### Glare Evaluation by UGR  
UGR is a glare quantification tool which is based on contrast noticeable by an observer. Figure 1 shows an example of a space with highly contrasting LED luminaires. For an indoor observer, the UGR formula is given as [8]:  
\[UGR=8\log\left[\frac{0.25}{L_{b}}\sum_{i=1}^{n}\frac{L_{i}^{2}\omega_{i}}{p_{i} ^{2}}\right] \tag{1}\]  
where \(L_{b}\) is the background luminance in cd/m\({}^{2}\), \(L_{i}\) is the luminance of the luminaire in cd/m\({}^{2}\), \(\omega_{i}\) is the solid angle of the luminaire in sr, \(p_{i}\) is the Guth position index of the luminaire, and n is the number of luminaires in the observer's field of view. Assumptions and limitations of the UGR method:  
* UGR cannot be applied for completely indirect lighting systems or luminous ceilings [8]. * UGR is not applicable for point sources [8; 9]. * UGR cannot be computed for very thin beam patterns pointing straight down [4; 7; 9; 10]. * It is assumed the room walls are uniformly illuminated [8]. * UGR is unreliable for luminaires with non-uniform luminous intensity distributions [9; 11]. Alongside the mentioned limitations, discomfort glare evaluation is a challenging effort. [PERSON] et al. said that the two categories of effects which lead to discomfort glare are saturation and contrast. UGR is only based on contrast effect. The performance of metrics using contrast effect is poor in brighter scenes. [PERSON] et al. also stated that applicability of metrics using two or more effects simultaneously is limited by their dataset development scope [12]. [PERSON] and [PERSON] argue that in experiments, biases can affect reliability and validity of data. They indicate that among four psychological procedures used for explicit  
Figure 1: Visual discomfort glare of luminaires installed in the ceiling in a closed space. quantitative measurement, adjustment and category are the most prevalent procedures in discomfort glare evaluation. The two mentioned procedures are subjected to biases such as stimulus range, anchor, and order effect biases [13]. Consequently, UGR as a metric which developed using adjustment procedure [14] is subjected to the mentioned biases. [PERSON] et al. suggested that there is possibility of the existence of some unknown psychological and physiological factors influencing discomfort glare perception. They also noted that the intensity of each influencing factor cannot be understood and explained accurately. Therefore, an exact correlation between discomfort glare as assessed by the metrics and as perceived by subjects cannot be expected. They also, based on previous research, reported that even age and culture have effects on discomfort glare [15]. [PERSON] et al. based on investigating 22 glare prediction metrics, stated that still there is not a reliable metric that considers all scenarios [16]. [PERSON] obtained UGR for bare and diffuser-covered LED luminaires. UGR was calculated using both luminance measured by an image luminance measuring device (ILMD) and luminous intensity numbers in the luminaire IES file. The results showed for both luminaires that the UGRs calculated by the two methods have substantial differences. He recommended correct luminance distribution measurement according to human eye geometry and adding a factor to the UGR formula related to luminaire construction, particularly for luminaires with non-uniform luminance distribution on the whole surface [9]. According to limitations of the existing UGR method as pointed out above, alternatives such as the model based on receptive fields introduced in [17] are proposed. The model better interprets visual discomfort, especially in the case of luminaires with non-uniform luminous intensity distribution. However, the model still must be improved for indirect viewing angles and different background luminance levels. As mentioned previously, UGR is a widely accepted standard visual discomfort rating index, but according to limitations, researchers are working to enhance its applicability [10]. [PERSON] et al.