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:

Material yellowness index and whiteness index testing

We are able to measure material yellowness index and whiteness index according to ASTM E313-20. The measurement part is the same as our usual material color measurement.

The spectral reflectance data in the 380 nm – 780 nm range are used in the calculation. Our in-house software Color@OTM has been upgraded for automated calculation.

As the spectral range is included in the daylight reflectance test and solar reflectance index (SRI) test, it is possible to re-use the spectral data collected in the two tests for yellowness index and whiteness index calculation, without re-testing.

Far infra-red radiation measurement with pyrgeometer and wireless data logger

The system below was wired and configured by OTM for far infra-red radiation measurement, with the following components:

  • Delta Ohm LPPIRG01S pyrgeometer with MODBUS-RTU output
  • Delta Ohm HD35EDW-MB wireless data logger supporting MODBUS-RTU output

Listed below are the key specifications:

  • Spectral range: 5.5 μm – 45 μm
  • Viewing angle: 160°
  • Protection degree: IP67, for outdoor use
  • Wireless connection between data logger and base unit
Top view
Side view
Software interface

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.

Daylight reflectance of partially fritted glasses

We have a detailed article on the testing procedures of partially fritted glass optical & thermal properties. For partially fritted glasses, the daylight reflectance property concerns many customers. This article aims to present some opinions from us on the daylight reflectance of partially fritted glasses.

Partially fritted glasses: glass or non-glass material?

If you are a textualist and adhere to the texts strictly, partially fritted glasses are obviously made of glass, as the word “glass” appears in the name, and you should stop reading this post from here.

If you are not a textualist and open to some discussions, below are some explanations on the differences between glass and non-glass materials in terms of optical characteristics.

There are 3 types of material surfaces, in terms of optical characteristics:

  • With specular reflectance only
  • With mixed reflection
  • With diffuse reflection only

In our opinion, conventional glasses and glasses with ceramic frit are distinct in optical characteristics. For a partially fritted glass, it is more reasonable to classify its clear part as glass material and classify its fritted part as non-glass material.

A surface with specular reflection only
Specular reflection only

Optical characteristics:

  • With specular reflection only
  • Diffuse reflection is negligible

For such materials:

  • Diffuse reflectance = 0%
  • Total reflectance = specular reflectance


  • Conventional glasses
  • Materials with mirror finish
  • Metallic coating on glasses
A surface with mixed reflection
Mixed reflection

Optical characteristics:

  • With both specular reflection and diffuse reflection
  • Both components are not negligible

For such materials:

  • Total reflectance = diffuse reflectance + specular reflectance


  • Most general facade and roof materials with certain glossiness
  • Glasses with ceramic frit
A surface with diffuse reflection only
Diffuse reflection only

Optical characteristics:

  • With diffuse reflection only
  • Specular reflection is negligible

For such materials:

  • Specular reflectance = 0%
  • Total reflectance = diffuse reflectance


  • Materials with matt and rough surfaces: e.g. roof tiles, rough granites


  • The information presented above is our opinion. It is not reviewed, agreed, or approved by any external parties.

Luminance contrast: calculation method and a few examples

Calculation of luminance contrast

Luminance contrast is a measure of the difference in brightness of two surfaces. People with vision impairment prefer higher luminance contrast to identify TGSIs (tactile ground surface indicators).

The surface brightness is measured by its luminous reflectance. The luminous reflectance of a surface is the fraction of visible light reflected by the surface and it is equivalent to the daylight reflectance, visible light reflectance or light reflectance value of the surface.

In our lab, the luminance contrast between 2 surfaces is calculated according to AS 1428.1:2009 or AS/NZS 1428.4.1:2009, with the following equation:

Luminance contrast = 125 x (luminous reflectance 1 – luminous reflectance 2) / (luminous reflectance 1 + luminous reflectance 2 + 25)

For example, the luminous reflectance of surface 1 is 50% (luminous reflectance 1 = 50); the luminous reflectance of surface 2 is 25% (luminous reflectance 2 = 25); the luminance contrast between them can be calculated as:

Luminance contrast = 125 x (50 – 25) / (50 + 25 + 25) = 125 x 25 / 100 = 31.25

The luminance contrast is therefore 31.25 or 31.25%.

A few examples of luminance contrast

Left: luminous reflectance = 63%; Right: luminous reflectance = 9%;
Luminance contrast = 69%

In the example above, the surface on the left (pale grey color) is significantly brighter than the surface on the right (deep grey color). The luminance contrast between them is large (69%).

Left: luminance reflectance = 27.4%; Right: luminance reflectance = 27.9%;
Luminance contrast = 0.7%

In the example above, the surface on the right is only marginally brighter than the one on the left. The luminance contrast between them is small (only 0.7%). When such a surface pair is used as TGSIs, it will be very hard for people with vision impairment to identify them.

Left: luminance reflectance = 23%; Right: luminance reflectance = 24%;
Luminance contrast = 1%

In the example above, the surface on the right (cyan color) is only slightly brighter than the one on the left (green color), the luminance contrast between them is small (only 1%), although they are quite distinct in color. Luminance contrast is different from color contrast. TGSIs with large color contrast but small luminance contrast are still not friendly to visually impaired people.

Daylight reflectance, visible light reflectance, luminous reflectance, and light reflectance value: are they the same?

They are the same in physical meaning: all of them are quantities representing the fraction of visible light reflected by a surface.

For general applications, the results are equivalent. For example: 0.50 (50%) of daylight reflectance = 0.50 (50%) of visible light reflectance = 0.50 (50%) of luminous reflectance = 50 of light reflectance value (LRV).

In practice, there are some subtle differences in the test results, due to the different test methods used. Below are the practices implemented in our lab:

Most of the time, the results obtained with different methods are very close (typically less than ±0.01 of variation).

Are solar reflectance, TSR, and albedo the same?

In the context of material solar reflectance index (SRI), solar reflectance, TSR (total solar reflectance) and albedo are the same.

Particularly, solar reflectance is the standard term used in all relevant ASTM standards. TSR and albedo are not used in the ASTM standards, but they are often used by the industry or some literature.

In our test report, we only report the solar reflectance. In case the TSR or albedo values are needed, the solar reflectance results can be directly used, without conversion.