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A procedure for determining target illuminances.


The Illuminating Engineering Society (IES) has a long history of recommending good practice for designing lighting. At the heart of these recommendations has been illuminance levels judged to be adequate, appropriate, and practical. IES Illuminance recommendations were first provided in the form of "Lighting Codes" [IES 1924] and later as an essential aspect of the 1st edition of the IES Lighting Handbook [IES 1947] and all subsequent editions. The need for a 10th edition of the Lighting Handbook provided an opportunity for the Society to re-evaluate illuminance recommendations. This included a determination and assessment of salient factors affecting recommendations: contemporary lighting design, energy and environmental constraints, and results of research on visual performance under mesopic adaptation. The result was a new system for determining recommended illuminance. We provide an historical context, report the background and development of this new system, and give examples of its implementation in the 10th edition of the Lighting Handbook.



The distinction between quantity and quality of illumination was given official status with the establishment of the IES Committee on Standards of Quality and Quantity for Interior Illumination in 1943 [IES 1944], eventually becoming the Committee for Recommendations of Quality and Quantity [IES 1964]. It was one of the longest-standing committees of the Society, working for nearly 50 years. Its original scope was to

Establish fundamental foundations for lighting practice based upon available experimental data pertaining to such matters as brightness and brightness-ratios in the visual field, brightness-levels, visibility-levels, footcandle specifications for various tasks and other factors dealing with visibility, comfort and ease of seeing. [IES 1944]

The initial distinction between quality and quantity appears not to have been used to segregate any of the elements of lighting listed in its scope, but rather to begin the task of defining what constituted "quality lighting". This is implied by the initial title of the committee: "Quality and Quantity for Interior Illumination", and made clear by the work it performed and the reports it issued during the 50 years subsequent to its establishment.

Unhappily, this is no longer how "quality" and "quantity" are used in the context of lighting. Inappropriately, "quantity" is now used to segregate illuminance and illuminance recommendations from other aspects of lighting--color, visual comfort, distribution, psychological setting, spatial ordering and so on--these being grouped under "quality". This use is erroneous and misleading. Sufficiency (or limitation) of the amount of light, expressed as illuminance or luminance at a defined location, is as important an aspect of a luminous environment as any other. In many cases, it is the most important, in some few it is the least important; but it is always an aspect of quality. In all cases sufficiency of light is an aspect of a luminous environment and should be used as one of the measures of the success, or failure, of the lighting system that produces it and the design process that conceived and specified that system. That is, illuminance is an aspect of the quality of a luminous environment.

What is required is an accounting of and distinction between what are quantifiable and nonquantifiable aspects of luminous environments. Of the many aspects that contribute to the quality of a luminous environment, only a very few are currently quantifiable. Luminous intensity (spatial luminous flux density) has been quantified since the middle of the 18th century [Lambert 1760, Thompson 1794]. Illuminance (incident surface flux density) has been quantified since the 19th century [Weber 1884]. With somewhat lesser generality, discomfort glare was quantified early in the 20th century [Petherbridge and Hopkinson 1950; Guth 1961], as was color rendering [Nickerson 1960]. Other aspects of luminous environments have not yet been quantified and are a standing challenge to lighting research.

An aspect of the luminous environment that is quantifiable is analytic in the following sense:

* The aspect can be defined with sufficient precision to permit the development and use of instruments and methods for its measurement,

* It can be expressed in mathematical form sufficient for predicting its value in most luminous environments, and

* Its quantity can be mapped by psychophysical experiment and experience onto a continuum that ranges from insufficiency to sufficiency, or in some cases, excess.

Examples are illuminance, color rendering index (CRI), and unified glare rating. Consensus can then be used to determine points on the continuum that define appropriate values of the aspect for different types of luminous environments, and those values can become recommendations.

A nonquantifiable aspect of the luminous environment is heuristic in the following sense:

* Only qualitative statements about it can be made,

* Absent quantification, only general affinities can be pointed out between the aspect and satisfactory lighting,

* Recommendations regarding it can only be descriptive, such as: emphasizing its importance, indicating its benefits, linking it to other aspects of the environment, or pointing to good examples of its use and presence.

Examples are the seeming spaciousness of an environment, the gradient and magnitude of luminance required to produce an adequate luminous accent, the need and extent to which walls should be lighted to avoid the perception of a dim interior environment.

Analytic aspects of the luminous environment can have recommended values. Heuristic aspects--however important--cannot. The specificity and quantifiability of analytic aspects of a luminous environment do not imply their greater importance, only that they can be a described with a definiteness that heuristic aspects do not permit. The Society can and does make recommendations about all aspects of the quality of the luminous environment, but it can be most definite about those that are analytic. That it cannot be so about heuristic aspects does not mean it should not prepare and present recommendations for analytic aspects in the fullest detail and with the greatest possible specificity. Such recommendations are often the best help for and service to lighting design that the Society can provide.


It is not possible, in any practical sense, to base illuminance recommendations only on vision research [Boyce 1996]. The complexity of visual response, the varying nature and size of the visual component of overall tasks, and the overlapping effects on the luminous environment produced by illuminance, make it necessary for experience and proven good practice to be the most important guides to establishing illuminance recommendations. The depth and diversity of experience of members of application committees of the IES has been and continues to be the source of this guidance.

The critical role of experience, as embodied in IES application committees, in establishing recommendations has been recognized from the very beginning.

The work of the IES Committee on Standards of Quality and Quantity for Interior Illumination made this clear. In its first report to the Society it wrote:
 Its viewpoint [that of the committee] is not influenced by the
 attainability of its specifications or the practicability of its
 conclusions. Committees dealing with actual lighting practice may
 be obliged to compromise between present practicability and
TABLE 1. Illuminance Recommendations (fc) for Schools

Year Area
 Assembly Lecture Drawing

1916* 3 5 10
1928* 7 12 25
1947 10 30 50
1952 10 30 50
1959 15 70 100
1966 15 70 100
1972 15 70 100
1981 10/15/20 50/75/100 100/150/200
1984 10/15/20 50/75/100 100/150/200
1993 10/15/20 50/75/100 100/150/200
2000 10 5 100

Entries marked with an * are from recommendations in reports from
IES committees. All other entries are taken from the IES Lighting
Handbook published in that year.

It is difficult to image any point in the foreseeable future when application committees will not be needed to provide a consensus interpretive blend of good lighting practice and scientific knowledge.


As the allotment of energy devoted to lighting continues to decline, it becomes increasingly important for illuminance recommendations to be a design tool that helps produce lighting systems that not just consume less energy, but also provide good luminous environments. Rather than simply lower values, recommendations must now identify factors of location, use, variability, observer, and environment that can more carefully determine the need for illuminance in space and in time.


There are several detailed analyses of the history of IES illuminance recommendations [Osterhaus 1993; Mills and Borg 1999; DiLaura 2006; Steffy 2006]. We present here only an overview sufficient to give background to the development of the new illuminance determination procedure. Tables 1 and 2 show school and office illuminance recommendations, in footcandles, as they developed in the 20th century. Recommendations have always been affected by what illuminance was considered necessary, what was observed as contemporary good practice, and what was technically and economically possible.
TABLE 2. Illuminance Recommendations (fc) for Offices

Year Task Difficulty
 Easy Medium Difficult

1912* 2 4 6
1925* 4 6 12
1947 10 30 50
1952 10 30 50
1959 30 70 150
1966 30 70 150
1972 30 70 200
1981 10/15/20 50/75/100 100/150/200
1984 10/15/20 50/75/100 100/150/200
1993 10/15/20 50/75/100 100/150/200
2000 5 30 100

Entries marked with an * are from recommendations in reports from
IES committees. All other entries are taken from the IES Lighting
Handbook published in that year.



In the first decades of the 20th century, the IES promulgated so-called "lighting codes" that described illuminances appropriate for various applications, including industry, offices, and schools [IES 1912; 1916; 1925; 1928]. In all cases, these were consensus values based on then current practice and what was achievable economically with incandescent lamps.


Beginning in 1937 and continuing until 1959, many IES recommended illuminance levels were based on visual acuity and a standard task and illumination condition. The task was 8-point black Biondi type on white paper and the illumination condition was 10 fc. This task, illuminated in this way, was considered to represent a conservative standard for illumination. A relatively simple contrast-reducing visibility meter [Luckiesh and Moss 1934], was used to determine the suprathreshold visibility of the standard task under this standard illumination condition. Recommended illuminances for other tasks were established by determining the illuminance required to raise them to the same suprathreshold visibility as that found satisfactory for reading 8-point Biondi type. Sample results are shown in Fig. 1.

These values were collected and formed the table of recommended illuminances in the first edition of the IES Lighting Handbook published in 1947. In most cases, the values were higher than they had ever been and were, in some cases, considerably above then-current practice. These values also appeared in the second edition of the Lighting Handbook [IES 1952]. These can be compared to British recommended illuminances produced at nearly the same time and based on early visual performance studies [Weston 1935, 1943, 1945 ] using a Landolt ring task. The IES recommendations were similar, but higher.


The recommended illuminances promulgated in the third edition of the IES Lighting Handbook published in 1959. The method, data, and instrumentation used to generate these recommended illuminances were developed by H.R. Blackwell [1955]. As in the system used previously, a standard task formed the basis of recommendations.

The standard task was a 4 arc-minute luminous disc. The standard performance data related the contrast exhibited by the disc, presented for 1/5 second, as a function of surround luminance, when the disc was at threshold visibility. Threshold was defined to be 50 percent detection probability. The function relating contrast and luminance of the standard task could be moved above threshold by factors that accounted for task movement and performance higher than 50 percent. An instrument was developed to visually equate practical tasks to the standard task. Equality meant having the same suprathreshold visibility, defined as requiring the same contrast reduction to produce threshold visibility. The standard task contrast that had this suprathreshold visibility was used with the standard function to determine the required background luminance. The use of an appropriate value of reflectance gave the illuminance required for the practical task.

In general, these recommended illuminances were higher than any ever recommended by the IES. These recommendations appeared in the fourth and fifth editions of the Lighting Handbook [TES 1966; 1972].


Based on the results of a series of experiments begun in the 1970s it became apparent that suprathreshold visual performance could not be predicted by threshold visibility [Smith 1975; Smith and Rea 1978]. Though subsequent research provided a model for suprathreshold visual performance, vision research as remained a guide for rather than a determinator of recommended illuminances [Rea 1982; Rea 1986; Rea 1987]. In 1980 the IES abandoned attempts to base recommended illuminance levels on the results of visual performance experimental data and adopted a consensus procedure [IES 1980]. This involved ranges of illuminance defined by three values. A range of illuminance was assigned by consensus to a task or area. Which of the three values in the assigned range were to be used for the recommended illuminance was determined by a consideration of the age of the observer, the reflectance of the task background, and task importance. This process formed the basis for the recommendations in the 6th, 7th, and 8th editions of the Lighting Handbook [IES, 1981, 1984, 1993].


In an effort to balance emphasis across a broad range of aspects of luminous environment quality, the 9th edition of the Lighting Handbook returned to a single value illuminance recommendation procedure. For each task or application, recommended values of horizontal and vertical illuminance were part of a list of design issues, each of which was labeled with a relative importance [IES 2000]. The lack of flexibility imposed by the single illuminance values was subsequently noted [Steffy 2006].


There are many aspects of lighting and its design that have arisen in the last decade, changed importantly, or become prominent aspects of public policy.

These all have important effects on recommended illuminances.


The decreasing energy allocated to lighting has significantly increased the amount of effort and information necessary to adequately allocate lighting to locations and times when it is required. A more careful delineation of visual tasks and their environments is necessary to refine and narrow recommendations. Age significantly affects the amount of light reaching the retina, and this affect, coupled with a more careful assessment of who is performing the visual task or occupying the lighted space, is required to wisely allocate lighting energy. Such refinements are the necessary anodyne to any ill-conceived, simple reduction in light levels.


New technology has produced new light sources, new tasks, and new forms of information delivery that have significantly altered not only the objects and spaces to be lighted, but also the equipment used to do so. New sources and luminaires are able to provide more efficacious generation and more efficient delivery of light to required locations. Control systems are better and more economically able to provide light only where and when it is needed. New and different tasks arising from developments in information technology and more information about work and leisure spaces require and permit a refinement of illuminance recommendations. Accounting for these developments means recommendations are better able to promote the change in emphasis from lighting power to lighting energy, while maintaining the quality of luminous environments in the face of diminishing energy allocation for lighting.


Outdoor illuminance recommendations need to reflect the heightened awareness of and desire for environmental factors that constrain lighting systems. These include urban sky-glow, light trespass, and accommodating the general luminous surround as expressed in lighting zones [IES 1999].


To respond to tightened energy requirements, effectively take advantage of new technology, and accommodate environmental concerns, requires not only flexibility in illuminance recommendations but also the information necessary to use that flexibility. This information can only come from the lighting designer and the client, and so recommendations must be cast in forms that not only require this information but also allow it to be used to carefully tailor recommended illuminances to a project. This information can include: assessing the age of the observers, establishing the activity level of or around the task, knowing the lighting zone in which the project resides, and determining the task locations where the illuminance criteria apply.


The new procedure for determining recommended illuminances has three aspects that help recommendations respond to the demands of reduced lighting energy.


The step size, or granularity, of the illuminances that constitute the possible values of recommendations has been made small. Increments between values are approximately 1/2 log unit and the resulting step size helps more carefully and parsimoniously allocate illuminance to tasks and spaces.



Guidance is provided to help use recommended illuminances in the form of a "gauge." Average, minimum, and maximum are possible gauges, and indicate the way to interpret recommended illuminances when applied over areas or when minimum or maximum illuminance values are necessary. In most cases, recommended illuminances are accompanied by a uniformity target. Ratios of average:minimum, maximum:minimum, and maximum:average are used to specify these targets. Gauge and uniformity recommendations increase the specificity of recommendations and identify and limit overlighting.


For many outdoor environments the activity level varies considerably with location and time. The density and speed of pedestrian and automobile travel and interaction, for example, changes with location within a parking lot or along a roadway and with the time of night. Allowing recommended illuminances to vary with activity level permits additional tuning of a lighting system and an additional way to reduce lighting energy.


In the general population, age reduces the transmittance of the lens and other intraocular media of the eye and significantly reduces the amount of light reaching the retina. Although the transmittance reduction varies considerably with wavelength, between the ages of 15 and 75, the intraolcular transmittance for optical radiation between 500 nm and 600 nm decreases by a factor of 6 [Ruddock 1964; Weale 1992]. It is neither practical nor appropriate to apply this factor directly to recommended illuminances, since habit and experience are known to mitigate some of the effects of reduced retinal irradiance.

To account for these effects in a practical manner, the following has been adopted. It is reasonable to assume that legacy values of recommended illuminances were considered to apply to a population age of between 25 and 65 years. If it is known that at least half of the observers are at least 65 years old, then the legacy recommended illuminance (if one exists) is doubled. If it is known that at least half of the observers are less than 25 years old, then the value is halved. Given the available data and the relatively stringent requirement regarding average population ages, these increments and decrements of a factor of two are modest.




It is well-known that the spectral response of the visual system changes with the state of adaptation. In all past cases, IES illuminance recommendations have been based on the assumption that the state of adaptation is photopic and so the photopic luminous efficiency function of wavelength has been used to define the lumen.

Recent research results have provided for the development of a method to account for the visual system's changing spectral response as adaptation moves through the mesopic range as it passes from photopic to scotopic [Rea and others 2004; Elohoma and Halonen 2006; CIE 2010]. This system of mesopic photometry determines multipliers that can be used to convert photopic luminances to approximate mesopic luminances. As adaptation changes, the spectral response shifts and so the amount of change from photopic luminance to mesopic depends on the spectral power distribution (SPD) of the luminance. The ratio of scotopic lumens to photopic lumens (S/P ratio) of the optical radiation involved is used as a reductionist characterization of the SPD.

The multipliers provided by the system are shown in Fig. 2. Multipliers are determined from photopic luminance anticipated to be present and is used to determine the state of adaptation. Each curve in Fig. 2 describes the degree of change from photopic to mesopic luminance for different adaptation luminances. The range of the multiplier (above and below 1.0) corresponds to the magnitude of the effect of spectrum, increasing as adaptation luminance decreases. Mesopic multipliers can be used to modify recommended illuminances: change the recommend value depending on the anticipated adaptation state and the SPD of the source involved. Multipliers should not be used in environments that have automotive traffic with speeds greater than 40 kph (25 mph), since the tasks involved require extensive use of and reliance on peripheral vision and it is it not yet clear that the current mesopic photometric system can be used in these cases.


Outdoor environments are now classified by Lighting Zones, indicating their general background luminance level and the adaptation state produced, their susceptibility to light pollution, and their sensitivity to and appropriateness for outdoor lighting [TES 1999]. Most outdoor recommended illuminances decrease as the Lighting Zone indicates a more rural environment.


The above considerations were brought together in a system for determining recommended illuminances and adopted by the IES Board of Directors in August of 2009. The principal characteristics of this system are as follows:

* Values are consensus based, expressed in lux, determined by the appropriate application committees, and approved by the IES Board of Directors.

* Central or "anchor" recommended illuminances are determined for a general population ranging from 25 to 65 years old. For the same task, recommended illuminances for an observer population with at least half being 25 years old or younger is one-half the anchor value, and for populations with at least one half being 65 years old or older, twice the anchor value.

* Recommended illuminances are to be considered as maintained, target values at the task.

* Recommended illuminances are accompanied by a gauge, indicating how the value is to be interpreted at the target point or over a target area.

* Where ever possible or appropriate, uniformity targets are also specified as part of the recommendation. These involve maximum:minimum, maximum: average, average:minimum illuminance ratios.

* All recommendations assume photopic adaptation. If it is known that the adaptation luminance will be at or below 3 cd/m2, mesopic multipliers can be used to transform recommended values from photopic to mesopic. These multipliers are not used in environments with automobile traffic traveling at greater than 40 kph (25 mph).

Figure 3 shows the fundamental ranges of values used in the recommendation system. Figure 4 shows a representative table of recommended illuminances.

10 THE FUTURE OF RECOMMENDATIONS Unless the Society's role as principle source of lighting recommendations is successfully challenged, it will continue to be the primary source of lighting design guidance embodied in recommendations.


It is clear that future national and international codes will further restrict energy devoted to lighting. It is anticipated that there will be a shift from limiting lighting power to limiting lighting energy. With this shift will come the need for recommendations regarding not only the quantity of light but its use in time. Recommendations regarding dimming, load shedding, and the use of sensors to control light are expected to become not just ancillary to but rather part of illuminance recommendations.


As knowledge of the relationships between optical radiation and human wellbeing deepen, it is anticipated that medicine, biology, and psychophysics will produce bases for recommendations involving the nonvisual effects of lighting systems. These are likely to take the form of specifying source SPDs, exposures, and quantities of irradiance.


Lighting research has the potential to transform many aspects of the luminous environment from heuristic to analytic, allowing these aspects to be part of recommendations that are specific and quantitative. The transformation will be difficult, since the psychophysical research required is challenging and the resources required to support the necessary experimentation are extensive. However difficult the process, the results will be of great value and worth the effort and political will required to produce the required resources.


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David L. DiLaura (1) LC FIES, Rita M. Harrold (2) FIES, Kevin W. Houser (3) PE PhD, Richard G. Mistrick (3) PE PhD FIES, and Gary R. Steffy (4) LC FIALD

(1.) Acuity Brands Lighting, (2.) Illuminating Engineering Society, (3.) The Pennsylvania State University, (4.) Gary Steffy Lighting Design

doi: 10.1582/LEUKOS.2011.07.03001
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