on-site glass optical & thermal property monitoring. The system uses off-shelf sensors and wireless data loggers and can be easily deployed for 24×7 glass performance monitoring projects.

Measurement principle

The system consists of two indoor measurement systems (reference room vs. test room) and one outdoor measurement system. Shown below are the system schematics.

Instruments in an indoor measurement system (in total two sets, one for the reference room and one for the test room)
Instruments in the outdoor measurement system (one set only)

The following quantities are measured:

  • Indoor side (4 quantities): daylight illuminance [lux], solar irradiance [W/m2], air temperature [°C] and glass surface temperature [°C]
  • Outdoor side (3 quantities): daylight illuminance [lux], solar irradiance [W/m2], air temperature [°C]

What cannot be measured?

The system is for glass optical & thermal performance monitoring only. The results are qualitatively correlated to glass visible light transmittance and solar energy transmittance.

However, it is not possible to quantitatively evaluate glass optical & thermal properties rated under standard conditions (e.g. visible light transmittance, U-value, and shading coefficient) from the monitoring results. If such quantities are required, please consider our laboratory glass testing service.


The following instruments from Delta Ohm were used in the monitoring system:

Necessary mounting accessories were included. The instruments can be conveniently installed on-site in less than 1 hour. Shown below is a photo of the outdoor side instruments in the demo installation.

Outdoor side instruments in the demo installation


Shown below is a screen capture of some monitoring results (in the wireless data logger control software).

An in-house software can be used to automate the data processing part, as shown below.

Shown below are some sample results: