## 1. 2.Solar Absorptance and Absorptance-Emittance ($\alpha_s/\epsilon_{ir}$ or a/e) ratio Absorptance of real materials might have a strong spectral dependency (i.e dependency on the wavelength). For example a substance might have very low emittance/Absorptance in the visible and near infrared spectrum but might have a high emittance/Absorptance in the far infrared spectrum. Such a material would be very good at reflecting most of solar radiation and also be great at emitting whatever radiant energy it absorbs at a higher wavelength as far infrared radiation. Such material with low a/e ratio (absorptance/emittance) ratio are great for keeping a surface cool under solar loading. Some types of white paint fall in this category. On the other hand a surfaces with high a/e ratio absorb solar radiation better than they are able to emit them. $\epsilon_{ir}=\frac{\int{\epsilon_\lambda E_{b\lambda}d\lambda}}{\sigma T_{surf}^4}$ $\alpha_s=\frac{\int{\alpha_\lambda E_{b\lambda}d\lambda}}{\sigma T_{sun}^4}$ where $E_{b\lambda}$ is the black body [[Emissive Power]] The can be different because surfaces may absorb some frequencies more/less than others Standard test method for solar absorptance is [ASTM E 903](https://www.astm.org/e0903-20.html)