Diffuse daylight reflectance of glasses

Related services Glass optical & thermal properties, Daylight reflectance

As discussed in the glass daylight reflectance page, the diffuse daylight reflectance of a glass is negligible, with the following two relationships:

Diffuse daylight reflectance = 0
Total daylight reflectance = Specular daylight reflectance

In the test report, we will report the glass daylight reflectance only. Below is an example:

There is only 1 result, without the separate total/diffuse/specular reflectance components.

If the result above is expressed in the conventional total/diffuse/specular daylight reflectance format, it would be:

  • Total daylight reflectance = 0.088 (8.8%)
  • Diffuse daylight reflectance = 0.000 (0.0%)
  • Specular daylight reflectance = 0.088 (8.8%)

The diffuse daylight reflectance is negligible, but not exactly 0. It is related to the haze level of the glass. The haze level of typical glasses is less than 0.5%. The diffuse daylight reflectance can be calculated as 0.088 × 0.005 = 0.00044 (0.044%) ≈ 0.000 (0.0%)

In practice, the instrument is not able to accurately measure (or resolve) the diffuse daylight reflectance of a glass. The test method of measuring glass diffuse reflectance is also not defined in the standards.

Therefore, there is only 1 result in the test report, without 3 separate components. The diffuse daylight reflectance of a glass can be estimated theoretically, but cannot the determined by an instrument.

Can my material comply with BCA daylight reflectance requirements?

Related services Daylight reflectance

The requirements on daylight reflectance by BCA are available in this circular dated 27/06/2016. Please refer to Section P.3.2 of the document (pages 5-6) for the requirements.

Below is a flowchart prepared by us for your easy reference:

For a specific scenario, you may click one of the blocks below for more elaborations.

My material is glass

For glasses, the requirement is that the daylight reflectance shall be less than 20%.

Please note the following:

  • For glasses, the concept of total/diffuse/specular daylight reflectance is not applicable, as glass is a material with specular reflection only (without diffuse reflection).
  • Daylight reflectance is also called visible light reflectance in some contexts. The “visible light reflectance, front” result in our glass test report is the same as the daylight reflectance result.
My material is not glass and is installed on a facade or horizontal/low-slope roof

For non-glass materials installed on facades or horizontal/low-slope roofs, the requirement is that the specular daylight reflectance shall be less than 10%.

Please note the following:

  • Specular daylight reflectance is also called specular reflectance in some contexts. They are the same.
  • Low-slope roof refers to a roof with less than 20 degrees of inclination angle (a horizontal roof is with 0 degrees of inclination angle).
My material is not glass and is installed on a steep-slope roof

For non-glass materials installed on steep-slope roofs, the requirement is that the total daylight reflectance shall be less than 20%.

Please note the following:

  • Total daylight reflectance is also called daylight reflectance in some contexts. They are the same.
  • Steep-slope roof refers to a roof with greater than 20 degrees of inclination angle (a horizontal roof is with 0 degrees of inclination angle).

Full-size solar panel daylight reflectance test

Related services Daylight reflectance

We are able to test the daylight reflectance of full-size solar panels. Shown below is an example.

All typical commercial solar panels can be tested, including panels greater than 1.2 m x 2.4 m in size. The results reported are the same as our standard daylight reflectance testing service (sample report). Due to the extra effort in handling such large and heavy samples, some large-size sample surcharge is applicable.

±0.001 rounding discrepancy in specular daylight reflectance calculation

Related services Daylight reflectance

In the daylight reflectance test, the specular daylight reflectance is calculated as:

Specular daylight reflectance = Total daylight reflectance – Diffuse daylight reflectance

Due to the rounding operations, the reported specular specular daylight reflectance could be:

Specular daylight reflectance = Total daylight reflectance – Diffuse daylight reflectance ± 0.001

Shown below is an example result set, with this ± 0.001 discrepancy:

  • Total daylight reflectance = 0.216 (21.6%)
  • Diffuse daylight reflectance = 0.200 (20.0%)
  • Specular daylight reflectance = 0.015 (1.5%)

Some customers expect that the specular daylight reflectance should be 0.216 – 0.200 = 0.016 (1.6%) and suspect that there is a typo in the test report.

Our daylight reflectance test reports are generated by in-house software automatically. The ±0.001 discrepancy is not due to a calculation mistake or a typo, but due to the rounding operations, as illustrated in the table below:

Before roundingAfter rounding
Total daylight reflectance0.21560.216 (21.6%)
Diffuse daylight reflectance0.20040.200 (20.0%)
Specular daylight reflectance0.01520.015 (1.5%)

Can daylight reflectance be tested with ASTM E903 only?

Related services Daylight reflectance

As suggested by the title of ASTM E903:

Standard test method for solar absorptance, reflectance, and transmittance of materials using integrating spheres

ASTM E903 is for the testing of solar absorptance, reflectance, and transmittance, but not daylight reflectance directly. The procedures of spectral reflectance measurement with integrating spheres are described in ASTM E903 in detail, but it is still insufficient to get the daylight reflectance results with ASTM E903 only.

Therefore, at OTM, ASTM E903 is supplemented with other test methods in daylight reflectance testing:

  • For general materials: ASTM E971 and CIE 130
  • For glass materials: NFRC 300

Please refer to our daylight reflectance page for more information.

Do you test daylight reflectance at different inclination angles?

Related services Daylight reflectance

According to BCA’s requirements on daylight reflectance, there are different requirements for the following installation locations:

  • Facade
  • Low-sloped roof, with less than 20 degrees of inclination angle from the horizontal plane
  • High-sloped roof, with more than 20 degrees of inclination angle from the horizontal pane

We were asked by many customers if we test the daylight reflectance at different inclination angles, in order to meet BCA’s requirements.

The short answer is: no, we do not test the daylight reflectance at different inclination angles.

Daylight reflectance, as a material property, is independent of its inclination angle. The angle in BCA’s requirement is the installation angle, it does not mean that daylight reflectance needs to be tested at various inclination angles.

More explanations

Daylight reflectance is a material property and it does not change with its inclination angle.

For example, for a surface with 10% of total daylight reflectance, its total daylight reflectance remains 10%, when the surface is tilted.

The test angle in the laboratory is dependent on the test instrument design and it is different from the installation angle in a building project.

Online total daylight reflectance calculator: V1.0.0

Related services Daylight reflectance

We are pleased to introduce our online total daylight reflectance calculator (V1.0.0, first version). Click the screenshot below to access this online calculator.

Calculation principle

The calculator simply converts an sRGB color (common in screen displays and websites) to a CIEXYZ color, whose Y component is equivalent to the total daylight reflectance of the color (refer to this Wikipedia article for more details).

This calculator calculates total daylight reflectance only and it cannot calculate diffuse and specular daylight reflectances, as the latter two components are dependent on surface finishing, but not on surface color.

The conversion is a theoretical conversion and does not introduce conversion errors. In practice, one needs to manually match a physical color with screen displayed color. This manual process introduces some errors. Nevertheless, this online tools is still useful in estimating the total daylight reflectance of color samples.

20% total daylight reflectance

BCA requires that, for roof surfaces with greater than 20° inclination angle, the total daylight reflectance shall be less than 20% (What is daylight reflectance?). The table below lists 4 colors (grey, red, green, and blue) with close to 20% total daylight reflectance.

If your color is brighter than the 4 colors (except blue color, read the explanation below the table), it is possible that it cannot meet the 20% requirement.

Color typeColor valueTotal daylight reflectanceColor display
GreyRGB (123, 123, 123)
Hex (#7B7B7B)
0.198 (19.8%)
RedRGB (248, 0, 0)
Hex (#F80000)
0.200 (19.8%)
GreenRGB (0, 144, 0)
Hex (#009000)
0.199 (19.9%)
BlueRGB (0, 0, 255)
Hex (#0000FF)
0.072 (7.2%)

Human eyes are less sensitive to blue and red colors, but more sensitive to green and grey colors. In the table above, the total daylight reflectance of the most saturated blue color in the sRGB color space is only with 7.2% of total daylight reflectance.

Daylight reflectance of partially fritted glasses

Related services Glass optical & thermal properties, Daylight reflectance

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.

Daylight reflectance, visible light reflectance, luminous reflectance, and light reflectance value: are they the same?

Related services Glass optical & thermal properties, Daylight reflectance, Luminance contrast

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).