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Answers .Task . .Lighting . for . Offices. Volume 1 Number 3 April 1994 Introduction How much light is needed in an office? In offices, most efforts to reduce lighting The Illuminating Engineering Society of energy costs have focused on maximizing the North America (IESNA) has established a efficiency of the general (or ambient) lighting procedure for determining how much illumi- system. The common approaches include nance is needed for a given task. The proce- installing energy-efficient lamps, electronic dure takes into account several factors, ballasts, specular reflectors, and occupancy including the type of activity, the characteris- sensors. Relatively little attention has been tics of the visual task, the age of the person paid to the potential for energy savings that performing the task, how critical speed and the effective use of task lighting offers. Task accuracy are, and the reflectance of the lighting can provide illumination where it is task’s background. most needed—on the work surface—more The procedure categorizes tasks by how economically than the most energy-efficient difficult they are to see. Small print tasks and ceiling luminaire (light fixture), simply be- low-contrast tasks fall into higher illuminance cause task lighting is located closer to the categories. Office tasks such as reading tele- work surface. When retrofitting the lighting phone books, maps, or handwritten notes in in an office, the general illuminance* can be No. 3 pencil fall into category “E,” where three reduced if task lighting is implemented prop- desk-height illuminances are suggested: 50, erly into the overall design. The result can be 75, or 100 footcandles (abbreviated fc; metric significant energy savings and improved equivalents 500, 750, or 1000 lux). In most visibility for workers. However, used improp- cases, the middle value is used. The lower erly, task lighting can result in wasted energy value should be used when at least two of the and poorer visibility. following are true: the office occupants are all Although for many people the term task under 40; speed and accuracy are not impor- lighting conjures up the image of a portable tant; or the reflectance of the background is desk lamp, products that are mounted under high (white paper, for example). The higher shelves or cabinets attached to office parti- value should be used if at least two of the tions are the most common type of task following are true: the office occupants are lighting used in offices today. One furniture over 55; speed and accuracy are critical (in manufacturer reports that approximately one- accounting, for example); or the reflectance third of all American office workers occupy of the task background is low (dark grey partitioned workstations, and under-shelf paper, for example). products account for approximately 90 per- Most office workers perform a variety of cent of all office task lighting sold. This issue visual tasks. The illuminance should be suffi- of Lighting Answers examines under-shelf and cient to facilitate the most difficult visual task portable task lighting and explains how to use performed. Thus, if workers spend part of the task lighting effectively to save energy and day reading category “D” materials such as improve visibility. typed originals or first-generation photo- copies, which would require only 20, 30 or 50 fc (200, 300 or 500 lux), and part of the day reading poor fax copies, category “E” illumi- nance should be provided. * All terms in italics are defined in the glossary on p. 7. 1 In a typical office, the general lighting often is Both reflected glare and veiling reflections designed to provide an illuminance of 75 fc (750 can be minimized by locating luminaires away lux) at desk height, the middle value from from the offending zone (the geometry of the category “E.” In partitioned workspaces, it is offending zone is illustrated in Figure 1). The difficult for general lighting to deliver this illumi- offending zone for a worker looking at materi- nance to the desk surfaces, because shadows als on a desk is immediately in front of the from shelves, cabinets, partitions, and the work- worker. Nevertheless, this often is where task er’s own body can block light from the ceiling- lights are located. To reduce reflected glare mounted luminaires. and veiling reflections, the light that exits a luminaire in the offending zone should be masked or redirected. The best solution is Can task lighting provide the necessary illumi- lighting that is directed toward the task from nance more efficiently than general lighting? either or both sides. Direct glare occurs when a bright light Yes. Rather than providing a high general source is within the field of view of an ob- illuminance all the time, task lighting can be server. A task light can be a source of direct used to provide the additional light where it is glare if its lamp or lens is visible from the needed on the desk, and only when the diffi- normal viewing position of either the worksta- cult task is being performed. Task lighting tion occupant or workers nearby. Mounting a provides illuminance more efficiently than task light above the eye height of the worker general lighting because it is closer to the can cause direct glare. A good task lighting work surface. system is shielded from direct view by opaque For example, in an office that requires 75 material, either built into the task light itself or fc (750 lux), a fluorescent under-shelf task as part of the shelf to which it is mounted. light can provide 40 fc (400 lux), which allows Figure 2 shows examples of both a properly the general lighting to be reduced to 35 fc (350 shielded task light and a task light mounted in lux). Even greater energy savings can be a position that causes direct glare. Reflected achieved if the task lights are switched off glare, veiling reflections, and direct glare can when not in use. hamper visibility and worker productivity. Task lighting can increase the energy use Complaints of eyestrain and headaches some- in an office if the general illuminance is not times are attributed to these problems. reduced when task lighting is added. Thus, task lighting should be carefully selected and implemented as an integral part of an office lighting system for either retrofit or new construction projects. How can visibility problems with task Figure 1 lighting be avoided? How a task light can create reflected glare and veiling reflections The image of a bright Proper illuminance is achieved easily with task lamp under the shelf can lighting. However, there are other visibility be reflected from a specu- problems that can result from using task light- lar task or desktop directly ing, including reflected glare, veiling reflections, into the worker’s eyes. Light direct glare, and extreme differences in bright- that strikes a task at angles ness in a workstation. that reflect directly into a worker’s eyes (such as Reflected glare is the reflected image of a θθ bright light source from a specular (shiny) angle θ) can create a veiling surface, such as a polished desktop or a glossy reflection that makes the magazine. Veiling reflections occur when re- print difficult to read. Optics built into the task light can flected light obscures the task by reducing its minimize reflected glare contrast. For example, light from a bright lumi- and veiling reflections by naire reflected from thermal fax paper can redirecting the light so that make the whole page (both the print and the it is not cast forward toward background) so bright that it is difficult to read. the worker at these angles. 2 Extreme variations in brightness at a work- 2. Luminaires that mask light. This cate- station, or strong patterns of light on the work gory includes luminaires that use a fixed surface, also can be a source of eyestrain or opaque shield to block light at offending distraction. An ideal task lighting system pro- angles (see Figure 2a) and products that can duces relatively uniform illuminance on the be adjusted to partially block the light in any work surfaces. As a rough guideline, the high- direction. An example of an adjustable prod- est illuminance on the work surface should be uct is a striplight with a rotatable plastic no greater than three times the lowest. sleeve on the fluorescent tube. A gradient pattern of black dots on the clear sleeve is used to mask the light. Because the effective- What types of under-shelf luminaires ness of this product depends on proper ad- are available? justment of the rotating sleeve, workers should be shown how to use it appropriately. Under-shelf luminaires usually are purchased as accessories to office furniture. They fre- 3. Luminaires with a small light source quently are supplied by the office furniture using asymmetrical optical control. manufacturer, but also are available from These luminaires use a compact fluorescent lighting equipment manufacturers. These lamp and an asymmetrically shaped reflector products can be divided into the following five that directs most of the light in one direction. categories, which are arranged roughly from They usually are located in pairs above the worst to best in terms of producing uniform principal work surface, one at each end of the illuminance and avoiding reflected glare and shelf. If the worker is positioned halfway veiling reflections. Most under-shelf lumi- between the two luminaires, veiling reflec- naires use fluorescent lamps. The luminaires tions are minimized because the light is in categories 1, 2, 4, and 5 use linear fluores- aimed toward the task from the sides. If only cent lamps, typically a 3- or 4-foot T12 or T8 one is used, it can create strong shadows on lamp.

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