SλΔλ: normalized relative spectral distribution of the UV radiation (part of the standard global solar radiation)
The wavelength range of interest is 300 nm – 380 nm. The term SλΔλ is the weights used in the weighted average of the spectral transmittance. The standard values of SλΔλ are plotted in the figure below.
The peak of the UV radiation distribution is at 375 nm. Glasses with high spectral transmittance near 375 nm are with high UV transmittance.
There are some small differences between the ISO 9050 and EN 410 UV distributions. The UV transmittances calculated according to the two standards could be slightly different.
Step 1. Set the user database as the optical data source
In the “File” -> “Preference” -> “Optical Data” tab, browse the Optics user database file and set it as the optical data source, as shown below.
Step 2. Import glass optical data from Optics user database
In the glass library (“Libraries” -> “Glass”), click “Import” (in the “List” view). In the pop-up window, set the format as “IGDB or Optics User Database”, as shown below. The optical data entries in the Optics user database can then be imported in the next pop-up window.
Typically, window film optical & thermal properties are tested with 3 – 6 mm clear or low-iron glasses as the substrate. Window film properties obtained with such high transparency glass substrates are more appropriate for product performance rating purposes.
In real buildings, window film products can be attached to all possible glass substrate types, such as tinted glasses, low-e coated glasses, laminated glasses, and double glazing units (DGUs). There are two methods to get window film optical & thermal properties with different glass substrates, as described below.
Option 1: direct physical test method
With the direct physical test method, the window film shall be attached to the actual glass substrate to be used. The whole glass system with window film is tested as usual.
This method is recommended for most applications, with a small number of glass substrate types.
Option 2: physical test + calculation method
With the physical test + calculation method, the following glasses need to be tested (based on the NFRC 304 method):
Window film on a reference glass substrate (typically a 3 – 6 mm clear or low iron glass)
The reference glass substrate (without window film)
Other glass substrates
With the test results of glasses 1 & 2, the window film only optical data can be calculated. The window film only optical data can then be added to all glass substrates tested in step 3 to get the combined glass with window film optical & thermal properties.
This method is recommended for product development applications, with a large number of glass substrate types.
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).
There are two steps in measuring glass color uniformity:
Determination a reference color
Determination of color differences between the sample glasses and the 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
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.
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.
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.
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.