Measure of Visual Fatigue as a Link Between Visual ...

Measure of visual fatigue as a link between visual environment and visual and non-visual functions of VDT users A review on what we have and what we need Merve Öner Dept. of Energy Engineering, Systems, Territory and Construction (DESTeC) School of Engineering – University of Pisa Pisa, Italy [email protected] Abstract—Over the last few decades, increasing number of VDT use in workstations has led to various health-related problems, among which visual fatigue is the most common. Within the framework of visual ergonomics of VDT workstations, lighting conditions are initially considered factors for being involved in the visual and non-visual functions on VDT users. Visual fatigue is a common eye-related phenomena among VDT users which manifests itself in visual and non-visual functions, but the origin is not always definite; could be associated with the VDTs, with the environmental lighting, or both. Up to now, it has been measured through objective and subjective methods in order to obtain the influencing factors; however, many of them were conducted under laboratory conditions. Moreover, the evaluation criteria were not in a multidimensional manner, which also ought to cover other visual and non-visual risks related to visual fatigue. This paper presents the overview of the existing literature to provide objective insights as to how lighting induces visual fatigue and related psychophysiological effects occurring in VDT users. In this way, the author points out the applicability of the findings of the existing literature for the implementation of visual fatigue as an indicator to be used in a broader sense; which may be implemented as a key component on identifying visual and nonvisual effects of lighting conditions in VDT workstations having different design parameters. Keywords—VDT workstations; visual fatigue; lighting environment; non-visual effects of light; visual effects of light.

I. INTRODUCTION The introduction of computers has brought about revolutionary changes in work practices of office workers. Even during 80s it was predicted that 75% of jobs would involve computer by the beginning of 2000 [1-2], which now makes it a commonplace rather than being a choice of device for conducting an office task. Several terms are used to refer to a computer in literature; cathode ray tube (CRT), visual display terminal (VDT), visual display unit (VDU) and video display terminal (VDT) are some of those. To avoid confusion, the term of video display terminal (VDT) is adopted in this paper.

Over the past two decades, changes in work practices of VDT users have been followed by a variety of health-related problems, among which eye discomfort is the most common and serious complaint [3-5]. The actual prevalence of discomfort symptoms generally remains ambiguous due to the origin of complaints and the various underlying factors. Uncertainty in the incidence of symptoms manifests itself as varying estimates from study to study and is reflected by researchers in the literature [6]; yet still, the mean percentage of incidence was found to be around 57% [7-8], which is quite considerable to realize the eye-related risks among VDT users. Aforementioned symptoms that induce visual discomfort of VDT users may be associated with an extended term called visual fatigue - also referred to as eye strain or asthenopia, which is defined by the World Health Organization (WHO) as a subjective disturbance (ICD-10, H53.1), manifested by a degree of visual discomfort typically occurring after some kind of prolonged visual activity, and characterized by fatigue, pain around the eyes, blurred vision or headache [9-10]. Visual fatigue is commonly accompanied by general fatigue in VDT users; that is to say, decrease in alertness, mood, and performance, increase in the risk of accidents, which in turn adversely impact the organizational productivity. Environmental factors in workstations are one of the most influential causes that lead to visual fatigue, which, on the other hand, can be prevented with adequate design choices. Among environmental factors, lighting of workstations is characterized as a physical factor that influences both visual i.e. task performance, visual comfort, and non-visual i.e. health, well-being, alertness attributes of VDT users. In order to deal with these complex and multi-dimensional phenomena, VDT workstations must provide well-designed visual environment that stimulates both visual and non-visual process of workers without inducing visual fatigue [11-12]. A few well-established standards exist by now although they are solely based on visual criteria that influence our visual capability. As for the non-visual process, a few modeling frameworks were proposed for static lighting conditions [1317] and have been practiced by a number of studies [18-20]. However, in the case of VDT workstations, or more generally,

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in the case of office environments having at least one window referring to approximately 97% [21], the inclusion of nonvisual effects of light in mathematical models remains challenging due to the dynamic behavior of daylight and its dependencies on i.e. architectural, climatic and geographic parameters [22].

contributors of general fatigue that influence efficiency, productivity, and accuracy in a negative way [25]. Leaving its complex and multi-dimensional form to one side, what is definitely known is that the incidence of visual fatigue among VDT users is highly common. According to a survey performed in Poland, 90% of VDT operators complained about visual fatigue, whereas other studies indicated that 75-80% of all VDT users have visual fatigue complaints [26-28]. The characteristics of lighting environment in VDT workstations, accordingly, carry a significant influence on the assessment of visual fatigue. Poor lighting environment triggers higher visual fatigue than ones that are able to meet the visual and non-visual demands of VDT users. With regard to the lighting-visual fatigue interaction, findings published in literature agree on the impact of lighting design upon the incidence of visual fatigue [28-33, 40]. Bearing in mind the type of task and the users’ demands, a well-designed luminous environment – e.g. adequate position of the workstation in accordance with the natural and artificial light sources to avoid glare or reflection is what we need for preventing visual fatigue and stimulating visual acuity and user satisfaction in VDT workstations [34-37].

Fig.1. Conceptual model representing the relationships between the lighting environment and visual fatigue. The predictor variables for lighting environment are indicated on the top whilst the visual and non-visual outcomes of visual fatigue are displayed at the bottom.

Therefore, this paper presents the overview of the existing knowledge to provide objective insights as to how lighting environment induces visual fatigue and related visual and nonvisual effects occurring in VDT users. In this way, the author elucidates the applicability of the current knowledge for the indication of “visual fatigue” in a broader sense; which may be implemented as an intersection between lighting environment and visual and non-visual effects of dynamic patterns of light in VDT workstations with various influential parameters (Figure 1). tables are not prescribed, although the various table text styles are provided. The formatter will need to create these components, incorporating the applicable criteria that follow. II. VISUAL FATIGUE Visual fatigue has been of interest to those concerning with visual ergonomics in office environment since 70s with the introduction of VDTs [23-24]. Generally, visual fatigue results from visual inefficiencies or eye-related problems, and the main causes can be divided into three categories: 1environmental i.e. ergonomics, lighting, 2- ocular i.e. binocular vision anomalies, 3- personal i.e. age, gender, general health [7]. It may manifest itself in several psychophysiological and behavioral forms such as sleepiness, negative mood state, drop in alertness, arousal, task performance etc. The symptoms of visual fatigue are the

A. Measure of Visual Fatigue A wide range of possible applications is used to detect visual fatigue in various fields. In 1990, [38] introduced 5 measures to identify visual fatigue: (1) accommodation, etc.; (2) vision measure including critical flicker frequency (CFF), transition in visual sensitivity; (3) the measure of visual fatigue; (4) the measure of subjective visual fatigue (SVF); and (5) other indications relating to vision, such as changes in visual range; which is followed and developed by the suggestion of [39], at this time including 7 items: (1) the capability of visual accommodation; (2) visual acuity; (3) the diameter of the pupil; (4) critical fusion frequency - CFF; (5) eye movement velocity; (6) the subjective rating of visual fatigue; and (7) task performance. These methods could be divided into two groups: subjective visual fatigue (SVF) and objective visual fatigue (OVF), and no matter which method is selected, the tests are ought to be conducted both before and after ocular light exposure [40]. To use OVF alone as a measure of visual fatigue may not be adequate to evaluate the subject’s state. SVF method, on the other hand, seems like the most accurate way to acquire subject’s fatigue level from the first person; however, the results could be affected by e.g. other types of emotions inducing similar feelings as visual fatigue [41]. In this regard, both subjective and objective measures should be conducted for providing a more solid body of knowledge. 1) Objective Visual Fatigue (OVF): Based on the optometric measurements, objective visual fatigue may be determined with the before/after characteristics of VDTinduced ocular indicators. There is a wide range of objective signs to measure visual fatigue, such as pupil diameter, accommodation speed, convergence, blink rate, eye closure time, heart rate, EMG, saccadic eye movement velocity, visual acuity, CFF and fixation to cite several of them. The eye itself

is identified as one of the most observable solutions to investigate visual fatigue [42], and this statement accordingly has been supported by prior research dealing with visual impairments during the use of VDT that are particularly associated with the movements of the eye. A set of ocular indicators are described as follows: a) Accommodation Speed: Accommodation is the ability of human eye to change the focus between objects of interest at different distances by adjusting the curvature of the lens. It is recognized as one of the most important elements of the oculomotor system, and slower accommodation responses are linked with the increase in visual fatigue [41]. Previous studies have shown that incidence of increased lag of accommodation among VDT operators is mostly associated with higher luminance contrast [5, 43-44]. b) Convergence: In general, accommodation and convergence responses are identified as highly correlated. According to the recent studies, higher incidence of visual discomfort symptoms occurs among individuals with convergence insufficiency, which also results with other symptoms such as headache and eyestrain [45-46]. Eye-screen distance and luminous distribution within the field of view should be carefully controlled in order to satisfy the convergence mechanism. c) CFF: Critical Flicker Frequency (CFF) is a highly sensitive measure method for assessing visual fatigue that detects the frequency at which a flickering light appears continuous and constant [39]. A decrease or increase in the CFF values is the detection way by using limits until a change is reported by a subject [47]. The CFF value decreases with increasing visual fatigue which is also accompanied by a decline in the retinal activity, ultimately results in a decrease in alertness [48]. d) Blink and blink rate: Blink, the rapid and repeated closure and opening of the eyelid, is considered as an important indicator of visual fatigue. Human eyes blink 10-15 times per minute under normal circumstances and decreased blink rate is mostly associated with visual fatigue which manifests itself as the symptoms of dry eyes [49-51]. Existing research in this area indicates that blink rate decreases when the luminous intensity on screen increases during VDT work [52-53]. e) Eye closure time: The duration of eye closure is defined as eye closure time. Prolonged eye closure time is generally a good indicator of decreased arousal occurring during visual fatigue. Alert eye closure time ranges from 100 to 300 ms with an average of 200 ms and increases to approx. 300 ms (between 200-450 ms) when experiencing fatigue [54]. Eye closure time of at least 400 ms is considered as a reliable signal for entraining visual fatigue [55]. f) Percentage of Eye Closure (PERCLOS): PERCLOS is the percentage of time that the eye is closed per unit of time, and that indicates slow eye closure rather than blinks. It is known as an accurate and effective way for drowsiness detection to assess the level of a driver’s alertness. PERCLOS

has three metrics for the identification of fatigue which are P70, P80 and EYEMEAS(EM). Of the three, P80 remains as the most reliable standard so far, where eye is considered to close when palpebral fissure is reduced to 20% and below [56]. g) Pupil Diameter: Pupil size is one of the most influential adaptation mechanisms of human visual system which mostly depends on the luminance received at the eye, modulated by the principal light/dark detectors (so-called ipRGCs) in the retina. Depending on the light reaching the eye, the diameter can vary from approximately 2 mm to 8 mm by contraction (miosis) and dilation (mydriasis) of the pupil. Generally, pupil size constricts as luminance increases and dilates under low light intensity [57-59]. Existing knowledge on the precise mechanism behind the fluctuations in pupil diameter, however, is still unclear and controversial. For instance, widely differing results were determined by [60-61], that found no significant association between pupil size variation and the amount of light received by the eye. This leaves more room for explanations of this complex mechanism in which cognitive and affective information processing also might be the contributors to the changes of pupil size. h) Reaction Time: Reaction time (RT) is a measure of the response to a stimulus which is potentially affected by personal and environmental factors. It is usually obtained by means of different tests that evaluate cognitive performance. Previous research on the light-related triggering effects of reaction time has revealed differing opinions. For example, [62] found no effect of color temperature on RT whereas [63] demonstrated that higher color temperature has a positive effect on RT. [64] found a significant positive correlation between decreased illuminance levels and faster RT, whereas, on the other hand, [65] discovered that RT do not depend on the amount of light. i) Visual Acuity: Visual acuity testing methodology is generally organized by reading projecting letters from a specific distance or using vision testers. Previous research indicates that luminous characteristics of VDT workstations are found to be highly related with the level of acuity, along with appraisal, well-being, cognitive performance, and perception [35, 66]. j) Fixation: Following a saccade, the eyes pause on a single location for a limited time which is termed fixation, and move again. Visual information is efficiently sent to the brain mainly during fixations due to the suppression of vision during a saccade [66]. Physical characteristics such as contrast within the gaze direction, font characteristics and resolution have an impact on the performance of fixation. It is assumed that longer fixations between saccades may be considered as indicators of level of arousal and alertness during VDT reading [9, 68-69]. k) Saccadic Movement Velocity: The quick eye movements between fixations are termed as saccades. Velocity of saccadic eye movements is known to be potential source for the measure of visual fatigue. Visual fatigue could be induced by decreased saccadic velocity which also

contributes to lower arousal and attention, attributable to a decrease in task performance during VDT work [70]. Saccadic movement velocity of the pupil constriction depends on the intensity of a light stimulus [71]. 2) Subjective Visual Fatigue (SVF): In this approach, subjective responses of VDT users are transformed into quantitative data. [72] suggested 5 methods to measure SVF: (1) ranking method; (2) rating method; (3) questionnaire method; (4) interview and checklist. So far, the questionnaire method has been the most preferred by the scientific community for enabling to acquire the visual fatigue state of the subjects directly. A subjective rating of VDT-induced visual fatigue was developed by [73] consisting of six items, which are listed as follows: • • • • • •

I have difficulties in seeing. I have a strange feeling around the eyes. My eyes feel tired. I feel numb. I have a headache. I feel dizzy looking at the screen.

III. RESEARCH GAP ASSOCIATED WITH THE VISUAL AND NONVISUAL EFFECTS OF VDT-INDUCED VISUAL FATIGUE UNDER THE DYNAMIC BEHAVIOUR OF DAYLIGHT For more than 150 years, rods and cones were considered as the only two photoreceptor cells in the human eye. Rods provide vision in dim light (scotopic) whereas cones are responsible for sharpness, detail, and color in bright light conditions (photopic vision) [74]. A decade ago, the discovery of a new class of photoreceptors in the retinal ganglion cells (the so-called intrinsically photosensitive Retinal Ganglion Cells, ipRGC) by [75] demonstrated that the human eye serves much further beyond the response of the visual system: regulating hormonal secretions, circadian rhythms, and the pupil light reflex [76]. Since then, it has become evident that light entering the eye has consequences not only visually but also biologically. In lighting design, the regulations for photobiological stimulation (e.g. vertical illumination, spectrum, duration, timing of light etc.) have thus to be taken into account including non-visual factors for human health and psychophysiological well-being. On the contrary, lighting standards and recommendations have been developed, and are still in use, based on the characteristics of light-responding receptors in the eye that focus exclusively on the photopic vision. Moreover, the evaluation criteria in most of the standards are established on horizontal illuminance, that is for traditional paperworks, disregarding extensive use of VDTs in work environments. However, as a result of an increasing recent research indicating that a synchronization of the biological system through its interaction with the external lighting conditions is possible, a start has been made by various researchers on a quantitative relationship between the light characteristics and non-visual response. While for the non-visual effects of daylight, there are limited number of studies attempted to introduce a framework because of the many factors involved in the evaluation process i.e. daylight characteristics (intensity,

duration, spectrum, quantity, directionality, history, and time of the day), architectural and climatic parameters- which pose new challenges in the evaluation of lighting quality, such studies are, accordingly, more for establishing mathematical models for quantifying the effects of artificial lighting design by means of the spectral sensitivity of static light sources and its relation with non-visual processes. In the previous section, it was briefly stated that VDTinduced visual fatigue affects VDT users visually and nonvisually i.e. the level of visual comfort, visual acuity, arousal, performance etc. In case of VDT workstations in which particularly dynamic patterns of daylight and artificial light dominate the space, then we can conclude that light factors triggering the visual and non-visual effects on VDT users are dependent on the climatic and architectural parameters determining the amount of light received, and also are able to be adjusted with adequate time-based and environment-based design choices. There is still a long way to establish the most accurate framework but the growing attention towards this new research interest in order to contribute to a more holistic approach can not be ignored. A key barrier to progress in nonvisual effects of daylight is the lack of estimating the contribution of light factors that best responds the users’ needs in real-world settings in which light characteristics vary with time of the day and are influenced by spatial and temporal factors. IV. DISCUSSIONS The aim of this paper was to evaluate possible associations between visual fatigue and psychophysiological responses of VDT users under the influence of dynamic behavior of lighting environment which is particularly dependent on architectural and climatic parameters. Based upon the existing literature, it can be assumed that visual fatigue triggers general fatigue and a change in general fatigue subsequently affects our alertness, health, well-being, and performance. The World Health Organization (WHO) defines health as “a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity” [77]. Current lighting recommendations are solely based on the visual criteria; however, the non-negligible fact is that VDT workstations need further recommendations involving many more factors for stimulating non-visual attributes for users’ health and well-being according to the definition of WHO. Setting the indication of “visual fatigue” as a starting point, if a framework is established, time and environment dependent visual and non-visual effects of light may be determined with a newly developed analytical model for the purposes of both preventing risk factors in VDT workstations and providing a healthy lighting setting for users. This overview lends further support to the idea that suggests a multi-dimensional framework by measuring visual fatigue for the identification of visual and non-visual functions of VDT users under lighting conditions influenced by various building parameters. At this point, possible solutions on what we need are listed as follows:

- To measure visual fatigue in static laboratory settings with short-term exposure to test conditions may not be adequate to make a generalization; therefore, measuring visual fatigue in a real-world setting and under various intraarchitectural and climatic parameters may be a good start. In order to determine the accurate influences of intraarchitectural and climatic variables, other possible determinants of visual fatigue have to be eliminated e.g. selection of healthy subject, proper VDT device, correct sitting posture etc. Both subjective and objective measurements are ought to be performed for a better understanding of visual and non-visual processes of VDT users under various conditions. - The following step may be implementing the identical scenario for the building scale to identify the influence of inter-architectural parameters on visual fatigue. - As perception of light also depends on the interindividual parameters, age, gender, chronotype and origin of the VDT user and his/her reaction against lighting environment may vary from person to person. Hence, the final step may be performing an extensive, cross-cultural experiment in order to analyze the impact of inter-individual differences on the evaluation process of visual fatigue. REFERENCES [1] [2] [3]

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