Workplace Illumination – Part 13: Daylighting

            Daylighting is a form of general lighting used to create aesthetically pleasing spaces that maintain visual performance capabilities while reducing energy consumption.  Effective daylighting requires integration of electric lighting and appropriate control strategies to compensate for the variability of daylight and for use of the space during evening, nighttime, or early-morning hours.
            Daylighting provides several advantages; however, the design of daylit spaces is fraught with challenges.  A number of tradeoffs among design parameters must be considered, including room size and orientation, window size and placement, energy consumption, psychological effects, and more.  This installment of the “Workplace Illumination” series explores design tradeoffs and the evaluation of a space’s potential for effective daylighting.

            Some fundamental terms of daylighting are at risk of confusion with colloquial use of these or similar words.  The meaning of daylight, of course, is central to this discussion.  Daylight is comprised of three components:  sunlight, skylight, and groundlight.
            Sunlight is the optical radiation received directly from the sun without reflection from another body.  Skylight is the optical radiation from the sun received after scattering in the atmosphere.  Groundlight is the optical radiation from the sun received after reflection from terrestrial surfaces, such as terrain, vegetation, and structures.
            The definition of daylight presented here is more-inclusive than may be provided by other sources, in which groundlight is often omitted.  However, the necessity of its consideration in daylighting calculations and the nature of its origin and existence render the inclusion of groundlight as a component of daylight logical and defensible.  Also, optical radiation is used, rather than visible light, to identify the relevant flux received from the sun.  Ultraviolet (UV) and infrared (IR) radiation also require consideration in daylit spaces; thus their inclusion here, serving as a reminder of such, is also logical.
            Moonlight is the optical radiation from the sun received after reflection from the moon.  Moonlight also contributes to groundlight; however, neither is sufficient for practical application to workplace illumination.  The illuminance provided by moonlight is insignificant; the fraction reflected as groundlight is negligible.
            Natural light is comprised of daylight and moonlight.  Because moonlight provides such low illuminance, its existence is often ignored and the terms natural light and daylight used interchangeably.  As colloquial uses of terms go, this one, in this context, is low-risk; moonlight will not be discussed in significant detail.
            A summary of these terms is provided in Exhibit 1.  Typical illuminance and luminance values, demonstrating the significance of groundlight and the insignificance of moonlight, are presented in Exhibit 2.

Daylighting Design Sequence
            The design sequence summarized in Exhibit 3 can be used to generate an initial concept for a daylit structure.  It is based on rules of thumb and approximations, providing a simplified assessment of each space’s feasibility or potential for daylighting.
            Use of this preliminary design sequence is also based on several assumptions, including:

• Overcast sky conditions provide uniform, diffuse light.  This assumption removes the effects of direct sunlight from the analysis; thus, orientation of the structure’s façade, relative to the sunpath, is not considered in the preliminary design phase.  Any direct sunlight that will be incident on a space must be addressed in the final design; some approaches are discussed later, in the context of design tradeoffs and daylighting methods.
• Spaces are sidelit.  Sidelighting is achieved with openings in the vertical walls of a space (i.e. conventional windows) adjacent to the workplane.  In contrast, toplighting, discussed later, is achieved with openings above the workplane, such as roof monitors.
• The approximate layout of furniture and equipment, or the intended primary use of the space is known.  The more detail that is known about the anticipated configuration of the space, the earlier effects on daylighting can be addressed (e.g. tall cubicles or numerous machines).  If no information is available, the preliminary analysis can be based on an empty room; however, this limitation should be clearly noted so that the proper adjustments can be made as information becomes available.
• Reflectances of room surfaces and anticipated contents are known.  An estimate of average room reflectance, based on experience with similar spaces, may suffice for a preliminary design.

Given the uncertainty introduced by these conditions, final design parameters should not be specified until a rigorous analysis or simulation has been conducted.

Step (1) Site conditions and programming needs
            The planned use(s) of a structure, and any number of spaces within it, provides first-stage differentiation of potentially daylit spaces.  “[C]omparable programmatic needs” may include similar contents, layouts, and occupants (e.g. children or elderly) of rooms, location on the same floor of a multistory building, and purpose or activities taking place (e.g. retail, industrial, healthcare, recreational).
            The sky angle (θ) is the angle between any horizontal obstruction of daylight (e.g. neighboring building) and the vertical obstruction created by any overhang of the window (e.g. recess in the façade), measured at the center of the window.  Shown pictorially in Exhibit 4, θ represents the vertical range of visibility of the sky from the window.  In a multistory building, the sky angle increases on each higher floor as the extent of obstruction declines.  If no obstruction is present, θ is measured from the horizontal.

            Daylight factor compares the illuminance of a point in a daylit space to the maximum possible in the given sky condition.  Formally, daylight factor (DF) is the ratio of the illuminance due to daylight at a point within an interior space to that on a horizontal plane with an unobstructed (i.e. hemispherical) view of the sky expressed as a percentage.  It is comprised of three components:

• sky component (SC), related to the sky angle;
• externally-reflected component (ERC), i.e. groundlight; and
• internally-reflected component (IRC), related to room surface reflectances.

DF = SC + ERC + IRC; Exhibit 5 depicts each component contributing to a space’s daylight factor.
            In the preliminary stages, for which this sequence is intended, the average daylight factor is both more useful and easier to estimate.  It is found using the Lynes formula:

where:

• DFavg is the average daylight factor of a rectangular daylit space (%);
  
• Aglazing is the net glazing area of daylighting apertures (i.e. area of windows less mullions and frames) (m^2);
  
• τvis is the visible transmittance of the glazed area (dimensionless);
  
• θ is the sky angle (degrees);
• Atotal is the combined area of all interior surfaces (m^2); and
  
• Rmean is the area-weighted mean reflectance of all interior surfaces, including windows (dimensionless).
  

            A modified Lynes formula can also be used:

While the Illuminating Engineering Society of North America (IESNA) has endorsed the modified Lynes formula, others cite the value of a safety margin provided by the original.  The marginal computation cost is vanishingly low, making it worthwhile to calculate both values; the additional “data point” may be edifying.
            IESNA has also provided a guideline to facilitate selection of daylight factor targets:  a space with DFavg ≥ 5% will appear well-lighted, while one with DFavg < 2% will appear dimly-lighted.  While this guideline provides insight into common perceptions of daylit spaces, it in no way limits the target values that can be pursued.  However, spaces with lower daylight factors, particularly those below 2%, will require more-frequent use of supplementary lighting if visually-demanding tasks are to be performed.

Step (2) Daylight feasibility test
            The glazing transmittance, τvis, is typically 0.80 – 0.85 for clear glass and 0.70 – 0.75 for double glazing.  If additional glazing treatments are anticipated, this value should be adjusted accordingly.  An allowance can also be made to represent the real-life performance of dirty windows.
            The feasibility of designing effective daylighting in a space is assessed by its window-to-wall ratio (WWR), the proportion of the wall area that must be glazed to achieve the desired daylight factor.  A space’s minimum window-to-wall ratio for effective daylighting is estimated as

where DFavg is the target average daylight factor in the space; τvis and θ are as previously defined.  Spaces with WWR > 80% are not considered good candidates for daylighting; adjustments should be considered to increase a space’s daylighting potential.  For example, selecting glazings with higher transmittance can reduce the area required; a refined local lighting plan could lower the target daylight factor.
            For any space that fails to pass the feasibility test, project stakeholders must choose a path on which to proceed.  Options include:

• Continue to seek adjustments, as described above, to specify attainable objectives.
• Accept a suboptimal daylighting design and adapt the electric lighting plan to compensate.
• Abandon daylighting as a significant component of the structure’s lighting strategy.

Spaces with borderline feasibility assessments (i.e. WWR ≈ 80%) can be pursued further, refining the analysis to achieve definitive results, or abandoned.  This decision is influenced by several factors, including the strength of stakeholders’ desire for daylighting, the balance between aesthetic and utilitarian values of the space, project budget, and time constraints.

Step (3) Room proportions
            To continue the preliminary daylighting design process, select values for the following parameters of the space to be daylit:

• Rmean:  mean reflectance of room surfaces, as defined previously (dimensionless).
  
• w:  width of the interior daylit space (m).
• hwindow-head-height or hw-h:  height of window head (i.e. top of window) from the floor (m).
  

            The depth of daylit area (Ddaylit-area) represents the extent to which daylight can be expected to penetrate into an interior space; it is the minimum of three calculated values determined by the parameters chosen above.
            The first value is the Lynes limiting room depth (DLynes or Dlrd), calculated as

It is used to limit excessive illuminance contrast ratios caused by excessively-high illuminances near the windows.  A steep illuminance gradient can result in a deceptively-high average daylight factor.
            The second value is the no skyline depth, calculated as Dno-skyline = (hw-h – hwp) tan θ, where hwp is the workplane height (m).  The no skyline depth defines the distance from the window beyond which there is no view of the sky on the workplane, as depicted in Exhibit 6.

            The third value, the daylight penetration depth, is dependent upon the need for shading devices, such as venetian blinds.  It is determined as follows:

Thus,

            If the depth of daylit area does not satisfy design objectives, room parameters can be adjusted and Ddaylit-area recalculated.  Several iterations can be completed quickly at very low computational cost.
 
Step (4) Required glazing area
            Setting room depth equal to Ddaylit-area (from Step 3), calculate the total surface area, Atotal, of the space.  Then calculate the minimum glazing area, Aglazing, required to daylight the space:

Verify that the Aglazing value obtained is consistent with the room dimensions selected and the WWR from Step 2.  Address any discrepancies and iterate as necessary.  Further analysis of design tradeoffs is required to ensure an optimal daylighting solution is developed.

Design Tradeoffs
            Effective daylighting requires a number of tradeoffs to be addressed in the design.  Some of the most salient tradeoffs are financial in nature; construction, operation, and maintenance expenses must be balanced to ensure a project’s viability.  There are many design choices to be made that affect each cost category, the possible combinations of which are limitless.  Therefore, a detailed discussion of all possible interactions is impossible.  Instead, some of the most-ubiquitous approaches to cost-effective daylighting are discussed.  Each presents advantages and challenges; designers must weigh these to choose appropriate solutions, as each project is unique.
            A key challenge for any daylighting project is to optimize energy consumption by balancing lighting requirements with those of heating and cooling the space.  In cold climates, solar heat gain afforded by large glazed areas may be advantageous.  However, large glazings can also cause substantial heat losses; the net effect must be determined to ensure the design specifications allow the project to meet energy efficiency targets.
            Likewise, solar heat gain must be controlled in warmer climates to maintain comfort.  In either case, the heating and cooling systems must be of sufficient capacity to accommodate the needs of the daylit space.  Reduced heat gain from electric lighting supplanted by daylight is a benefit in warmer climates, but the heating system must compensate for it in lower temperatures.

            The selection of glazings strongly influences the balance between lighting and heat load.  Glazing choices are differentiated by a number of characteristics.  Visible transmittance, τvis, is used in the determination of glazing area needed, as described above.  The solar heat gain coefficient (SHGC) quantifies the proportion of incident UV and IR radiation, as well as visible light, that is transferred to the interior by any means (i.e. transmission, reradiation, conduction, convection).  SHGC ranges from 0.0 to 1.0, though the end values are more theoretical than practical.  An SHGC of 0.0 implies the existence of an opaque perfect insulator and an SHGC of 1.0 implies the absence of any material (i.e. no glazing present).
            The ratio of visible transmittance to solar heat gain coefficient (τvis/SHGC) is called the light-to-solar-gain ratio (LSG).  It can be used to quickly evaluate a glazing’s compatibility with a project’s climate.  High-LSG glazings reduce cooling requirements, while low-LSG glazings are preferable in cold climates, as they reduce heating costs by exploiting solar heat gain.
            A glazing’s U-factor (U) is a measure of its heat conductivity (W/h/m^2/K); it is the inverse of R-value, a measure of insulating performance.  U-factor is influenced by the glazing material, thickness, and treatments, the number and spacing of layers, and the contents of the gaps.  Catalog values typically refer to heat conduction at the center of a pane.  However, this does not represent the total heat transfer by conduction through a window assembly, as frames and mullions can create thermal bridges.
            Low-emissivity (low-E) coatings can be applied to glazing surfaces to reduce reradiation, usually on an internal surface of a multilayer assembly.  Which direction the coating should face depends on the heat-gain or heat-rejection properties desired.
            Other treatments can also be applied to glazings, such as tints or spectrally-selective coatings.  A spectrally-selective coating can be particularly helpful in hot climates.  Heat gain is primarily associated with infrared radiation, which comprises nearly half of the solar radiation received.  While UV radiation comprises a comparatively small portion of solar radiation (~7%), it is notoriously detrimental to many materials, particularly synthetics.  Reflection of incident radiation outside the visible spectrum maximizes illuminance while minimizing detrimental effects of extraneous radiation.
            Typical transmittance and U-factor values for several glazing types are compared in Exhibit 7.  Using these values, preliminary design variables can be specified rapidly and refined as the project progresses.

            Another glazing-selection criterion to consider is the extent of image preservation or diffusion required.  A single layer of clear, untinted glass provides the greatest image preservation.  However, there are cases in which image preservation is a lower priority than other objectives and may even be undesirable.  Spaces in which privacy is paramount can still take advantage of daylight by using diffuse glazings that transmit light while obscuring vision.
            Intermediate conditions can be achieved with multilayer glazings, tints, or other treatments, such as fritting.  Fritted glass preserves the view through the glazing while diffusing some of the incident light.  At sufficient distance, fritting becomes unnoticeable; however, that used on the periphery of automotive windshields is quite familiar to many.
            Image preservation is relevant to the psychological benefits sought by many daylighting projects.  The ability to see the outside world can profoundly influence morale, positive affect, and overall job satisfaction.  Daylit spaces can also reduce the occurrence or severity of seasonal affective disorder (see Part 4).  In some cases, however, the outside world can encroach on concentration and productivity; for example, nearby activity can be distracting.

Daylighting Methods
            The preliminary design sequence presented was developed for sidelighting applications; this may be considered “traditional” construction practice.  Similar procedures can be followed for toplighting and hybrid applications, which are often incorporated into contemporary designs.
            Toplighting can be achieved in various ways.  A skylight is a glazed area built into a flat or pitched roof.  Some are domed to capture more low-angle sunlight (i.e. early morning and/or late evening).  Efficiency of a skylight well is heavily influenced by its size and shape, as shown in Exhibit 8.  In addition to improving transmission efficiency relative to a straight-walled well, a splayed skylight well can also improve the light distribution, reducing the luminance contrast of the well and surrounding area.

            A light pipe, or “tubular skylight,” can also be used to transmit light from a rooftop to an interior space.  A significant advantage of light pipes over conventional skylights or other glazed apertures is their flexibility.  The path from the external collector to the internal light-distribution device (e.g. diffuser) need not be a straight line, nor are complex structures required to direct light into the interior space.  Exhibit 9 depicts how a light pipe accommodates varying external and internal geometries of a structure.

            A roof monitor is a section of a structure raised to accommodate daylighting apertures above the surrounding roof.  Examples of a standard roof monitor and a sawtooth roof configuration are depicted in Exhibit 10.  A roof monitor directs light deep into the interior space with greater uniformity than can be achieved with sidelights.

            Clerestories are usually considered sidelights; however, they blur the line between toplights and sidelights.  Any sidelight located above eye-level can be called a clerestory, though the term commonly refers to windows placed immediately below the roof.  As these are often installed in tall spaces, the effect is more akin to toplighting.  This is particularly true when additional design elements, such as sloped ceilings, are incorporated to extend daylight penetration and improve light distribution uniformity.

            Techniques for controlling glare are not daylighting methods per se, but without them, daylighting would be much less effective and appealing.  The worst-case glare scenario occurs when the sun is directly visible in the workspace.  Shades, blinds, or other window coverings can be used to block the sun during these periods.  They are adjustable from the interior of the space and under the control of its occupants.
            An awning installed above a window is not adjustable.  Therefore, its benefit during periods of direct glare may be offset by undesirable illuminance reductions at other times.  In some applications, a light shelf provides a “best of both worlds” solution.
            A light shelf can be mounted on the exterior or interior of a daylit space.  It is a passive device (i.e. not adjustable) that shades the lower section of a window or adjacent area while reflecting daylight onto the ceiling through the upper section.  This increases daylight penetration and improves light distribution throughout the space.
            Baffles can be installed in skylight wells or roof monitors to protect against direct glare while minimizing the overall impact to illuminance.  Likely lines of sight to clerestories can be similarly baffled.
            These techniques are in addition to any glazing characteristics specified to aid glare control.  The techniques described are common, but not exclusive.  Designers should be encouraged to develop alternative solutions when needed to ensure an optimal daylighting solution.

Daylighting in Brief
            This installment is intended to be only an introduction to daylighting, the challenges it presents, and the benefits it offers.  For it to be manageable, the scope has to be very limited, leaving much for the interested reader to explore.
            Key topics for daylighting that could not be adequately addressed here include the use of sunpath and weather data to predict levels of solar radiation throughout the year.  The myriad building codes and other regulations in effect across various jurisdictions renders meaningful treatment of the topic impossible.  Let its mention here serve as a reminder to thoroughly research all requirements applicable to a specific site as early as possible in a project.
            Discussion of financial considerations could also be expanded.  Beyond the generalized expense categories mentioned, there may be energy and tax credits or other incentives available to improve a project’s financial viability.  These also vary by location and time and are, therefore, best left to project- and site-specific research and planning.
            Though psychological effects of daylighting were discussed only briefly, they can have an outsized impact on a project’s perceived success.  So important are these factors to productivity and morale that a view of the outdoors may be maintained even when it is contraindicated by other, usually financial, metrics.


            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
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000.
[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] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] “Guide for Daylighting Schools.”  Daylight Dividends, 2004.
[Link] “A rules of thumb-based design sequence for diffuse daylight.”  CF Reinhart and VRM LoVerso.  Lighting Research & Technology; March 2010.
[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] “Architecture Terminology Lighting.”  What Art.  HubPages, March 21, 2019.
[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]