# Material solar reflectance index (SRI) testing procedures

Material solar reflectance index (SRI) test is a test regularly performed by our lab, following ASTM E903, C1371, and E1980. This article aims to explain the principles and step-by-step procedures of the SRI test.

SRI testing principles:

SRI testing steps:

## SRI testing principles

### SRI calculation

The SRI is defined as the temperature of a sample surface (Tsample) scaled by the temperatures of a reference black surface (Tblack) and a reference white surface (Twhite), under the standard environmental conditions.

The reference black surface, reference white surface, and standard environmental conditions are defined in ASTM E1980.

Listed in the table below are the properties of the reference black surface and reference white surface:

Listed in the table below are the standard environmental conditions:

The solar flux, air temperature, and sky temperature are the same in the 3 standard environmental conditions.

There are 3 SRIs for each sample (low-wind SRI, medium-wind SRI, and high-wind SRI). The surface temperature calculation method is explained in the section below.

### Surface temperature calculation

Please refer to this post for more information on the surface temperature calculation model in ASTM E1980. The equation is shown below:

The model is based on the heat balance principle that the absorbed solar radiation is either transferred to the sky via long-wave radiation or to the ambient air via convection. The conduction to the material underneath the surface is ignored.

The unknown is surface temperature (Ts). The convective coefficient (hc), solar flux (I), air temperature (Ta), and sky temperature (Tsky) are defined in ASTM E1980 and listed in the table above.

The solar absorptance is calculated from the solar reflectance with the simple relationship:

Solar absorptance = 1 – Solar reflectance

The solar reflectance calculation method is described in the next section.

The emissivity is the same as the emittance, which is directly measured in the lab.

### Solar reflectance calculation

The solar reflectance is calculated from the spectral reflectance measured in the 300 nm – 2500 nm range with weighted averaging. The standard AM1.5 solar spectrum is used as the weights. Shown below is the calculation formula:

In the formula above:

• ρ: solar reflectance
• ρλ: spectral reflectance
• Eλ: standard solar spectrum
• λ: wavelength (300 nm – 2500 nm)

By default, the AM1.5 direct normal solar spectral irradiance distribution defined in ASTM E891 is used as the weighting spectrum. It is possible to use other standard solar spectra, as described in this post.

Essentially, spectral reflectance and emittance are the two quantities measured in the lab, and the details are described below. The other quantities are defined in the standards or calculated according to the standards.

## SRI testing steps

The UV/VIS/NIR spectrophotometer available at OTM lab is used in spectral reflectance measurement. The emissometer is used in emittance measurement. The in-house software SRI@OTM is used in SRI result calculation and report generation.

### Step 1. Sample preparation

Upon receiving an SRI 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 of the sample is recorded in the list of test samples.

Shown below is a sample with labels.

### Step 2. Spectral reflectance measurement

This step is a physical measurement step. The spectral reflectance of the sample in the 300 nm – 2500 nm range is measured with the UV/VIS/NIR spectrophotometer (photo shown below).

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, with randomly selected measurement locations. For uniform samples (e.g. PVDF coated metal cladding or painted board), typically 2 measurements are sufficient. For non-uniform samples (e.g. natural granite or concrete), more 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)

### Step 3. Emittance measurement

This step is a physical measurement step. The emittance of the sample in measured with the emissometer (photo shown below).

The emissometer is firstly calibrated by a high emittance standard and a low emittance standard. After the calibration, the sample is measured, with the reading directly displayed on the screen of the meter.

### Step 4. Report generation

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

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

The test sample is in white color and has very low surface temperatures and very high SRIs. Please refer to this link for a full sample report.

### What are the differences between ASTM E903 and C1549?

ASTM E903 is based on the spectral reflectance measurement in the range of 300 nm – 2500 nm. If the measurement interval is 5 nm, there are 441 spectral reflectance results. The instrument used is a UV/VIS/NIR spectrophotometer.

ASTM C1549 is based on the broadband reflectance measurement results at four nominal wavelengths: 380 nm, 500 nm, 650 nm, and 1220 nm. The instrument used is a portable solar reflectometer.

The typical measurement uncertainty of solar reflectance measurement at OTM is ±0.007 for uniform samples. As presented in Table 1 of ASTM C1549, the reproducibility was between 0.0043 and 0.0201. In general, the measurement accuracy of ASTM E903 is better than that of ASTM C1549.

Some result comparison was presented in Table 2 of ASTM C1549. The maximum result bias between the ASTM E903 and C1549 was 0.029. In practice, The results obtained with ASTM E903 and C1549 are considered equivalent.

In summary, ASTM E903 and C1549 are based on different measurement principles. ASTM E903 has a better accuracy. The results obtained with ASTM E903 and C1549 are in general considered equivalent.

### There are 3 SRIs. Which one should be used?

There are 3 SRIs for a sample: low-wind SRI, medium-wind SRI, and high-wind SRI. If it is not explicitly specified, we recommend using the medium-wind SRI as the representative sample SRI.

### For paint/coating materials, are the SRI results affected by the substrate?

For paint/coating materials, the paint/coating needs to be applied onto a substrate for testing and we cannot directly test the wet materials.

Regarding the influence of the substrate, there are two scenarios:

• If the paint/coating layer is opaque, the SRI results are not affected by the substrate.
• If the paint/coating layer is not opaque, the SRI results are affected by the substrate.

Shown below is an example of a translucent (non-opaque) coating layer.

In practice, the SRI results are strongly dependent on the top layer and the dependence on the substrate is weak, particularly when the paint/coating is thick or with a primer.