Both quantities are about the reflectance of material surfaces. However, they are different.
Daylight reflectance is about the reflectance of a surface to visible light.
Solar reflectance is about the reflectance of a surface to solar energy.
Shown below is the solar radiation spectrum (red color part is for the sunlight at sea level). The solar radiation consists of 3 parts: 1) ultraviolet (UV) radiation; 2) visible light and 3) infrared (IR) radiation.
Daylight reflectance is in terms of visible light only.
Solar reflectance is in terms of UV radiation, visible light, and IR radiation, i.e. the entire solar radiation spectrum.
Additionally, in the lab, the daylight reflectance test and solar reflectance test are two different tests:
Daylight reflectance test: only the spectral reflectance in the 380 nm – 780 nm range (the visible light range) is measured. Typically, we also separate the total, diffuse and specular components of it.
Solar reflectance test: the spectral reflectance in the 300 nm – 2500 nm range (the entire solar spectrum, including UV radiation, visible light, and IR radiation) is measured. Typically, we also measure the material thermal emittance and calculate the solar reflectance index (SRI).
Unlike paints, membrane products can be free-standing. A membrane product sample can be prepared either as a standalone membrane layer or with a substrate. The question from many customers is whether the SRI of membrane products can be tested without substrate.
To answer this question, two examples are illustrated below:
The sample on the left is translucent (with large transmission). For such samples, a substrate is required.
The sample on the right is opaque (with zero transmission). For such samples, a substrate is not required.
For a material, we have the following relationship:
Solar transmittance + solar reflectance + solar absorptance = 1
In SRI calculations, the solar transmittance is assumed 0 (solar transmittance = 0). The solar reflectance is directly measured by the instrument. Therefore, we use the following relationship to calculate the solar absorptance:
Solar absorptance = 1 – solar reflectance
However, if the material is not opaque (solar transmittance ≠ 0, for example, the left example), the equation above is not valid.
For such translucent samples, the calculated solar absorptance is higher than the actual solar absorptance, as the solar transmittance is counted as part of the solar absorptance, and the resultant SRI is therefore lower.
In order to eliminate this error, translucent samples shall not be used for SRI testing. If a membrane product is translucent, it shall be applied onto a suitable substrate for testing.
To determine if a membrane sample is translucent, a simple method is to check if the flashlight from a phone can transmit through the sample, as shown in the examples above.
In our website, we explain that the solar reflectance index (SRI) is the surface temperature in a 0 – 100 scale. In the same section, we also mention that It is possible for SRI to be negative or larger than 100. So, what is the theoretical range of SRI? What is the possible minimum SRI and maximum SRI?
Minimum SRI: the minimum SRI is the SRI of a surface with solar reflectance = 0 (perfect black color in the solar radiation spectrum) and emittance = 0 (perfect white body in the infrared radiation spectrum)
Mininum SRI = -244.6 (low-wind), -99.7 (medium-wind), or -44.9 (high-wind)
Maximum SRI: the maximum SRI is the SRI of a surface with solar reflectance = 1 (100%, perfect white color in the solar radiation spectrum) and emittance = 1 (perfect black body in the infrared radiation spectrum)
Maximum SRI = 133.0 (low-wind), 129.6 (medium-wind), or 128.0 (high-wind)
Therefore, the theoretical range of SRI is:
Low-wind: -244.6 ~ 133.0
Medium-wind: -99.7 ~ 129.6
High-wind: -44.9 ~ 128.0
Shown below are the calculation screenshots.
With the theoretical SRI range presented above, is it still valid to say that the SRI is the surface temperature in a 0 – 100 scale?
Yes, it is still valid. As most natural surfaces are with high emissivity (emittance > 0.8), the SRIs of most natural materials are in the range of 0 – 100.
Surfaces with low emissivity (low-e, e.g. emissivity < 0.2) are typically bare metal surfaces (e.g. aluminum or stainless steel). In practice, they are rarely directly used as the top layer of roof or pavement materials. When such low-e surfaces are painted, the emissivity is high (as the paint layer becomes the top layer).
Why the SRI of a low-e surface is low?
There are 3 heat transfer modes: conduction, convection, and radiation. For a low-e surface, the radiative heat exchange between the surface and the ambient environment is weak (i.e. less heat transfer via radiation). More heat is kept on the low-e surface and it results in higher surface temperature and, therefore, lower SRI.
In the low-wind condition, the convection is weak and the radiation is more dominant. This is the reason that the dependence of SRI on emittance is stronger at the low-wind condition.