Daylight reflectance of glasses
In Singapore, a 20% daylight reflectance limit for glasses is in place to reduce glare caused by glass reflection to sunlight. This article aims to explain the characteristics of glass daylight reflectance, as well as the laboratory testing method.
- Glass reflection and daylight glare
- Daylight reflectance of glasses
- BCA daylight reflectance requirements on glasses
- Laboratory testing of glass daylight reflectance
More information:
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Glass reflection and daylight glare
Glare is a visual discomfort or disability caused by excessive brightness. In the example below, the flashlight is very bright, and it causes glare.
Similarly, the sunlight reflected by building glasses can be very bright too and causes glare, and below is an example.
Daylight glare may cause annoyance and visual discomfort to occupants in neighboring buildings and may even pose safety hazards to nearby vehicle drivers. It is therefore important to minimize daylight glare in building design and façade material selection.
Daylight reflectance of glasses
There are 3 types of material surfaces, in terms of the optical characteristics in light reflection:
- With specular reflection only
- With mixed reflection
- With diffuse reflection only
They are compared below:
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
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
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
Glasses are with specular reflection only, with the following two relationships:
Diffuse daylight reflectance = 0
Total daylight reflectance = Specular daylight reflectance
Therefore, for glasses, we use the term “daylight reflectance” only, without making a distinction between the total, diffuse, or specular components.
Additionally, due to the relationship:
Transmittance + reflectance + absorptance = 1
Glasses are not as reflective as they appear, contrary to popular brief, as a significant amount of incident light is transmitted through the glass without being reflected. Glasses look reflective because they only reflect light in one direction (i.e., mirror-like reflection).
For example, the daylight reflectance of an uncoated clear glass is only around 8%, which is only slightly higher than the total daylight reflectance of a black color surface (typical value 5%).
Low-e coated glasses are in general with higher daylight reflectance than uncoated glasses. Fortunately, most low-e coatings are deliberately engineered with high reflection in the near infra-red (NIR) range yet low reflection in the visible light range.
BCA daylight reflectance requirements on glasses
Please refer to the circular issued by BCA about the requirements on daylight reflectance of facade and roof materials in Singapore.
A flowchart is created by us for easy understanding of BCA’s requirements.
For glasses, the requirement is that the daylight reflectance shall be less than 20%, regardless of the glass installation position.
It should be noted that the flowchart uses the term visible light reflectance, which is the same as daylight reflectance (to be explained in the next section).
Laboratory testing of glass daylight reflectance
According to BCA’s requirements on daylight reflectance testing, the ASTM E903 method shall be used, and the instrument shall be equipped with a 150 mm integrating sphere. OTM can perform glass daylight reflectance tests in compliance with the requirements.
The spectral reflectance of a glass sample in the range of 380 nm – 780 nm is measured, and the daylight reflectance is calculated from the measured spectral reflectance.
A double glazing unit (DGU) glass can be directly tested, without separating the inner and outer panes (in contrast to the practice in DGU glass full optical & thermal property testing). With careful alignment, we can test very thick DGU glasses (e.g. thicker than 60 mm) with good accuracy.
For single glazing glasses, the daylight reflectance test is part of the full optical & thermal property test (Step 3. Spectral transmittance/reflectance measurement with UV/VIS/NIR). The “visible light reflectance, front” result in a glass optical & thermal property test report (an example is shown below) is the same as the daylight reflectance result. They are simply different names in the standards. On request, it is possible for us to issue an additional report of glass daylight reflectance if the glass full optical & thermal property test is performed by us.
More information
Daylight reflectance of partially fritted glasses
For partially fritted glasses, we recommend the following practices:
- The clear part and the fritted part of a partially fritted glass should be considered separately.
- The clear part should be evaluated as a glass material, i.e. with glass daylight reflectance.
- The fritted part should be evaluated as a non-glass general material, i.e. with total, diffuse and specular daylight reflectance.
More details are available in this blog post.
Last update: 17/10/2022
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