Workplace Illumination – Part 14: Task Lighting

            A local lighting strategy relies on area lighting (see Part 12) to provide minimum levels of illumination required to maintain safety and task lighting to provide the higher illuminances required for satisfactory performance of demanding visual tasks.  This strategy provides great flexibility in lighting small areas, allowing customization of the visual environment to suit the task and individual performing it.
            Customization of a task-specific area presents its own challenges, in part due to the proximity of luminaires to individuals performing the task.  Maintaining visual performance requires ensuring that sufficient light is provided where it is needed without sacrificing visual comfort.  Balancing these competing objectives requires another level of analysis and planning, which is the subject of this installment of the “Workplace Illumination” series.

Visual Task Characteristics
            Visual performance is influenced by several factors.  In general, these factors can be controlled through job design; however, the nature of a task may limit control of one more factors.  Where control of any factor is limited, the creation of compensatory conditions should be pursued.  Likewise, compensation for any visual deficiency that may occur in those performing a task should also be pursued to the extent possible within the limits of control.
            The luminance, or brightness, of a visual task target is a critical factor; if it is insufficient to activate the eye’s photoreceptors, the target is imperceptible.  There also must be sufficient contrast between the target and its background for an observer to differentiate them.  Differentiability is usually achieved via luminance contrast; recommended maximum luminance ratios are given in Part 12 Exhibit 12.  Very low luminance contrast increases the utility of chromatic contrast (see Part 6) and it may become critical to visual performance.
            A target’s size, relative to the visual field, is another key visual performance factor.  The greater acuity required to accurately interpret a visual scene (see Part 6), the longer it takes an observer to reach a conclusion about the correct interpretation.  Thus, the duration of a target’s presentation (i.e. time) is another critical factor.
            Other factors also impact visual performance.  Rapid motion of a target may make it difficult to discern details.  If motion itself is the visual stimulus, rapidity may not be detrimental and may even reduce response time and increase accuracy.
            While objective measures of a target’s visibility (e.g. size, luminance contrast) may predict high performance on a visual task, visually “busy” or otherwise distracting surroundings can negatively impact task performance.  Conversely, monotony can cause vigilance to falter, increasing response times and error rates.  Consistency in presentation is helpful to the extent it ensures recognition of a target; beyond that, it provides diminishing returns.  If presentations are too consistent, an observer may begin to respond to a learned temporal pattern, rather than the visual stimuli presented.

Visual Comfort
            The objective of task lighting is to sustain high levels of visual performance in a small area or workstation.  Key to meeting this objective is maintaining visual comfort or, alternatively, avoiding visual discomfort.  There are several potential causes of visual discomfort; the more that exist simultaneously, the greater the compound effect.
            Glare is a common cause of visual discomfort; the greater its severity, the more detrimental it is likely to be to visual performance and physical well-being.  Light sources must be carefully placed relative to observers to minimize glare.  Light directed onto a workplane over the shoulder of an observer prevents direct glare; however, if the observer is not stationary, the movement could create shadows on the task.  Variation in positioning and/or physical stature of multiple users of a workstation could cause similar issues.
            Light sources forward of an observer should be placed outside the “forbidden zone” depicted in Exhibit 1.  The lower position depicted represents task lighting; the upper position can represent task or area lighting.  Task lighting is best placed below eye level or above 45° from the horizontal.  Area lighting should also be placed above the 45° line, unless at sufficient distance or sufficiently shielded to prevent glare at the observer’s position.

            Beyond this basic guideline, other features of the task and workstation should also be considered.  The layout of tools, materials, displays, etc. on the workplane may impose limitations on luminaire placement; for example, two light sources may be needed when only one would normally be used.  Luminaire placement may also be dictated by a need to highlight specific physical features or potential defects of a component or assembly.
            The need to highlight a part feature is an example of advantageous application of nonuniform illumination.  Without careful planning and execution, however, inconsistency or nonuniformity in lighting conditions can lead to discomfort and reduced performance.  Both spatial and temporal inconsistencies can be detrimental to response times and error rates.
            Temporal nonuniformity can distort a target’s appearance and cause eyestrain.  Potential issues associated with flicker are presented in Part 8, including the stroboscopic effect.  Exceeding the critical flicker frequency (CFF) at all times is critical to task lighting effectiveness.
            Spatial nonuniformity can lead to perceptual confusion (see Part 9), or misinterpretation of a visual scene.  The potential for this to occur can be assessed by the uniformity ratio and the diversity ratio, as follows:

• Uniformity ratio = minimum illuminance/mean illuminance.
• Diversity ratio = maximum illuminance/minimum illuminance.

In general, lighting is to be designed such that these ratios are as near to 1.0 as is practicable.  However, the practical limits of each ratio will vary with the tasks performed in a space, the individuals performing those tasks, and the size of the space evaluated.  The ratios may provide the greatest utility when comparing potential lighting strategies in a planned workspace, rather than as absolute measures.
 
Visual Task Classification
            Planning for task lighting can be facilitated by categorizing visual tasks according to physical properties of the targets and their backgrounds.  IESNA has published, and ANSI has adopted, recommendations for various combinations of target and background attributes.  Recommendations for flat surfaces of varying transmittance are given in Exhibit 2.  Recommendations for lighting three-dimensional (3D) objects are given in Exhibit 3, again arranged by level of material transparency.
            In the “luminaire location” columns of Exhibits 2 and 3, there are several references to “Fig. 19-15,” shown here as Exhibit 4.  In it, examples of luminaire types and installation positions relative to workplane and observer are shown for various task classifications.

            Common physical structures of luminaire types referenced in Exhibits 2 and 3 are shown at the top of Exhibit 4.  A brief description of each type and its key characteristics follows:

• Type S-I (Directional):  concentrating luminaires using collimating lenses or reflectors.
• Type S-II (Spread, High-Luminance):  small-area light sources with incandescent or HID lamps.
• Type S-III (Spread, Moderate-Luminance):  fluorescent luminaires with luminance variance across the light-emitting surface greater than 2:1.
• Type S-IV (Uniform-Luminance):  luminaires with variance across the light-emitting surface less than 2:1, often achieved via a diffuser.
• Type S-V (Uniform-Luminance with Pattern):  Type S-IV luminaires, sans diffuser, configured to project precise, high-contrast lines on the workplane.

            No set of tabulated recommendations can perfectly address every potential lighting scenario; they are best used as the first iteration in the design process.  Though imperfect, the recommendations are well-considered and can significantly reduce the effort required to plan a work area’s task lighting.  Evaluating deviations from recommendations can often be completed more rapidly than a clean-sheet design process.

Lighting Techniques
            Positioning luminaires above or forward of an observer requires consideration of viewing angles and reflectances of all items in the workspace; this challenge is depicted in Exhibit 5.  Lighting a workspace with a single luminaire may require a compromise mounting location, leading to veiling reflections and visual discomfort.  Often, it is advantageous to use multiple luminaires, each of lower luminance, to achieve the total illuminance required on a workplane.  This approach allows strategic placement of luminaires to achieve proper object modelling, maximizing visual comfort and performance.

            An alternative light source placement position exists below the work surface.  Backlighting a workplane, as depicted in Exhibit 6, can provide high contrast at relatively-low luminance.  The combination of high luminance contrast and minimum visual discomfort maximizes visual performance.  Backlighting is particularly helpful when working with small parts for which adequate modelling is difficult to achieve by other means.  Vertical surfaces can also be backlit to highlight features of larger objects.
            Color can be used to enhance visual performance; if neglected, poor color characteristics of a task environment can be detrimental to performance and comfort.  Contrasting colors can be used to maintain high visibility of tools, to quickly locate needed materials in storage bins, and similar process-aiding functions; see Part 10 Exhibit 1 for high-contrast color-pairing recommendations.  Like most things, color should be used in moderation; too many colors or color transitions, or too vibrant an environment can cause distraction or perceptual confusion.
            When the colors of materials cannot be modified, such as components of an assembly, modifying the spectral content of workspace illumination may be a feasible alternative.  To accentuate a color, a light source with output biased toward its wavelength can be used.  Alternatively, viewing objects through a filtering medium can achieve a similar effect without changing the overall quality of lighting.  Review the “Spectral Distributions” section of Part 7 for more information on how manipulation of light source power distribution, surface reflectances, and media transmittances can create visual stimuli that enhance task performance.  Though it does not provide a precise definition of a light source’s spectral power distribution (SPD), its correlated color temperature (CCT) may be a useful metric in early iterations of lighting system design.
            The use of color discussed thus far is for the purpose of maximizing visibility or differentiability of objects.  However, an object’s color, itself, may be the most important characteristic of the visual task.  In this case, the color rendering index (CRI) may be a useful metric.  However, the more critical is color identification, the more detail is needed to make appropriate lighting decisions.  For example, the size and shape of the color gamut (see Part 7) provides much more information about the appearance of colors under a considered light source.

The Design Guide Revisited
            The IESNA Lighting Design Guide is introduced in Part 12 (see Exhibits 6 and 7) as a helpful reference for planning facility lighting systems.  When local lighting is in use, area lighting is typically employed only for tasks with descriptors such as “coarse” or “simple,” while task lighting is crucial to those described as “fine,” “difficult,” or “exacting.”
            Planning for workspace lighting system design can be significantly accelerated by applying the Design Guide methodology.  Assessments, categories, and recommendations provided in the Guide are valid for any lighting strategy pursued.  A summary of the design process is provided in the “Design for Illuminance” section of Part 12.

Physical Well-Being
            Throughout this installment, task lighting decisions have been discussed with regards to their impact on visual comfort.  However, subsequent effects on physical comfort and well-being should not be overlooked.  The potential for visual discomfort to cause eyestrain and other physiological detriments is discussed in Part 9.
            Avoiding visual discomfort can cause physical discomfort, potentially escalating to aches, strains, and chronic conditions.  Exhibit 7 depicts the potential influence of task lighting on a person’s seated posture.  Upright posture, encouraged or facilitated by proper task illumination, reduces neck and back pain.  Unhealthy posture could also result from efforts to reduce glare and eliminate veiling reflections.  That is, an individual may shift body position to one not ideal for the task to compensate for poor lighting conditions.

            If an evaluation of the workspace does not reveal significant issues with viewing angles, illuminance, or other parameters, it may be appropriate to refer the individual for an eye exam.  It is often surprising how much our vision deteriorates before we realize it is happening.  The negative impacts of attempts to compensate for visual deficiencies provide warning signs that should be heeded.  Restoring an individual’s visual capabilities provides immeasurable benefits to both the individual and those around him/her.


            For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

            For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).

References
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[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] Lighting Engineering:  Applied calculations.  R. H. Simons and A. R. Bean.  Butterworth-Heinemann; 2001.
[Link] “Lighting.”  Hugh King in Plant Engineer’s Reference Book, 2ed (Chapter 25).  Dennis A. Snow (ed).  Butterworth-Heinemann; 2002.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Handbook of Human Factors and Ergonomics, 4ed.  Gavriel Salvendy (ed).  John Wiley and Sons; 2012.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.
[Link] “ANSI/IES RP-7-21 Recommended Practice: Lighting Industrial Facilities.”  ANSI.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
[email protected]