Ann. occup. Hyg., Vol. 48, No. 1, pp. 29-38, 2004
© 2004 British Occupational Hygiene Society
Published by Oxford University Press
Photometry in the Workplace: The Rationale for a New Method
1 Department of Occupational Health, Universita’ Degli Studi, University of Milan, Via S. Barnaba 8, Milan 20122; 2 Department of Electrotechnology, Polytechnic of Milan, Piazza Leonardo da Vinci 32, Milan 20153, Italy; 3 Department of Public Health, University of Adelaide, South Australia 5005, Australia
Received 12 October 2001; in final form 25 April 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHOD FOR ASSESSMENT OF...DISCUSSIONCONCLUSIONSREFERENCES |
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Objectives: The assessment of lighting conditions in workplaces has traditionally focused on the measurement of illuminance. The rationale for a new method for the detailed evaluation of natural and artificial light in ‘near work’ situations, involving the assessment of luminance, is described. Methods: The procedure comprises four successive phases: (1) identify object/images observed during work tasks; (2) outline the area of the operator’s visual field where gaze is predominantly directed; (3) measure luminances in the visual field, pin-pointing all sources of primary and secondary luminance, and constructing iso-luminance maps; and (4) compare luminance ratios. Results: The procedure was illustrated using the common example of near work in an office environment. Illuminance was found to be inadequate to evaluate the effects of natural and artificial environmental light in the workplace. This is due to the fact that the luxmeter is designed to integrate the light detected over a large angle, whereas in near work the operator’s retina is mainly stimulated by light originating from objects/images placed in the occupational visual field. Conclusions: A detailed measurement of luminance within the occupational visual field is consistent with ocular anatomy and physiology, and can be used as part of a risk assessment for visual disturbances and to rationalize lighting at workstations.
Keywords: asthenopia; lighting conditions; photometry; VDU work; veiling glare
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INTRODUCTION |
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Lighting in a work environment may influence both efficiency and visual comfort (Weston, 1962; Hopkinson and Collins, 1970; Grandjean, 1987; Begemann et al., 1997; ISO, 1997; IESNA, 2000; Knez and Kers, 2000; CIE, 2001). These issues have become more obvious in recent decades because of the greater demand for computer-related ‘fine work’, and the high contribution of artificial lighting in modern buildings, related to open space design, security considerations and sub-optimal furniture layout. Fine work refers to the predominant observation of small objects and images, normally within 1 m, whereas ‘near work’ implies activation of ocular accommodation and convergence mechanisms typically within 1 m, regardless of the size of the object.
According to the recent Second European Survey on Working Conditions, involving 15 800 workers, it was found that 39% use a computer, 9% complain of ocular problems and 50% have no personal control over comfort factors at their workplace, such as lighting, ventilation and temperature (European Foundation for the Improvement of Living and Working Conditions, 1997). It was also reported that the pace of work seems to be increasing, dictated by external demand. Finally, a majority of workers believe that job demands are high, stemming from complex tasks and quality standards.
The consequences of these working conditions are very significant by virtue of the fact that they affect tens of millions of workers throughout the world. Furthermore, any problems are likely to be exacerbated by the shift from manual work to ëconceptual work’, i.e. from blue collar to white collar. As such, lighting conditions and visual well-being are increasingly important aspects of occupational hygiene, medicine and ergonomics.
OCULAR PHYSIOLOGY AND PHOTOMETRY IN THE WORKPLACE
It is evident from the literature on illumination engineering and architecture that there is a focus on one parameter, i.e. illuminance, when evaluating lighting conditions in the workplace (Hopkinson and Collins, 1970; ISO, 1989; IESNA, 2000). It is argued here that this parameter, by itself, is inadequate for the purpose of evaluating conditions that can potentially lead to visual disturbance and inefficiency.
This may be better understood if some technical information about the instrument used to measure illuminance, i.e. the luxmeter, is given. In its most simple and common form, the luxmeter is composed of a photovoltaic cell, the output of which is colour-corrected to match human visual sensitivity. The measurement is strongly influenced by the angle of incidence (cosine law) of light on the photocell (Fig. 1), but is largely non-specific in terms of the direction of light. This is because the photocell gathers and integrates a great number of primary (direct) and secondary (indirect) light vectors, but gives only one average reading (Fig. 2).
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These features mean that the readings taken for instance from a desk-plane or a computer keyboard include and integrate all light coming vertically, even though the operator’s eyes may perceive it only inadvertently. The predominant source of the vertical light stream in offices is typically the lamp mounted in the ceiling, and although characterizing this illuminance is important in engineering terms, it is not particularly important with regard to ocular exposure, unless the worker is constantly looking towards the ceiling. In contrast, there is only partial integration of non-vertical light by the luxmeter, but light from this direction is commonly perceived by the operator’s eyes. If the luxmeter photocell, instead of being positioned parallel to the work-plane, is positioned with any inclination to it (Fig. 2), the readings would again be inadequate because they are still not representative of that bundle of light admitted to the eye. In fact, only a minimum part of light present in a given environment is able to reach the operator’s retina. Overall, it is important to consider the following:
- the shielding action of the osteo-cutaneous orbitary protuberances (eyebrows, nose) and eyelids;
- the selective action of the pupil associated with both light intensity and ‘near reflex’ (accommodation-convergence reaction);
- the position of the head which, in modern work, is frequently predetermined and fixed in relation to the different tasks (D'Orso et al., 1995; Salerno et al., 1998; Rulli et al., 1999), but is mismatched to the normal usage geometry of the luxmeter.
- the selective action of the pupil associated with both light intensity and ‘near reflex’ (accommodation-convergence reaction);
To reinforce the issue of geometry, the structure of the retina should be considered in more detail. The centre of retina is occupied by a small area (5.5 mm of diameter) called the macular region. In this portion of the retina, composed of four concentric areas (perifovea, parafovea, fovea and foveola), a great differentiation in density, structure and peculiarity of nervous connection of photoreceptors (cones) allows high detail discrimination to be achieved. In contrast, the rest of retina, due to a prevalence of rods, has a high light sensitivity, but poor form discrimination (Anderson, 1987; Cohen, 1992). In this regard, it is worth noting that at 20° from the centre of foveola the resolution power is reduced by 90% (Cohen, 1992) and the global sensitivity to ‘the stimulus’ (photopic condition) is diminished by 30% (Saraux and Bias, 1983; Anderson, 1987).
Finally, the impact of the Stiles–Crawford effect should be considered. This phenomenon is related to the directional sensitivity of retinal photoreceptors. Parallel rays of light entering the pupil through its centre are more effective in stimulating retinal cones than are those that enter the eye near the edge of a dilated pupil, as they reach the retinal cones somewhat more obliquely (Hart, 1992).
The effect is appreciable if the pupil is dilated (>5 mm in diameter), for instance because of lens opacity, owing to age or pathology, or because of relatively dim conditions, e.g. in computer-aided design and manufacturing (CAD, CAM), air traffic control, work with photosensitive materials, pre-press editing or photo-retouching. Despite there being a natural age-related reduction in pupil size, an increase in opacity reduces the amount of light reaching the fovea which increases the likelihood of pupil dilation beyond 5 mm.
Although, in theory, enlargement of the pupil diameter from 6 to 7 mm would contribute 6.5 times more additional light than enlarging it from 2 to 3 mm, the Stiles–Crawford effect reduces this by a factor of about two in the case of cone vision.
Thus, while the retina, and in particular the macular region, is subject to continuous stimulation by light, whose intensity and origin is highly variable, there is considerable discrimination of light, due to anatomical shielding, physiology and head position. The luxmeter, on the other hand, is designed to gather and integrate into one summary value the light detected over a large angle.
Overall, two conclusions can be made. The first is that the luxmeter readings do not correspond to the light that enters the operator’s eye. The second is that the effect of environmental light on human eyes does not depend only on its magnitude, but also on the region of the retina reached (Westheimer, 1992). In fact, the summation of light originating from objects unrelated to the work-tasks, which we shall term ‘parasitic straylight’, with that produced by the objects actually observed during work, is much more disturbing for perceptive and cognitive processes if they occur in the macular region than in the rest of the retina.
Building-related ocular symptoms are common, but the origin is not always apparent (Bachmann et al., 1995; Hempel-Jorgensen et al., 1997). However, any rational assessment of lighting that seeks to account for symptoms or loss in productivity should specifically address the occupational visual field, which is defined as the zone in which the worker, because of task constraints, must direct his/her gaze for extended periods of time (D'Orso et al., 1995; Salerno et al., 1998; Rulli et al., 1999). For near vision work in office environments, this visual field can be readily identified.
Detailed photometric analysis in the occupational visual field is impossible using a luxmeter, but is feasible with a luminance meter (Grieco and Piccoli, 1982; Piccoli et al., 1988; Halonen, 1993; Halonen et al., 1993). This kind of photometer, which is similar in shape, size and mode of use to a video camera, has an optical system with a viewfinder, which allows the region of interest to be framed while readings are taken. If placed in the same position as a worker’s head, and oriented according to gaze, the luminance meter permits photometry that is more detailed, selective and precise, compared with the luxmeter, because of its optical system and design.
A practical illustration of the advantages of a luminance meter over a luxmeter is given in Figs 3 and 4. These figures are taken from laboratory experiment with a visual display unit (VDU) workstation. The laboratory is equipped with black curtains and panels for the roof and walls, in order to eliminate daylight and minimize reflectance (below 5%). Luminance and illuminance measurements have been carried out by simulating two different lighting conditions:
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The luxmeter and the luminance meter are fixed in the same position during both conditions.
The difference between the two conditions is evident: while the illuminance varies minimally (within 30 lux), the luminance ratios increase hundreds of times within the occupational visual field. The subject in condition (b) has parasitic stray light of many thousands of cd/m2, interfering with the relatively low luminances of the occupational targets (screen, 69 cd/m2; keyboard, 40; document, 32).
Ideally, light on the fovea should be measured, but if there is a choice to be made between a luxmeter and a luminance meter, it would be more logical to use the latter because it more closely matches the structure and function of the human eye.
Illuminance measured with a luxmeter is still valid if the purpose is to gauge the effectiveness of the lighting system, its temporal variation and to obtain an overall measure of light. However, the luminance meter is more appropriate if the purpose is to assess the relationship between visual comfort and lighting.
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METHOD FOR ASSESSMENT OF LUMINANCE IN THE OCCUPATIONAL VISUAL FIELD |
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The luminance meter should have the following characteristics: a photocell, spectrally corrected according to the spectral sensitivity function of the human eye; a wide range of measurement (e.g. up to 100 000 cd/m2); and an output that can be directed to a storage oscilloscope or other device for longer-term (hours/days) measurements and for flicker evaluation. Suitable luminance meters are available from many commercial suppliers of photometric instruments (e.g. Hagner Instruments, Solna, Sweden). During measurements, the photometer is placed on a tripod, equipped with a graduated pivoting head.
The method comprises four successive phases, and is applicable for ‘near work’.
Phase 1 (operator task analysis)
The identification of objects/images (occupational targets) that must be regularly observed in order to perform the task. These objects might typically be a computer screen or a book.
Phase 2 (determine the occupational visual field)
This can be described as follows, with reference to Fig. 5.
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- (a) Identify the ‘occupational fixation axis’ (o.f.a.), represented by that theoretical segment which extends from the centre of the macular region to the centre of each occupational target.
- (b) Identify the ‘occupational fixation zones’ (o.f.z.) by determining a 40° angle space around each o.f.a. Note that the real external projections of the macular region are represented by a 18.4° angle (Tychsen, 1992), but due to the fact that the occupational targets are rarely single points in space, it is appropriate to widen the angle to include all light actually reaching the macular region. The figure of 40° is based on experience (Piccoli et al., 1988, 1995), and approximates the theoretical value of 36.8° (i.e. 2 x 18.4). This zone can easily be circumscribed by (i) centering the occupational target inside the circle (1°) situated in the middle of the viewfinder (Fig. 6), and then (ii) rotating the pivoting head by the appropriate amount. The use of self-adhesive markers could be helpful.
- (c) Outline the space that encloses the entire occupational fixation zones involved, which then becomes the occupational visual field (Fig. 5). This field roughly approximates a cone (Fig. 7) with its vertex at the midpoint of the worker’s eyes (nasion in medical terminology) and with an irregular base.
- (b) Identify the ‘occupational fixation zones’ (o.f.z.) by determining a 40° angle space around each o.f.a. Note that the real external projections of the macular region are represented by a 18.4° angle (Tychsen, 1992), but due to the fact that the occupational targets are rarely single points in space, it is appropriate to widen the angle to include all light actually reaching the macular region. The figure of 40° is based on experience (Piccoli et al., 1988, 1995), and approximates the theoretical value of 36.8° (i.e. 2 x 18.4). This zone can easily be circumscribed by (i) centering the occupational target inside the circle (1°) situated in the middle of the viewfinder (Fig. 6), and then (ii) rotating the pivoting head by the appropriate amount. The use of self-adhesive markers could be helpful.
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Phase 3 (measure luminance and construct isoluminance maps)
Photometric measurements should be taken, but those in the occupational visual field must be particularly detailed in order to pinpoint all significant (tens of times higher than the average of the ‘occupational targets’) sources of primary or secondary luminance. Measurements can also be carried out within the somewhat broader ‘kinetic’ visual field, whose borders are, in each eye, 100° temporally (i.e. towards the relevant temple), 60° nasally, 60° superiorly and 75° inferiorly (Harrington and Michael, 1980; Michaels, 1980; Rosenberg, 1980; Anderson, 1987), when strong sources (hundreds of times higher than the average of the ‘occupational targets’) of luminance exist outside of the occupational visual field. Considering that the fields of view of the two eyes overlap in the central portion, the bi-ocular kinetic visual field is 180° (Fig. 8).
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A digitized photograph of the work area allows ‘false colour’ (brightness-banded) representation, which helps to show up the distribution of luminance, as in Fig. 9. Commercial image-processing software can create brightness-banded (posterized) images from common digital pictures. In our experience, luminances of 50–500 cd/m2 in the occupational visual field, and of several thousand in the kinetic visual field, represent a quite common working condition in offices and commercial industry.
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The illustrated procedure can be carried out thoroughly and analytically, e.g. for research purposes, legal court cases, peculiar work situations, elaboration of standards and new workstation design. Alternatively, a simplified procedure may be chosen if the investigation is introductory or generalized. In this case, spot measurements can be performed from the operator’s position, over a 120° angle (apex at the nasion) to assess or quantify sources of luminance believed (intuitively) to be the cause of possible discomfort. This simplified procedure, which may typically entail the use of a Polaroid camera, is mainly applicable for tasks requiring fixed and close vision, as in Table 1, and for which most of the occupational targets are placed within an area of
2 m2.
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Phase 4 (compare luminance ratios)
Once the luminance data have been gathered as a result of phases 1–3, phase 4 is the evaluation, which consists of identifying high luminance (contrast) ratios.
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DISCUSSION |
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International standards (ISO, 1989; CIE, 1995) make reference to luminance ratios but do not provide a rational basis, and do not explicitly refer to the occupational visual field. It is our experience that luminance ratios in excess of several hundred to a thousand are not uncommon and are very likely to contribute to visual disturbance, also termed occupational asthenopia (Piccoli et al., 1993, 1995). Grandjean (1987) has suggested maximum luminance ratios of 3:1 in the ‘middle of the visual field’, 10:1 for the ‘marginal areas of the visual field’ and 40:1 as ‘maximum brightness contrast within the entire room’. These proposals, which consider ‘the distribution of luminances in the visual environment of crucial importance for both visual comfort and visibility’, suffer from two major inadequacies. Firstly, they do not provide clear anatomical and physiological definition of what is meant by ‘middle of visual field’ and ‘marginal areas of visual field’, which are essential requisites for the application of these criteria. Secondly, a luminance ratio of 3:1 appears to be unnecessarily restrictive. Indeed, luminance ratios exceeding 10:1 are common for ‘near’ tasks in office and industrial environments, both within the occupational visual field and the kinetic visual field. The ratios may arise because of the presence of small objects or surfaces with high reflectance in the working plane or on walls. For example, a bright knob or a shiny watch or simply a white sheet under a table lamp may result in luminance ratios exceeding Grandjean’s criteria. In addition, there is no scientific evidence that such low luminance contrast ratios are responsible for visual disturbances or inefficiency.
Indeed, lighting-related visual disturbance (asthenopia) is a phenomenon that needs to be better studied in terms of prevalence, incidence, severity, work relatedness, etc. Subjective questionnaires, particularly self-administered questionnaires, which have been the main tools used to study this issue, are considered scientifically inadequate (Festinger and Katz, 1963; Osgood, 1967; Blalock, 1968; Argyle, 1972).
Thus, we argue that risk assessment of near work, which may largely entail lighting-related visual disturbances, should include an evaluation of luminance contrast ratios.
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CONCLUSIONS |
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The proposed method is not intended for design purposes, but rather it supplements conventional illuminance measurements and allows for more thorough assessment of lighting conditions, especially in the context of occupational visual performance and asthenopia.
The method represents a more robust tool in occupational epidemiological studies which seek to elucidate the relationship between the indoor lighting and visual wellbeing. Traditionally, such studies have suffered from the lack of an objective photometric method that takes account of the anatomy and physiology of the human visual system.
Furthermore, there has not been a consensus between the various disciplines, in respect of aims, terminology and procedure, and this has inhibited co-operation in developing a shared method.
In the absence of consensus on a valid, objective method, the uncritical use of illuminance as a ‘comfort parameter’ may lead to inappropriate interventions and ineffective health surveillance programmes (Reading and Weale, 1986).
It could be argued that occupational health and safety professionals recognize the inherent limitations of the current approach and adopt pragmatic strategies, e.g. making a judgement based on their own personal experience and capacity to interpret workers’ complaints.
Further work needs to be done on the development of the method, but we believe it will assist in a better understanding of lighting-related visual disturbances and possibly provide a basis for the development of more rational standards. The application of the method in other office scenarios and in commercial, workshop and industrial situations will be illustrated in subsequent publications.
Supplementary data—Colour versions of the figures are available as supplementary data with the on-line version of this paper.
Acknowledgements—We thank Dr Daniele Grosso for advice on data acquisition and with the literature review. We are also indebted to Mr Alessandro Spallanzani and Mr Guido Lanfrit for assistance with the bibliography.
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FOOTNOTES |
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* Author to whom correspondence should be addressed.
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