# A Novel Approach for the Definition of an Integrated Visual Quality Index for Residential Buildings
## 3 Methods
### Metrics  
Each physical dimension can be measured with different metrics. They usually present both advantages and disadvantages and may be used in different situations [49]. Concerning visual comfort, there is still no consensus on the metrics to consider, and, moreover, there is the challenge to synthetize in a single value the conditions that change in relation to space and time [38]. Here, a trade-off between precision and possibility to measure/simulate is necessary. The metrics chosen for the proposed VQI are:  
* sDA--spatial daylight autonomy [50] (for daylight access)
* sDGP--simplified daylight glare probability [51] (for glare probability from daylighting)
* E\({}_{\text{m}}\)--maintained illuminance (for illuminance level from electric lighting)
* UGR--unified glare ratio
* CCT--correlated color temperature (for color appearance of electric lighting)  
Daylight factor (DF) is the most used metric for the assessment of daylight access. It is very easy to calculate but it has several limitations: it is a static metric that does not depend on weather conditions, the period of the year, or even the orientation of the window [49]. To overcome these limitations, daylight autonomy was introduced by the Association Suisse des Electriciens in 1989: it represents the percentage of the occupied hours of the year when a minimum illuminance threshold is met solely by daylight. sDA is a further step and allows the use of a single value to give information about the daylight illuminance of the entire room [52]: it is the ratio of space that receives a daylight illuminance greater or equal to 300 lux for at least 50% of the occupied hours. The main criticism of this metric is that it doesn't have an upper limit, leaving the possibility of an excess of light that could cause glare. In our case the risk was overcome, since we also considered glare probability with a separate parameter. In the LEED v4 protocol [33], sDA is paired with the measurement of annual sun exposure (ASE), which has to be inferior to 10%. This limitation is highly restrictive and is adequate for spaces with delicate visual tasks [53] which have been generally used for office buildings.  
According to some authors, daylight glare probability (DGP) is the most appropriate metric to with which to analyze absolute glare issues [54], but it has the limitation that the computation is very demanding. sDGP is a simplified version of daylight glare probability proposed by [PERSON] et al. in 2007 [51]. The simplification consists of neglecting the influence of peak glare sources, and significantly reduces the computational effort [49]. The discomfort glare index (DGI) is considered less precise [55] while the measurement of glare through HDR (high dynamic range) images [29; 56] is too complicated for extensive monitoring.  
For electric lighting it was decided upon to select the metrics that are more widely used in professional practice and which are also regulated in the standard UNI EN 12464 of illumination for workplaces [31]. Maintained illuminance (E\({}_{\text{m}}\)) describes the illuminance on a given surface considering the natural decay of performance of the illumination devices. The unified glare ratio (UGR) was introduced by CIE (Commission Internationale de l'Eclairage) in 1995 for determining the glare of light sources. It has several limitations [49], but it is widely used, and is considered suitable to assess glare due to artificial light sources of medium size. Correlated color temperature (CCT) measures the apparent color of the light emitted.