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).