Daylight reflectance

Can daylight reflectance test be performed on curved surfaces?

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Specular reflection follows the law of reflection, which states that the angle of incidence is equal to the angle of reflection, measured with respect to the local surface normal.

For a flat surface, the surface normal is uniform across the entire area. As a result, incident light is reflected in a single, well-defined direction, allowing the specular component to be reliably captured by the instrument.

For a curved surface, the surface normal varies from point to point. According to the law of reflection, the direction of reflected light therefore also varies across the surface. Instead of a single direction, the specular reflection is spread over a range of angles.

Daylight reflectance instruments measure reflection within a defined geometry. For curved surfaces, part of the specular reflection falls outside this geometry. As a result, the instrument misses part of the specular component, leading to an underestimation of the specular daylight reflectance.

In practice:

  • For curved surfaces with a strong specular component, the measurement error can be significant. Results should therefore be interpreted with caution.
  • For curved surfaces with a weak specular component, the impact is negligible, and measurements may still be acceptable

In summary, daylight reflectance testing can be performed on curved surfaces, but the accuracy, particularly for the specular component, may be limited.

How to measure daylight reflectance with a portable colour meter?

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Related services Daylight reflectance

It is possible to measure the daylight reflectance of a surface using a portable colour meter. For best results, a portable colour meter equipped with an integrating sphere is recommended.

The preferred measurement settings are:

  • Illuminant: D65
  • Observer: 10° standard observer
  • Colour space: Yxy

Many colour meters provide two measurement modes:

  • SCI (Specular Component Included): The Y value obtained in this mode is equivalent to the total daylight reflectance.
  • SCE (Specular Component Excluded): The Y value obtained in this mode is equivalent to the diffuse daylight reflectance.

The difference between the SCI and SCE Y values is equivalent to the specular daylight reflectance.

Q: Can the results measured with this method be used for BCA submission?

A: No. The Building and Construction Authority (BCA) requires testing in accordance with ASTM E903. Portable colour meters do not meet this requirement and therefore cannot be used for BCA submissions.

Q: How accurate are the results?

A: Accuracy depends largely on the performance of the instrument.

As a general guideline:

  • For colour meters based on the spectrophotometric method, the total daylight reflectance results are reasonably accurate, especially for neutral colours.
  • The diffuse and specular daylight reflectance results are typically less accurate, since the integrating spheres in most portable colour meters are relatively small.

Despite these limitations, the measurements remain useful for preliminary assessments and internal evaluations.

Q: What if my colour meter measures only L*ab?

Some online tools are available to convert L*ab results into Yxy colour values. One example is: Convert Lab to Yxy

Diffuse daylight reflectance of glasses

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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?

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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

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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

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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?

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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?

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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

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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.