Lighting for work: A study of the relationships among discomfort glare, physiological responses and visual performance
Introduction
Exposure to appropriate amounts of natural light elevates occupant mood, alertness [1,2] and overall health, and reinforces synchronising of our circadian rhythms to day and night [3,4]. The existing building practice represents a trend towards direct sunlight avoidance in order to ensure energy-saving and visual comfort. In contrast, more recent standards with a greater focus on occupant health and wellbeing, that is, the WELL Building Standard [5] and the new European standard (EN 17037) [6], require a minimum amount of exposure time to natural light in order to fortify occupant physical and psychological health. However, excessive sunlight remains problematic in terms of glare and undesirable visual discomfort.
In daylit workplaces, visual discomfort can occur due to glare, veiling reflections, shadows or too much non-uniformity in the created visual field [7]. Avoiding glare is considered as one of the key features in addressing visual discomfort in office buildings with high daylight availability and clear skies. This phenomenon can occur due to either high luminance contrast or an unsuitable range or distribution of luminance, leading to discomfort sensation in or around the eyes without necessarily impairing the vision [8,9]. Hopkinson [10] proposed the first model to assess the perceived discomfort glare from windows by using the Cornell equation [10], which was later modified by Chauvel [11] and introduced as Daylight Glare Index (DGI): indicates the luminance of the glare source(s) (cd/m2); shows the solid angle subtending each source from the viewer's line of sight, and the position index of each luminaire (Pi); Lb is the luminance of the background (cd/m2); represents the solid angle of the window; and is considered as the luminance of the window (cd/m2). This index is not related well with direct sunlight or interior specular reflection. In addition, it fails to perform well when the glare source fills almost the entire field of view or when the background luminance equals to the source luminance due to the focus on window luminance as a part of background luminance.
To account for DGI limitations, Wienold and Christoffersen [12] incorporated vertical eye illuminance (Ev) as an adaptation level into their equation and proposed the Daylight Glare Probability (DGP) index:where represents the vertical eye illuminance received from the light source (lux); Pi represents the position index with respect to the glare source; indicates for the luminance of the source [cd/m2]; shows the solid angle of the source seen by an observer. DGP values ranging from 0.3 to 0.45 indicate the progression of glare evaluations from imperceptible glare to intolerable glare respectively. In addition, it can be interpreted as the percentage of people perceiving discomfort in a lighting situation [12].
Quantifying visual discomfort has been considered as a controversial and much-disputed subject within the field of lighting research [13,14] despite myriad studies conducted on glare and visual discomfort [13,[15], [16], [17], [18], [19]]. That is, in all existing models, the physical quantities of the luminous environment are attributed to discomfort sensation through psychophysical procedures such as subjective rating scales [20,21]. Although subjective evaluations facilitate broadening research understanding of the topic, there is always a degree of uncertainty and bias associated with these subjective measures [22]. Further, the subjective perception of lighting environment has been argued to be inextricably interwoven with the subject's preferences, background, culture, and physiological differences, and is consequently susceptible to individuals' to Refs. [7,23]. The inconsistency indicated by a number of validation studies [13,14] has emphasised this deficiency. Thus, pairing subjective assessments with objective measures is suggested as the more promising research method [[24], [25], [26]] in order to quantify the individual's sensation under discomfort glare conditions and compensate for the subjectivity of glare perception.
Although the human visual system responses have been shown to be sensitive to the lit environment and could be considered as objective measures, to date, physiological measurement has received scant attention in the lighting research literature. Hamedani et al. [25] conducted a thorough review of physiological responses studied in lighting research as an objective measure of visual discomfort sensation. According to this literature review, measures of pupil size [[27], [28], [29], [30], [31], [32], [33], [34], [35]], eye movement [27,36], eye blink [37] and degree of eye-opening [34,38] have been investigated previously. Pupil size was shown to be sensitive to the overall background luminance and illuminance at eye level; however, relative measures of pupil size characterising pupil oscillation (fluctuations in pupil size) indicated a better correlation with subjective evaluations and glare indices. Further, they identified eye movements and spontaneous blink rate as potential indicators of visual discomfort; however, more research was recommended in this regard. Finally, a holistic approach that includes two main objective measures, coupled with common methods in lighting research, light-induced physiological responses and visual performance, was suggested to overcome the limitations associated with the subjectivity and individuality aspects of visual discomfort evaluations [25]. Thus, in this research, this approach was undertaken in order to objectify the individual's visual discomfort sensation, which may provide a higher predictive reliability.
Visual performance has been defined as the speed and accuracy of performing a visual task. Speed and accuracy are considered primary requirements for worker productivity as they engage visual and motor factors of task performance [39]. To this end, Boyce et al. [40] utilised a timed vision test and recorded the accuracy and speed of performing the test as quantitative measures in their analyses. Despite the objective nature of task performance, to the best of our knowledge, no study has incorporated visual performance measures into physiological research.
To date, little experimental evidence has been reported on the full range of known light-induced physiological responses. The present study coupled a wide range of physiological measures and visual performance evaluations with the common lighting research method concerning photometric measurements and subjective evaluations. In particular, the present study focused on examining mean Pupil Diameter (PD), Pupillary Unrest Index (PUI), Blink Rate (BR), Blink Amplitude (BA), number of fixational eye movements during reading (Fixation Rate) (FR), and average Fixation Duration (FD), as well as Combined Visual Performance (CVP). Therefore, this study is the first to bring together a wide range of objective, subjective and photometric measures. This holistic approach offers new insight into the application of objective measures in the assessment and prediction of visual discomfort, by advancing knowledge on various involuntary physiological responses and identifying the most sensitive indicators. Further, these indicators can create more definitive glare markers, which can in turn lead to the development of efficient predictive models and responsive lighting solutions.
Section snippets
Experimental design
The present study sought to evaluate the effect of luminous conditions on involuntary physiological responses and performance among the participants. With this aim, a between-subjects experimental design was implemented, which included lighting conditions as between-subject factors at three levels. Each participant experienced one lighting condition, with either low, medium or high visual discomfort level. Main daylight conditions were initially identified using a parametric lighting simulation
Dependent variables
The multidimensional approach was based upon physiological measures, visual performance and subjective responses. Physiological measures were selected according to their sensitivity and reliability in detecting visual discomfort sensation or fatigue. One combined measure of visual performance was captured for each participant. Subjective responses were collected at different steps of the experiment utilising questionnaires comprised of demographic questions, visual discomfort and glare ratings,
Results
This study set out to assess the extent to which visual discomfort sensation can be operationalised as an objective measure, through a multi-dimensional method employing physiological measurements and visual performance. To this end, the first set of analyses examined the impact of experimental conditions on each physiological measure as well as on visual performance. Further statistical tests examined the relationship between subjective responses and photometric measurements, glare indices,
Discussion
The present study was designed to determine the effect of lighting conditions on involuntary physiological responses, subjective responses and visual performance. Physiological responses included Fixation Rate (FR), Fixation Duration (FD), Blink Rate (BR), Blink Amplitude (BA), Pupil Diameter (PD), Pupillary Unrest Index (PUI), and Visual Performance (CVP).
The current study found that the FR was significantly higher under high discomfort glare conditions, by indicating that participants
Conclusion
This study set out to objectively measure and assess discomfort glare sensations through examining user involuntary physiological responses and visual performance. To this end, the eye-tracking method was coupled with photometric measurements and subjective evaluations. Light-induced physiological responses, namely Pupil Diameter (PD), Pupillary Unrest Index (PUI), Blink Rate (BR), Blink Amplitude (BA), Fixation Rate (FR) and average Fixation Duration (FD) were then calculated from the
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