The traditional focus of office lighting design has been to ensure the visibility of paper-based tasks carried out on horizontal desktops. However, there are many other issues involved in good lightin...
The traditional focus of office lighting design has been to ensure the visibility of paper-based tasks carried out on horizontal desktops. However, there are many other issues involved in good lighting design for today’s office, where most work is performed on computer screens.
These issues have been addressed by the Illuminating Engineering Society of North America (IESNA) in its Lighting Design Guide. The guide is included in both the IESNA’s Lighting Handbook 9th Edition (New York, NY: 2000) and its American National Standard Practice for Office Lighting (New York, NY: 2004)
The National Research Council of Canada’s Institute for Research in Construction has conducted several projects specifically addressing lighting for partitioned, open-plan offices with cubicles. IRC’s research supplements or modifies the information provided by the Illuminating Society of North America. From its research, IRC has developed recommendations for enhancing lighting quality in open-plan offices.
Computers, workstations and iIlluminance
Until the advent of the IESNA Lighting Design Guide, horizontal illuminance on the desk surface was the primary numerical design criterion for office lighting.
With the shift in office work from paper to computers, the recommended horizontal illuminance level has been reduced. The IESNA recommends 300 lux if computer use is intensive and 500 lux if it is intermittent. By comparison, in studies in an office set-up where occupants used dimmers to select their preferred lighting conditions, IRC found that average chosen illuminances on the desk surface were in the range of 400-500 lux, even for those who are intensive computer users.
Workstation design can have a large effect on office lighting. Small workstations, high panels, and dark-coloured panels all reduce the illumination reaching the desk. Increasing the panel height from 1 m to 1.7 m (42″ to 66″) can decrease illuminance on the desk surface by about 20%. The effect varies according to the type of ambient lighting fixture, or luminaire, used. Luminaires with a parabolic louvre direct light straight down to the desk and are therefore affected least by panels. Indirect or direct/indirect luminaires rely on reflections from the ceiling and other surfaces for light to reach the desk and are therefore affected most by panels. Luminaires with prismatic lenses are somewhere in between. Reducing the reflectance of workstations from 50% to 20%, or decreasing the workstation area size from 3.05m x 3.05m to 2.4m x 2.4m (10′ x 10′ to 8′ x 8′) can further decrease illuminance by about 10%.
An office worker’s field of view usually includes vertical surfaces behind the computer screen, such as panels and walls. The IESNA recommends 50 lux for vertical surface illuminance. However, IRC research suggests that workers prefer higher levels, averaging around 200-300 lux on surfaces of mid-reflectance.
The average illuminances presented above do not take into account the wide range of individual preferences. In one experiment in an open-plan office set-up, 94 participants were given dimming control over separate direct and indirect luminaires, and switching control over an under-shelf task light. The lighting conditions chosen yielded desktop illuminances ranging from below 100 up to 800 lux.
Failure to meet workers’ lighting needs can adversely affect their satisfaction and mood. It is best to provide some form of individual control over lighting — ideally, dimming control over the lighting associated with each workstation. Individual control can also deliver energy savings of 10% to 30%.
With good design, some or all of the prescribed illuminance can be provided by daylight, providing several benefits. With appropriate control systems, lighting energy savings in the range of 25%-60% near the building perimeter are achievable. With daylight and access to a view, occupant satisfaction increases and there is evidence that daylight is associated with health benefits, benefiting proper functioning of circadian rhythms and associated hormone production.
It is luminance, not illuminance, that causes the sensation of brightness. In general, people prefer bright spaces, provided glare is limited. The luminance of major surfaces in an office should be at least 30 cd/m2. Light-coloured surfaces are required to achieve both the recommended luminance and illuminance values. (Luminance is the amount of light emitted by a surface and is measured in candelas per square metre (cd/m2). It is a function of the light falling on a surface — its illuminance — and the surface’s reflective properties.)
Glare is too much luminance in the wrong place and should be eliminated at its source by selecting luminaires that do not emit light at angles likely to create glare directly or through reflection, particularly on computer screens. Appropriate sunshades can eliminate glare from outside.
People generally prefer some variation in luminance across a room. The IESNA recommends that across a person’s field of view the ratio of maximum luminance to minimum luminance should not exceed 10:1. IRC research suggests, however, that ratios up to 20:1 might be acceptable, provided there is a good reason for this — for example, highlighting an area or objects of visual interest. People are also more tolerant of high luminance ratios if they are caused by daylight.
In the vicinity of the occupant’s primary task, lower ratios are preferred. The IESNA recommends that the luminance ratio between a task and immediately adjacent surroundings should not exceed 3:1 (or vice versa); IRC research suggests lower ratios. When participants chose office lighting levels using dimmers, their choices resulted in an average desk to background luminance ratio of 1.5 – 2 : 1, and an average computer screen to background luminance ratio of 0.8 – 1.2 : 1.
This article is adapted from National Research Council of Canada/Institute for Research in Construction (NRC/IRC) Construction Technology Update No. 62 written by G.R. Newsham, J.A. Veitch, C.F. Reinhart and D.M. Sander. The full study, which took a more comprehensive view of issues such as ventilation and acoustics, is reported in Construction Technology Update No. 60. See http://irc.nrc-cnrc.gc.ca.