Workplace Illumination – Part 15: Office Lighting

            The importance of customizing lighting for individuals, tasks, and spaces has been conveyed in preceding installments of this series.  That theme continues here, where the unique characteristics of office spaces and their impact on lighting design decisions are explored.
            In most offices, the range of activities, or tasks, is relatively narrow, aiding specification of appropriate lighting parameters.  However, other factors, such as the type of displays in use, the proportion of time spent on each task type, visual capabilities of users, and psychological influences of lighting complicate system design.
            Competing task requirements and varying needs of occupants creates tradeoffs that must be considered.  Where balance cannot be achieved, priorities must be established.  Understanding the characteristics of office spaces and tasks is paramount if one hopes to design a lighting system that suitably balances its objectives.

The “Office”
            The first step in designing effective office lighting is to understand what constitutes “office space.”  The definition used here is more inclusive than is typical to maximize the benefit to be gained from the discussion.  For purposes of this presentation, a space accurately described by any of the following three statements is treated as an office:

1. The primary task performed requires viewing a self-luminous display (SLD) (see below for more information on SLDs).  Examples include computer-aided design and drafting (CADD) and air-traffic control.
2. A significant portion of task performance relies on viewing an SLD, while the remainder requires switching between the SLD and other visual tasks, such as reading papers on a desktop.  Examples include engineering and production management.
3. Task performance requires frequent or critical interaction with readouts, gauges, and control mechanisms, including digital LCD displays, analog gauges, switches, knobs, and levers.  Examples include various types of control centers, such as those in power-generation facilities or aircraft cockpits.

            A self-luminous display (SLD) is one in which the presentation of information also provides the light required for its visibility.  Most often, this refers to computer monitors and similar screens, alternatively called visual display units (VDU), video display terminals (VDT), or similar term.  For purposes of this presentation, backlit displays and lighted gauges are also considered SLDs to ensure they receive proper attention.
            Information can be displayed with negative or positive polarity.  Negative polarity describes the display of bright text on a dark background.  Early “monochrome” computer monitors and digital alarm clocks (i.e. red numbers) are examples of negative polarity displays.  Positive polarity describes the display of dark text on a light background.  LCD screens common on calculators and digital watches are examples of positive polarity displays.  Though polarity has been defined using its common application, text, this definition should also be expanded to include the appearance of an indicator needle relative to a gauge face, graphical displays, and other non-text information.  Exhibit 1 provides examples of graphical displays in positive and negative polarity.

Effects of Office Lighting
Non-Visual Effects
            Much of the discussion of workplace lighting is centered on its stimulation of the visual system.  However, its effect on the non-visual system must also be considered to ensure effective lighting.  The nature of “office work” can make the non-visual impacts more relevant than in other types of activities.  The non-visual system, introduced in Part 4, is revisited in the context of office lighting before returning to visual system-relevant topics.
            One goal of office lighting is to generate positive affect in the space’s occupants.  A generally-favorable opinion of a space improves mood, collaboration, and overall morale with a subsequent improvement in productivity.  A popular method of increasing positive affect is to incorporate daylight and a view of the outdoors in the office plan.
            Studies and surveys have shown that the retained connection to the outside world provided by windows is highly desirable.  It can reduce the occurrence and severity of seasonal affective disorder (SAD) and can mitigate feelings of isolation that sometimes occur in “cube farms” or similarly partitioned workplaces.
Windows can also provide a physical benefit in the form of eyestrain relief.  The opportunity to focus on a distant object allows eye muscles to relax, providing a momentary reprieve from close-up tasks and intense concentration typical of a workday.
            It should also be noted that deviation from recommended lighting parameters, e.g. illuminance and luminance contrast, are typically deemed less objectionable or more acceptable when caused by daylighting than when the result of an electric lighting mishap.  Occupants of these spaces value daylight so highly that they are willing to overlook lighting conditions that would otherwise be unacceptable.  This is a testament to daylighting’s contribution to positive affect.
            The spectral content of light has a significant impact on occupants’ perception of a space.  Curves representing subjective assessments of a space’s illuminance at various lighting color temperatures are plotted in Exhibit 2.  In general, high-color temperature lighting at low illuminances is considered cold and drab, while low-color temperature lighting at high illuminances is deemed unnatural and “overly colorful.”  The area between the curves represents preferred combinations of color temperature and illuminance.  Not surprisingly, the “preferred” area expands rapidly in the region of high color temperature and high illuminance, a combination commonly associated with daylight.

            The scales of color temperature and illuminance in Exhibit 2 seem to suggest well-defined relationships between these parameters and occupant assessments.  However, these curves do not fully account for several variables, including light distribution characteristics, color gamut, task requirements, age and visual capabilities of occupants, and state of visual system adaptation.  Thus, the value of the plot lies in its conceptual framework, described above, rather than the values indicated on its axes.
            In addition to the aesthetic impact of light’s spectral content, it can also influence observers’ circadian timing (see Part 4).  High illuminance, short-wavelength light contributes to melatonin suppression, enhancing alertness.  Given that many office environments require long periods of sedentary activity that may also be repetitive, this is an important aspect of office lighting to consider.  Achieving balance among lighting objectives, including both subjective and objective measures, is the key to maximizing productivity and satisfaction in an office space.

Effect of Lighting on Displays
            Light incident on an SLD screen can be detrimental to visual performance in three ways.  First, excess illumination reduces screen contrast and, therefore, visibility of the information displayed.  Depending on the task, eyestrain, increased error rates, and reduced processing speeds may result.
            Second, images reflected in a screen can distract the user from the required task.  Attention is naturally drawn to the brightest object or area in the visual field, requiring frequent refocusing or intense concentration to stay on task.
            Third, the different distances at which the display and reflected images exist cause adjustments in accommodation.  This can cause the task to be out of focus for significant amounts of time, again increasing processing times and error rates.
            In the case of an analog gauge, there is another mode of error introduction.  An observer changing his/her viewing position to eliminate veiling reflections may obtain erroneous readings due to induced parallax.
            The extent to which incident light creates these detrimental effects is strongly influenced by the type of SLD screen in use.  Screens are divided into three categories, as shown in Exhibit 3.  Panel (a) depicts a matte-finish screen (Type I) characterized by diffuse reflection of incident light that minimizes reflected glare.  Type I screens are the most accommodating of various lighting conditions.
            Type II screens have a semi-specular finish, as depicted in panel (b).  This reduces glare by partially diffusing reflected light, but some specular reflection still occurs.  For best results in most lighting conditions, positive polarity is recommended.
            As depicted in panel (c), Type III screens are most susceptible to veiling reflections due to their specular finish.  This is particularly true when used with negative polarity or in high illuminance.  For best results, use in low to moderate illuminances with positive polarity.

Lighting Recommendations
            While those cited in previous installments of this series remain valid and generally applicable, recommendations customized for office lighting applications have also been developed.  Several important design factors are identified in the image in Exhibit 4, including lighting and control strategies.

            To minimize visual system adjustments required when switching between tasks, luminance ratios of the SLD, other (i.e. paper) tasks, and other surfaces within and surrounding a workstation must be controlled.  The recommended maximum luminance ratios for important surfaces in an SLD-centric workspace are shown in Exhibit 5.  These ratios are similar to those given in Part 12 Exhibit 12 for Environmental Classification A (i.e. controlled reflectances).  Recommended reflectances of surfaces throughout office spaces are shown in Exhibit 6.  The key difference between these recommendations and those given in Part 12 Exhibit 11 is the higher ceiling reflectance recommended in office spaces.

            To minimize reflected glare in SLD screens, indirect lighting is highly desirable.  A high-reflectance ceiling increases the efficacy of indirect luminaires by distributing light in the room while preventing direct glare.  To prevent the ceiling from becoming a source of glare or distraction, its luminance should be maintained below 850 cd/m^2 with a uniformity ratio no greater than 8:1.  Uniformity less than 4:1 is preferred; if negative polarity screens are used, 2:1 is the recommended maximum.
            If direct luminaires are used, the installation must be designed carefully to minimize direct and reflected glare.  Direct luminaires intended for office spaces limit the luminance above a defined critical angle, as shown in Exhibit 7.  A critical angle of 65° is often used; for applications known to be susceptible to lighting issues (e.g. reflective screens), a 55° critical angle may be substituted.  When SLD use is only incidental, luminaires with a 75° critical angle may be installed to achieve other lighting objectives.

            The table in Exhibit 8 provides light distribution guidelines for direct luminaires used in office spaces.  As can be seen in Exhibits 7 and 8, recommendations vary by source; it is imperative that requirements for a specific site be verified before a lighting system design is finalized.
            If both direct and indirect lighting is used, guidelines for each must be followed simultaneously.  A highly-effective lighting system can result from the combination, but an iterative design process may be required to achieve desired results.
            An excerpt of the IESNA table of recommended illuminance targets for offices is shown in Exhibit 9.  In the table, office tasks are broken down in more detail, with lighting recommendations defined for each.  This provides additional information to facilitate selection of a lighting strategy and design of a corresponding lighting system.  Additional explanations of the table’s contents are provided in the footnotes shown in Exhibit 10.