Daylight reflectance testing procedures

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Façade & roof material daylight reflectance testing procedures

Façade & roof material daylight reflectance test is a test regularly performed by our lab. This article aims to explain the principles and step-by-step procedures to test total/diffuse/specular daylight reflectance.

Daylight reflectance testing principles:

Daylight reflectance testing steps:

More information:

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Daylight reflectance testing principles

Daylight reflectance is defined as the fraction of daylight reflected by a material surface. Daylight reflectance consists of two components, namely diffuse daylight reflectance and specular daylight reflectance. For more information, please visit what is daylight reflectance.

The sum of the diffuse daylight reflectance and the specular daylight reflectance is the total daylight reflectance.

Total daylight reflectance = Diffuse daylight reflectance + Specular daylight reflectance

Glasses are with negligible specular daylight reflectance. Glass daylight reflectance is not discussed in this article. Please visit the glass daylight reflectance page for more information.

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Daylight reflectance calculation

Daylight reflectance is calculated from the respective spectral reflectance (i.e. total, diffuse, and specular) with weighted averaging. Please expand the box below on the weighted averaging method.

Weighted averaging in daylight reflectance calculation

Daylight consists of light in the 380 nm – 760 nm range (visible light in the solar spectrum). Our instrument measures the spectral reflectance of monochromatic light in the 380 nm – 760 nm range at 5 nm intervals. Weighted averaging is used to calculate the daylight reflectance, with the standard solar spectrum and human eye sensitivity as the weights.

Below is the daylight reflectance calculation equation:

In the equation above:

  • ρ: daylight reflectance
  • ρλ: spectral reflectance
  • Eλ: terrestrial direct normal solar spectral irradiance (standard solar spectrum)
  • Vλ: photopic spectral luminous efficiency function (standard human eye sensitivity)
  • λ: wavelength (380 nm – 760 nm range)

ASTM E971 is used by our lab for daylight reflectance calculations. Shown below are the spectral curves of the standard solar spectrum and human eye sensitivity.

It is clear that human eyes are most sensitive to light close to 555 nm wavelength (green color) and least sensitive to light close to 380 nm (blue color) and 760 nm (red color). The daylight reflectance results are therefore strongly dependent on the spectral reflectance near 555 nm.

Standard solar spectrum
Standard human eye sensitivity

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Specular spectral reflectance calculation

The instrument measures total and diffuse spectral reflectance only. The specular spectral reflectance is calculated with the following relationship:

Specular spectral reflectance = Total spectral reflectance – Diffuse spectral reflectance

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Total and diffuse spectral reflectance measurement

The total and diffuse spectral reflectance in the 380 nm -760 nm range is physically measured with the instrument UV/VIS/NIR spectrophotometer with 150 mm integrating sphere accessory. Please expand the box below for the measurement principle.

Total and diffuse spectral reflectance measurement principle

An integrating sphere is an optical device with a hollow spherical cavity. The interior surface of an integrating sphere is with high diffuse reflectance. Due to multiple scattering reflections on the interior surface, an integrating sphere collects the light scattering in all directions and gets an integrated total value.

In reflectance measurement, an integrating sphere can be configured to two measurement modes, as shown in the figure below:

  • Total reflectance measurement mode: in this mode, the specular light port is covered and the specular component is included (SCI).
  • Diffuse reflectance measurement mode: in this mode, the specular light port is open and the specular component is excluded (SCE).

With the two measurement modes, the total and diffuse spectral reflectances are measured.

Shown below are the photos of the specular light port (in the covered and open states) and the plug.

Specular light port covered (specular component included, SCI, mode)
Specular light port open (specular component excluded, SCE, mode)
Specular light port plug

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Daylight reflectance testing steps

The UV/VIS/NIR spectrophotometer available at OTM lab is used for total and diffuse spectral reflectance measurement. The in-house software DLR@OTM is used for daylight reflectance result calculation and report generation.

Step 1. Sample preparation

Upon receiving a daylight reflectance test sample, the sample is cleaned and labeled.

For sample labeling, each sample is assigned a unique sample ID. Only one side of a sample will be tested. The side to be tested (the outdoor facing side of a sample) is labeled “Front”. Alternatively, the side not to be tested is labeled “Back”.

The sample dimensions are measured. As the sample dimensions are for reference only and do not affect the test results, sample dimensions may be rounded to a low resolution (e.g. cm for length and width).

The photos of the sample are taken, and the essential information about the sample is recorded in the list of test samples.

Shown below is a sample with labels.

A sample for daylight reflectance test

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Step 2. Total and diffuse spectral reflectance measurement

This step is the physical measurement step. The total and diffuse spectral reflectance of the sample in the 380 nm – 760 nm range is measured with the UV/VIS/NIR spectrophotometer (photo shown below). The measurement principle (link) is discussed above.

UV/VIS/NIR used at OTM (model: PerkinElmer Lambda 950)

The sample is attached to the reflection port of the integrating sphere, as shown in the photos below. The measurement port size is around 1 inch.

A sample is measured multiple times. For uniform samples (e.g. PVDF coated metal cladding), typically 2 sets of measurements are sufficient. For non-uniform samples (e.g. natural granite), more sets of measurements are required to reduce the measurement uncertainty caused by sample surface non-uniformity.

A sample on the reflection port (view from the integrating sphere)
A sample on the reflection port (view from the external)

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Step 3. Report generation

The UV/VIS/NIR spectrophotometer instrument only measures the total & diffuse spectral reflectances and saves the raw data as text files but does not directly calculate and report the daylight reflectance results.

An in-house DLR@OTM software has been developed to automate the result calculation and report generation part. Shown below is a screenshot of the software.

DLR@OTM software

Shown below is the result section captured from the test report generated.

Sample result section

The test sample is white and has a very high total daylight reflectance. Please refer to this link for a full sample report.

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If you are interested in more in-depth information, please continue reading this article. If your concerned questions are not explained, please feel free to leave a comment at the end of this page. This article will be reviewed and updated regularly.

What is the accuracy of daylight reflectance test?

The measurement uncertainties are automatically calculated for each sample.

For uniform test samples (e.g. PVDF coated metal cladding), the typical measurement uncertainties are:

  • ±0.006 (±0.6%) for total and diffuse daylight reflectances
  • ±0.003 (±0.3%) for specular daylight reflectance

For non-uniform test samples (e.g. natural granite), the measurement uncertainties are much greater and could be ±0.100 (±10.0%) or greater for samples with strong non-uniformity. For such samples, we will measure them multiple times to reduce the uncertainties.

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What is the maximum sample size for daylight reflectance test?

We can test very large samples. For example, we can test the daylight reflectance of full-size solar panels longer than 2 m.

As mentioned above, the reflection port size of the integrating sphere is around 1 inch. For large test samples, a small area of around 1 inch in diameter is sampled in each measurement and the same procedures are followed.

For samples larger than 300 mm x 300 mm, an over-size surcharge is applicable, due to the extra effort required to handle such heavy and bulky samples. Please contact us for more information.

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Last update: 28/10/2023

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