Daylight reflectance is an important material surface property influencing the visual comfort of people.

What is daylight reflectance?

Daylight reflectance is the fraction of daylight falling onto a surface that is reflected back.

Example: out of 100% of daylight falling onto a surface, 60% of it is reflected back. The daylight reflectance of this surface is then 60% / 100% = 60%.

Apparently, the range of daylight reflectance is between 0% and 100%

• 0%: a perfect black surface
• 100%: a perfect white surface

Surfaces with lighter colors are more reflective, i.e. larger daylight reflectance.

There are a few synonyms of daylight reflectance, i.e. visible light reflectance, luminous reflectance and light reflectance value (LRV). The term daylight reflectance is mainly used in Singapore.

Daylight reflectance & glare

In the example shown on the right, the flash light is very bright and it causes visual discomfort or glare.

For surfaces with high daylight reflectance, such as white color cladding or reflective glasses on building exterior, they reflect an excessive amount of daylight to the surroundings.

High daylight reflectance surfaces behave in the same way as the flash light and they cause glare too. It is quite disturbing to the neighbors of buildings installed with such materials, as they suffer from the glare effect on daily basis.

Materials with high daylight reflectance are prohibited for building exterior installation in Singapore and a few countries.

What is total / diffuse / specular daylight reflectance?

As illustrated on the right, for the incident light (black color ray) reflected by a surface, there are two components:

• Specular reflection (blue color ray): the mirror-like reflection in one single direction, opposite to the incident light
• Diffuse reflection (red color rays): the general reflection scattered in all directions
• Total reflection: the sum of the specular reflection and diffuse reflection

With the understanding of total / diffuse / specular reflection, it is easy to understand total / diffuse/ specular daylight reflectance:

• Specular daylight reflectance: the fraction of daylight falling onto a surface that is reflected back as specular reflection.
• Diffuse daylight reflectance: the fraction of daylight falling onto a surface that is reflected back as diffuse reflection.
• Total daylight reflectance: the sum of the specular daylight reflectance and diffuse daylight reflectance

Example: out of 100% of daylight falling onto a surface, 10% of it is reflected back as specular reflection and 20% of it is reflected back as diffuse reflection. The specular daylight reflectance of this surface is then 10% / 100% = 10%; its diffuse daylight reflectance is 20% / 100% = 20%; its total daylight reflectance is 10% + 20% = 30%.

As diffuse reflection is scattered in all directions, for an observer at a particular position, the amount of daylight seen by him is small. In other words, diffuse reflection does not cause glare in general.

On the other hand, as specular reflection is in a particular direction, glare is resulted when an observer is in the same direction. In other words, specular reflection causes glare in general.

This is the reason that, in BCA’s regulations, specular reflectance is paid special attention to, because of its direct impact on glare.

What is BCA’s requirements on daylight reflectance?

Please refer to BCA’s circular for the original texts. We prepared a flowchart as shown below for easy interpretation.

Question 1: is your material glass?

Glass is a special material, with specular reflection and negligible diffuse reflection. This is the reason we can see clear images reflected by a glass, whereas other general materials do not reflect images.

For glasses, its diffuse daylight reflectance is always 0% and its specular daylight reflectance and total daylight reflectance are equal.

It is therefore meaningless to split the daylight reflectance of a glass to 3 components.

If your material is glass, BCA requires that its daylight reflectance shall be less than 20%, regardless of its installation location (e.g. facade or roof).

In the glass industry, instead of using the term daylight reflectance, its synonymy visible light reflectance is more commonly used.

Question 2: for non-glasses, where is the material installed?

For non-glass materials, BCA’s requirements are dependent on the location of installation.

Scenario A: on a facade or on a roof with less than 20° of inclination

For roofs, the inclination angle is measured from the horizontal plane. Roofs with less 20° of inclination means horizontal or low slope roofs.

For this scenario, BCA requires that its specular daylight reflectance shall be less than 10%. There is no requirement on total / diffuse daylight reflectance.

To meet this requirement, materials with strong specular reflection should be avoided. Materials with glossy and shiny texture (e.g. bare stainless steel and aluminium surfaces) are with high specular daylight reflectance and they should be avoided.

Scenario B: on a roof with more than 20° of inclination

Roofs with more than 20° of inclination are steep roofs.

For this scenario, BCA requires that its total daylight reflectance shall be less than 20%.

As explained above, perfect black color is with 0% total daylight reflectance and perfect white color is with 100% total daylight reflectance.

To meet this requirement, materials with dark colors should be used and material with light or medium colors should be avoided. As the limit is 20%, most colors seen in typical color charts cannot be used.

How to improve material daylight reflectance?

As daylight reflectance is a measure of human eyes’ response to the daylight reflected by a surface. Our eyes are naturally a qualitative daylight reflectance meter, though our eyes cannot quantify daylight reflectance.

Our visual examination of a material can tell us if its surface is dark or bright and if it is glossy or matte.

• To reduce specular daylight reflectance: use less glossy surfaces. Many bare metal surfaces cannot meet the 10% specular daylight reflectance requirement, whereas nearly all painted or coated surfaces can.
• To reduce total daylight reflectance: use dark color surfaces. Nearly all light color surfaces cannot meet the 20% total daylight reflectance requirement.

More importantly, there is no way to improve daylight reflectance without affecting surface appearance. Changing daylight reflectance means changing the visual appearance.

How is daylight reflectance tested in the lab?

Please refer to page 39 of our lab profile for the details.

Full compliance with BCA requirements

The test methods used at OTM are in full compliance to BCA requirements, i.e. with ASTM E903 method and with 150 mm integrating sphere.

For general facade & roof materials

The following test methods are used:

• ASTM E903-20 Standard test method for solar absorptance, reflectance, and transmittance of materials using integrating spheres
• ASTM E971-11 Standard practice for calculation of photometric transmittance and reflectance of materials to solar radiation
• CIE 130-1998 Practical methods for the measurement of reflectance and transmittance

The following results are reported:

• Total daylight reflectance
• Specular daylight reflectance
• Diffuse daylight reflectance

This test is SAC-SINGLAS accredited.

For glass materials

The following test methods are used:

• ASTM E903-20 Standard test method for solar absorptance, reflectance, and transmittance of materials using integrating spheres
• NFRC 300-2020 Test Method for Determining the Solar Optical Properties of Glazing Materials and Systems

The following result is reported:

• Glass daylight reflectance, front

UV/VIS/NIR spectrophotometer with 150 mm integrating sphere

The instrument used at OTM for daylight reflectance test is PerkinElmer Lambda 950 UV/VIS/NIR spectrophotomer, with 150 mm integrating sphere.

For daylight reflectance test, the instrument produces monochromatic light in the wavelength range of 380 nm – 780 nm, at every 5 nm interval. An optical device called intergrating sphere is used to measure the light reflected by the test sample. Shown below is the interior of the integrating sphere. The bright spot in the center is the sampling beam.

The instrument needs to calibrated prior to sample tests. A spectralon diffuse reflectance standard is used as the reference material. The spectralon material needs to be well maintained and regularly calibrated. Shown below is the spectralon reference material.

Facade & roof materials

We can test nearly all types of facade & roof materials. Shown below are some examples:

Sample requirements

The test samples to be tested shall be flat (at least in a small patch) and meet the following size requirements:

• Minimum size: 40 mm x 40 mm
• Preferred size: 100 mm x 100 mm
• Maximum size: 300 mm x 300 mm

It is still possible to test samples larger than 300 mm x 300 mm, but surcharge is applicable. The largest sample tested by us so far is a full size solar PV module, of the size around 0.5 m x 1.5 m

Only 1 sample is needed and it is optional for the client to provide additional samples as the backup.

Cost

Please contact us for the testing fee.

Lead time

The typical lead time is 5 working days.

Procedures

Our sales personnel will guide you through the necessary procedures.

Upon your request, the following three documents will be emailed to you:

1. Official quotation
2. Standard terms & conditions
3. Test request form

The client needs to return us the following documents:

1. Official purchase order (PO) or signed quotation
2. Completed test request form

The client needs to submit the test samples to our laboratory. The test will commerce as soon as possible after the test samples are received.

• Laboratory profile
• SAC-SINGLAS accreditation certificate and schedule
• Daylight reflectance test sample report

• 19/09/2022: Full-size solar panel daylight reflectance test
• 18/07/2022: ±0.001 rounding discrepancy in specular daylight reflectance calculation
• 18/04/2022: Can daylight reflectance be tested with ASTM E903 only?
• 16/08/2021: Do you test daylight reflectance at different inclination angles?
• 21/06/2021: Online total daylight reflectance calculator: V1.0.0
• 05/04/2021: Daylight reflectance of partially fritted glasses
• 01/03/2021: Daylight reflectance, visible light reflectance, luminous reflectance, and light reflectance value: are they the same?
• 04/01/2021: Are daylight reflectance and solar reflectance the same?
• 28/03/2018: Daylight reflectance, color and gloss