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.

All-in-one weather station for weather and air quality monitoring

Shown below is an all-in-one weather station supplied by OTM for weather and air quality monitoring.

The system consists of the following components:

The system is fully integrated and completely independent. The data are uploaded to Delta Ohm cloud every 2 minutes. OTM also provides 2-year maintenance services to the instruments.

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.

Instruments for on-site solar irradiance and daylight illuminance monitoring

A monitoring system was integrated and configured by OTM for on-site solar irradiance and daylight illuminance monitoring. The following Delta Ohm products were used:

The system is a fully automated system. The results are automatically logged and uploaded to the cloud, without site visits and manual operations.

The instruments (left: two sensors; right: wireless data logger)
Result page on Delta Ohm cloud

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.

Thermal comfort data logger for PMV/PPD/WBGT/turbulence measurement

Shown below are two sets of Delta Ohm HD32.3TC thermal comfort data loggers, with the following features:

  • For PMV/PPD/WBGT/turbulence measurement (ISO 7726, 7730 and 7243)
  • 4 quantities measured: air temperature, humidity, speed and globe temperature
  • Portable
  • With large touch screen
  • With built-in Li-ion battery (chargeable via USB)
  • With voice recording and camera function
  • With wi-fi connection for FTP upload

Visit our thermal comfort instrument page for more products.

Career opportunity with OTM: sales executive

We are looking for a sales executive for better sales flow and customer experience.

About us

OTM Solutions Pte Ltd was founded in 2013, with the following two operations:

  • Optical & thermal testing laboratory: we provide third-party material testing services to the building & construction industry (e.g. architectural glass optical properties, insulation material thermal conductivity). Our lab is SAC-SINGLAS accredited and with customers from more than 20 countries.
  • Instrumentation services: we are also the distributor of a well-established instrument manufacturer. We supply measurement instruments (e.g. weather stations and data loggers) and provide integration and measurement services.

For more details, please visit our website: www.otm.sg

Job description

We are looking for a sales executive, with the following duties:

  • Pre-sales enquiries: communicate with customers on service/product specifications. Technical knowledge on our services/products is required. Training will be provided.
  • Official sales documents: generate official documents (e.g. quotation, tax invoice, delivery order) required in a typical sales cycle.
  • Post-sales coordination: communicate with customers on job progress and other customer enquiries (e.g. customer feedback and satisfaction survey).
  • Sales records: maintain sales related records in the Customer Relationship Management (CRM) system and other relevant tools (e.g. customer loyalty program, sales records).
  • Quality document management: help laboratory manager maintain quality documents (i.e. standard operating procedures, SOPs) and attend quality audits. Training will be provided.

Job requirements

  • Education: diploma or degree is preferred
  • Language skills: proficient in English (both oral and written) is essential, most communications with customers will be over phone and emails
  • Computer skills: emailing with MS outlook, document editing with MS Word & Excel, quoting and invoicing with software (training will be provided)
  • Professional working and learning attitude: able to learn and comprehend our service/product specifications, able to comply with company requirements
  • Good inter-personal skills and responsive
  • Willingness in working in a small size company
  • Able to work in Woodlands area
  • We welcome fresh graduates to apply

Employment Benefits

  • Fixed basic salary with sales incentives and performance bonus
  • Short office hours (Monday to Friday, 9am – 4pm)
  • 14 days of annual leave
  • Medical benefits

How to apply

Please email your CV and cover letter to career@otm.sg. If you are shortlisted, we will contact you within 1 week.