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.
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.
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.
Specular reflection only
With specular reflection only
Diffuse reflection is negligible
For such materials:
Diffuse reflectance = 0%
Total reflectance = specular reflectance
Materials with mirror finish
Metallic coating on glasses
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
Diffuse reflection only
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.
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:
The luminance contrast is therefore 31.25 or 31.25%.
A few examples of luminance contrast
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%).
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.
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.
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:
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).
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.
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.