## 4 Evaluating the Indoor Luminous Environment and Visual Comfort Perception
### Visual Comfort Evaluation Metrics
#### 4.1.2 Direct Light Discomfort  
Direct light discomfort is commonly referred to as glare. It is meant to quantify the disturbance that a building occupant might perceive based on how the occupants' location and view/sight are exposed to excessive light. It mainly considers the \(L\) intensity, concentration, location and the effect of its generated contrast. It can be assessed either directly or indirectly:  
* Using direct correlation with \(E_{v}\) on a vertical plane corresponding to the occupant's eyes height, lo location and orientation, neglecting the contribution of local quantities as presented by [64; 65]. In Equation (3), the probability was re-calibrated with the vertical illuminance values only estimated from rendered images [64], while Equation (4) validated in a virtual environment a simplified \(DGP\) definition computing illuminance values only using ray-tracing methods (contribution of contrast is neglected). \[DGP_{Wienold}=6.22\times 10^{-5}\times E_{v}+0.184\] (3) \[DGP_{Hviold}=5.87\times 10^{-5}\times E_{v}+0.16.\] (4)
- Using correlation with the computed \(L\) from a radiance [66] simulated fish-eye image captured for the desired location, eye-height and gaze orientation using tools such as _evalglare_ to compute it [63]. * Glare can also be directly computed using the **CIE Glare Index (_CGI_)**: born as a correction of the British Glare Index (\(BGI\)), presented by Einhorn [67]; and then upgraded into the **Unified Glare Rating (_UIGR_)**. It is a short-term, local and one-tailed glare index based on the split contribution of the direct and diffuse \(E_{h}\). * Finally, glare can also be addressed indirectly by identifying the areas of a building which are subjected to \(E_{h}\) levels over certain threshold, that is either:
- Over an upper \(E\) comfort threshold (\(>\)2000 \(\div\) 3000 lux is typically used), which can be described using the overlit portion of \(UDI\);
- Over a maximum \(E\) comfort threshold, but sustained for a determined number of hours, as the defined in the case of **Annual Sun Exposure (_ASE_)**, indicates the possibility of glare occurrence (\(>\)1000 lux over 250 h per year, is often used) [59].  
Tables 2 and 3 represent a comprehensive summary of the guidelines that have embraced the aforementioned metrics, together with their advantages and disadvantages. These metrics are generally utilized and monitored (if possible) on visual comfort assessment methods under a high degree of building design development, or level of detail (LOD).  
Nevertheless, all the analysed metrics and suggested values, already established and included in different regulations and design guidelines, do not consider the adaptation level (or capacity) of the eye, nor the variability of occupant's sensibility to light. Thus, they are inaccurate and occupant-centred. Moreover, there are no clear suggestions or procedures developed on how to include them in the building operation phase when monitoring visual comfort (which in some cases is unpractical). Consequentially, a need to further include the occupants' true perception and response to visual stimuli within the loop of building performance design and operation strategies is clearly identified.