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

Examples:

  • 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

Examples:

  • 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

Examples:

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

Disclaimer

  • 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.

Agar gel thermal conductivity testing

Shown below are the photos of agar gel thermal conductivity testing using a thermal needle, according to ASTM D5334. The size of the thermal needle is 1.6 mm in diameter and 12 cm in length.

The thermal conductivity of agar gel (with 5-gram agar powder per liter of water) is tested, as part of the quality control measures when we test soil or similar gel-like materials.

Needle probe in agar gel
Standalone needle probe

Are daylight reflectance and solar reflectance the same?

Both quantities are about the reflectance of material surfaces. However, they are different.

  • Daylight reflectance is about the reflectance of a surface to visible light.
  • Solar reflectance is about the reflectance of a surface to solar energy.

Shown below is the solar radiation spectrum (red color part is for the sunlight at sea level). The solar radiation consists of 3 parts: 1) ultraviolet (UV) radiation; 2) visible light and 3) infrared (IR) radiation.

  • Daylight reflectance is in terms of visible light only.
  • Solar reflectance is in terms of UV radiation, visible light, and IR radiation, i.e. the entire solar radiation spectrum.

Additionally, in the lab, the daylight reflectance test and solar reflectance test are two different tests:

  • Daylight reflectance test: only the spectral reflectance in the 380 nm – 780 nm range (the visible light range) is measured. Typically, we also separate the total, diffuse and specular components of it.
  • Solar reflectance test: the spectral reflectance in the 300 nm – 2500 nm range (the entire solar spectrum, including UV radiation, visible light, and IR radiation) is measured. Typically, we also measure the material thermal emittance and calculate the solar reflectance index (SRI).

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