*3.4. Sky Model*

We should employ direct measurements if available from the meteorological institute, which we have proved fare well with the algorithm proposed by Pierpoint et al. [32] to obtain daylighting intensities for vertical and horizontal surfaces depending on the latitude of the place. Such clear sky-model regulates the vertical component of illuminance (in lux) as a function of the solar altitude (*α*) and the azimuth (*β*), by virtue of the equation below.

$$E\_{\mathcal{D}} = 4000a^{1.3} + 12,000 \sin^{0.3}a \cos^{1.3}a \left[\frac{2 + \cos\beta}{3 - \cos\beta}\right],\tag{16}$$

If we have to deal with the CIE standard overcast sky, the former equation simplifies to:

$$E\_{\upsilon} = 8500 \sin a\_{\prime} \tag{17}$$
