Guide to Mechanical / electrical equipment for buildings: PART III: ILLUMINATION: Lighting Design Process

1. GENERAL INFORMATION

LIGHTING DESIGN IS A COMBINATION OF applied art and applied science. There can be many solutions to the same lighting problem, all of which will satisfy the minimum requirements, yet some will be dull and pedestrian, whereas others will display ingenuity and resourcefulness. The competent lighting designer approaches each problem afresh, bringing to it knowledge of current technology and years of background and experience, yet rarely being satisfied with a carbon copy of a previous design. It is these years of background with their successful and not so successful designs, coupled with a constant striving for improvement, that are the characteristics differentiating the competent lighting consultant, designer, or engineer from the person who attempts to force each new job into the unwilling mold of a previous design.

Because of the large number of interrelated factors in lighting, no single design is the correct one; for this very reason, it is not entirely desirable to solve a lighting problem with a step-by-step technique. However, since experience has shown this technique to be a good approach for the uninitiated who lack the experience necessary to view an entire solution, we have adopted it.

2. GOALS OF LIGHTING DESIGN

Simply stated, the goal of lighting is to create an efficient and pleasing interior. These two requirements-that is, utilitarian and aesthetic- are not antithetical, as is demonstrated by every good lighting design. Light can and should be used as a primary architectural material.

1. Lighting levels should be adequate for efficiently seeing the particular task involved. Variations within acceptable luminance ratios in a given field of view are desirable to avoid monotony and to create perspective effects.

2. Lighting equipment should be unobtrusive but not necessarily invisible. Fixtures (luminaires)

can be chosen and arranged in various ways to complement the architecture or to create dominant or minor architectural features or patterns. Fixtures may also be decorative and thus enhance the interior design.

3. Lighting must have the proper quality, as discussed previously. Accent lighting, directional lighting, and other highlighting techniques increase the utilitarian as well as the architectural quality of a space.

4. The entire lighting design must be accomplished efficiently in terms of capital and energy resources, the former determined principally by life-cycle costs and the latter by operating energy costs and resource-energy usage. Both the capital and energy limitations are, to a large extent, outside the control of the designer, who works within constraints in these areas. These constraints are generally maxima.

With these goals before us, we can write a lighting design procedure, keeping in mind that the order of steps shown is not necessarily the same in each lighting problem and that, since all factors are closely interrelated, it is often necessary to address several of the stages simultaneously before arriving at a decision.

It is appropriate to note at this point that the lighting design approach and procedure that we explain in this Section is primarily an analytic one; that is, the design procedure establishes requirements primarily in numerical form and then manipulates the variables of sources, fixtures, placement of units, and so on, to arrive at a design solution. There exists an alternative approach, frequently referred to as brightness design, in which the designer labels surfaces on a perspective plan of a space with desired luminances as established by a mental picture (or by other means) and designs the lighting accordingly. This approach, which can be very effective, is highly intuitive (i.e., requires considerable prior experience on the part of the designer). For this reason, and because that approach requires a good deal of hands-on, field trial-and-error work, we feel that it is not appropriate to a textbook, and therefore is not used here.

The only exception to this statement occurs in our discussion of daylight models in Section 14.

There, the entire purpose of the model is to give the designer a visual/mental picture of the space's brightness patterns on the basis of which windows, window treatments, and possible exposures can be varied to achieve the desired light and shadow patterns (luminances and luminance ratios).

3. LIGHTING DESIGN PROCEDURE

(a) Project Constraints

The flowchart in FIG. 1, which represents the design procedure and its interactions, should be referred to throughout the necessarily lengthy discussion that follows in order to maintain perspective. It is important that the reader be aware of job constraints and of the interactions between the lighting designer and the remainder of the design team. We deliberately emphasize this to demonstrate the interdisciplinary nature of lighting design in general and its connection with HVAC and day light (fenestration) in particular. This approach, which is most often referred to as the systems design approach, is followed throughout the discussion.

Item 4 of the list in Section 13.2 referred to constraints. These can be related to the owner-designer user team and/or the jurisdictional authorities. In some detail, these are:

1. Owner-designer-user group. The owner establishes the cost framework, both initial and operating. A part of both of these may be a rent structure, which in turn determines and is determined by the space usage. If the owner is also the occupant, the cost factors change somewhat but remain in force. The architect determines the amount and quality of daylighting and the architectural nature of the space to be lighted.

Many of these data are detailed in the building program. Obviously, the architect and lighting designer (who may be the same person) should interact in this aspect of building design.

2. The jurisdictional authorities may include:

DOE-U.S. Department of Energy GSA-General Services Administration NFPA-National Fire Protection Association (via its codes) ASHRAE-American Society of Heating, Refrigerating and Air-Conditioning Engineers IESNA-Illuminating Engineering Society of North America NIST-National Institute of Science and Technology (formerly National Bureau of Standards) Most of these are jurisdictional by reference; that is, the actual authorities may specify that the lighting system meet the requirements of ASHRAE, IESNA, and so on. If federal funds are involved, DOE/GSA standards will probably be involved. The principal areas of involvement are energy budgets and lighting levels, both of which affect every aspect of lighting design, including source type, fixture selection, lighting system, fixture placement, and even maintenance schedules. For this reason, the first step in the lighting design procedure is to establish the project lighting cost framework and the project energy budget.

FIG. 1 Lighting design procedure chart.

(b) Task Analysis

As shown in FIG. 1, this step essentially deter mines the needs of the task. Factors to be considered in addition to the nature of the task are its repetitiveness, variability, who is performing the task (i.e., physical condition of the occupant), task duration, cost of errors, and special requirements.

Several of these factors have been discussed in the preceding sections dealing with quality of light.

These are cross-referenced in the appropriate sections in the following analysis.

(c) Design Stage

This is the stage during which detailed suggestions are raised, considered, modified, accepted, or rejected. This is also the most interactive stage, as is clearly seen in FIG. 1. At its completion, a detailed, workable design is in hand. The critical interactions here are with the architect in daylighting and with the HVAC group in power loads. The former may result in relocating a space within the building, the latter in making a change in a lighting system or HVAC system. In brief, this stage consists of the following steps:

1. Select the lighting system. Select the type of light source and the distribution characteristic of fixture(s) or the area source and consider the effects of daylighting, economics, and electric loads.

2. Calculate the lighting requirements. Use the applicable calculation method and establish the fixture pattern, considering the architectural effects.

3. Design the supplemental decorative and architectural (built-in) lighting.

4. Review the resultant design. Check the de sign for quality, quantity, aesthetic effect, and originality.

(d) Evaluation Stage

With the design on paper, it can now be analyzed for conformance to the principal constraints of cost and energy. If the design stage has been carefully completed, with due attention to these factors, the results of the final evaluation should be gratifying.

The results of this stage are fed to the architectural group for use in the final overall project evaluation.

In the following sections, we consider in detail each of the steps in the design procedure.

4. COST FACTORS

This is a particularly difficult item for a novice lighting designer because it requires experience in the field and an acquaintance with commercially available equipment. Also, the inevitable trade offs between first cost and operating cost cannot be made intelligently unless the cost structure is clearly understood. The following guidelines should be of considerable assistance both in avoiding unpleasant surprises when a job is estimated and in preparing cost analyses:

1. Decide at the outset what cost criteria will be applied-that is, the relative importance of first cost, operating costs, annual owning costs, and life-cycle costs.

2. Trade-off decisions are required between first cost and operating costs. For example, incandescent lamps and fixtures are low in first cost and high in operating cost, and so on. Dimming and control equipment falls into this area of decision making.

3. Manufacturers' catalog items are always cheaper than specials and can be priced more readily.

4. Compare the annual owning costs of two systems or methods. Conversion of these data to life-cycle cost comparisons is straightforward.

5. The impact of lighting energy on the operating cost of the entire building must be studied and the apportionment of costs determined. The only practical means of accomplishing this is by using a computer program. Programs can be readily adjusted to reflect the effect of the lighting system on building costs, and in particular on HVAC first cost and operating costs.

It is incorrect to artificially separate the lighting system from the HVAC system, with which it intimately interacts.

The lower the lighting system's energy usage, the lower the building's overall operating cost. The argument that heat from a lighting system is fully utilized to heat the building and is therefore not wasted is a specious one that has been refuted on many counts:

• HVAC system first cost is higher.
• HVAC year-round cost is higher.
• Lighting energy cost is higher.
• Life-cycle costs are higher.
• Energy resource use is higher.

5. POWER BUDGETS

The requirement to establish a project lighting power budget in accordance with a specified procedure has now been incorporated into the building codes of many states.

The purpose of this budget determination procedure is not to dictate the design procedure. Indeed, all standards explicitly so state. Instead, the purpose is to develop an overall maximum power budget within which the designer is free to do as he or she wishes. Obviously, extravagance in one area is necessarily at the expense of another area, as maximum power is inflexible, and the entire power budget is based on reasonable design techniques. Still, there is enough leeway in the budget and enough exceptions so that the designer is not overly restricted.

The nationally accepted standard that defines the establishment of a lighting power budget is ANSI/ASHRAE/IESNA Standard 90.1: Energy Efficient Design of New Buildings Except Low-Rise Residential Buildings, published by ASHRAE and regularly updated.

ASHRAE 90.1 sets forth design requirements for the efficient use of energy in new buildings intended for human occupancy. Specifically excepted in the current edition are single-family and multifamily residences of three or fewer stories above grade and buildings whose primary function is directed to a specific purpose, for which human occupancy may be required but is secondary. These include industrial buildings, buildings with very low energy use, and very small buildings (<100 ft^2 [10 m^2] area). The standard encompasses energy use requirements for all of a building's environmental systems (i.e., HVAC, lighting, electrical, power, water, envelope, and energy management).

The standard recognizes the advances that have taken place in the field of lighting controls, as a result of which it is able to focus on energy use limitation (which is its specific purpose) without necessarily limiting power use. For details on how this is accomplished, the reader is referred to the current issue of Standard 90.1.

6. TASK ANALYSIS

Refer to FIG. 1. This is the stage at which the quantity and quality of lighting required for the tasks are decided. The factors affecting this choice, as shown in FIG. 1, are difficulty, time factor, occupant, cost of errors, and special requirements.

(a) Difficulty

The components of visual difficulty were discussed at length in Sections 11.16 to 11.22, and the results in terms of lighting levels appear in Tables 11.4 to 11.9. Essentially, the designer examines the type of task involved, and after determining the applicable authority, he or she selects the required illuminance.

In the absence of specific instructions or reasons to the contrary, the North American designer uses IESNA recommendations. If there are several tasks to be performed at the same point and the most difficult one occurs infrequently, it may be reasonable to provide supplementary portable lighting or even to suggest moving to another brighter location. If it is the major task, lighting should be based on it and provision made for intensity reduction for less demanding work.

Variation in task difficulty is particularly common in spaces in public buildings. Thus, a school gym can be used for athletics, band concerts (despite the acoustics), and town meetings-activities with totally disparate lighting requirements. In these and similar instances, it is common practice to treat the space as essentially three different spaces and design lighting for each, with a careful eye to maxi mum common equipment usage. Similar problems are encountered in basements, multipurpose rooms, and conference/meeting/lecture/exhibition rooms.

Fortunately, most such spaces do not have severe seeing tasks.

The task variation referred to here is the variation that occurs in one very specific location and is not to be confused with task variation in an area, however restricted. Thus, a small private office of, say, 8 × 8 ft (2.4 × 2.4 m) has a desk, file cabinet, and circulation space, involving three tasks of differing but constant difficulty in one small space. The corresponding lighting for these is also fixed and varies with the task severity. The values listed in Tables 11.4 to 11.9 represent the required illumination on the surface in question, whether horizontal, vertical, or in between. Inasmuch as the flux method of calculating illuminance normally yields the 30-in. (760-mm), horizontal-plane illuminance value, it is helpful in the early design stages to know the ratio of horizontal to vertical illuminance for various lighting systems. This ratio is approximately Narrow distribution (direct and semi-direct) 3:1 Wide distribution (direct and semi-direct) 2.5:1 General diffuse (indirect) 1.5:1 Once the design is advanced, computer calculation will yield exact illuminance data on any desired surface, including important vertical surfaces. As the illuminance values listed assume adherence to both recommended luminance ratios and reflectances (see Section 11.32), it is necessary to select, in conjunction with the interior designer, finishes and reflectances for surfaces within the area.

If, for instance, in a private office a dark wall finish of 10% reflectance is chosen, it will be necessary for the lighting designer to compensate for this by additional wall lighting to maintain the recommended maxi mum 10:1 brightness ratio (see Table 11.11 and the discussion of point I, item G, in Section 13.7). The atmosphere created by vertical surface luminances is discussed later. TABLE 1 lists the reflectances of some common interior paint finishes.

(b) Time Factor

As discussed in Section 11.20, the length of time in which the task must be accomplished is important in exacting work. Beginning with moderately difficult tasks-that is, luminance of about 170 cd/ m^2 (50 fL)-prolonged intensive application or rapidly changing tasks would require illumination to be raised one (or more) levels. Alternatively, the quality could be improved by increasing daylight utilization or task contrasts.

TABLE 1 Approximate Reflection Values (c) Occupant

Inasmuch as the age and other specific characteristics of the worker are usually not known, a standard population distribution is assumed, and the illuminance recommendations as tabulated take account of this. However, if there is a high percentage of older workers, as is the case in certain industries, lighting should be raised one level. This compensates for the inability of aging eyes to accommodate and for their tendency to tire easily.

(d) Cost of Errors

This involves an economic trade-off between savings resulting from improving visual accuracy and the cost of the improved lighting. Performance can be brought close to perfection, but the cost of so doing increases much more rapidly than the proportional increase in performance. Lighting is usually designed to provide approximately 90% work accuracy.

Thus, this step is basically an economic calculation, the criteria for which must come from the owner or user. Tasks in which this problem is encountered include inspection, proofreading, textile matching, very fine machining, and jewelry manufacture.

(e) Special Requirements

These include any nonstandard task lighting requirements. Some of these are specific illuminant color, directionality for shadowing, reflections as required for inspection, polarization, and controlled variations, as required in a space with varied tasks or a varying daylight factor. In addition to these, the physical dimensions of the task often create special requirements of their own. We tend to assume a small object in the horizontal plane because that is the normal office task. However, there are exceptions, such as a drafting board, a large machine and an inspection bench, or a cutting table. Consequently, these special requirements arise:

1. Large tasks. With large tasks, the angle of seeing varies from 20º to 70º from the vertical, resulting in radically changing glare angles and reflections from the task.

2. Three-dimensional tasks. These tasks shadow themselves, particularly when containing under cuts and reveals. An architect's model shop presents such tasks. When it is necessary to see into an opening, an intense narrow beam is required.

3. Tools. Tools cast shadows below and in front when lighted from above and behind. A fabric cutter must see ahead of and below the cutting machine.

4. Nonhorizontal tasks. These must be calculated for the plane in which they stand. As noted in Section 13.6(a), the ratio between horizontal and vertical illumination varies between 1.5:1 and 3:1, depending upon the system.

Task lighting requirements are stated in the plane of the task. This can have a pronounced effect on the lighting system selected and its arrangement.

5. Task observed from various positions. There are instances in which a fixed task is observed from several angles, such as a drawing in a conference room or a wall display. Illumination must be adequate for all viewing angles.

7. ENERGY CONSIDERATIONS

Energy considerations must pervade every aspect of the design process. Some background material is in order here to place the lighting energy subject in proper perspective. Best current estimates indicate that lighting consumes approximately 25% of the electric power generated in the United States. In terms of resources, this amounts to approximately 4 million barrels (636,000 kL) of oil per day. The same sources indicate usage by occupancy as approximately:

Residential 20%

Industrial 20%

Stores 20%

Schools and offices 15%

Outdoor and other 25%

In commercial buildings, lighting consumes about 20% to 30% of the building's electric energy, more in residences and less in industrial facilities.

By judicious design, a reduction of 40% to 50% in lighting energy is attainable. Translated into resources, this reduction can readily amount to more than 1 million barrels (159,000 kL) of oil per day. Few will disagree that such a goal is well worth the effort. Every 1.0 W/ft 2 (10.7 W/ m^2) reduction in lighting energy results in at least 1.25 W/ft^2 (13.5 W/ m^2) savings in air-conditioned buildings. It has been demonstrated by actual designs that offices and schools can be well lighted with less than 1.5 W/ft^2 (16.1 W/ m^2). The question to be answered then is: What design guidelines can be followed to effect this energy-conscious design? The following are general recommendations regarding the design of energy-efficient lighting systems. For specific design requirements and details, refer to Standard 90.1 (ASHRAE, 2007), the Advanced Energy Design Guides (ASHRAE, 2004 et al.), and LEED (USGBC).

I. Conceptual-level approaches to energy conscious lighting design include:

A. Design lighting for expected activity. This is the task lighting approach. It is wasteful of energy to light any surface to a higher level than it requires. Nonuniform lighting is recommended where high illuminance levels are required for selected tasks in multitask spaces.

One way to accomplish this for areas where an exact furniture layout is not available is to use readily movable fixtures. Providing over all high-level illumination with provision for switching to reduce lighting levels is not advisable because of the increased first cost and the psychological impetus to operate at maximum levels. Another solution is to use fixed luminaires for general low-level lighting and supplementary task lighting. Other factors and techniques to be borne in mind are:

1. Group tasks with similar lighting requirements.

2. Place the most severe seeing tasks at the best daylight locations.

3. Improve the quality of difficult visual tasks. This is more energy-economical than providing additional light.

4. The advantages of nonuniform lighting increase as the space between workstations increases.

5. When using the task-ambient design approach, keep in mind that a nonuniform ceiling layout may give a chaotic appearance to a space.

Therefore, the preferred approach is uniform ambient light ing and local task lighting.

B. Design with effective, high-quality, efficient, low maintenance, thermally controlled luminaires.

"Effective" means providing useful light and minimum direct glare. In cases where much of the viewer's time is spent in a head-up position, as in schools, or where the viewer can compensate for veiling reflections, the decision should lean toward high visual com fort probability (VCP; see Section 11.28). Where work and viewing position are fixed and most of the viewer's time is spent head down, the decision should lean toward low reflected glare (see Sections 11.29 to 11.31).

1. A high-quality luminaire is made with permanent finishes such as Alzac or multicoat baked enamel or any of the high quality permanent aluminum finishes.

This ensures that its performance after 8 to 10 years of service will be comparable to the original.

2. An energy-efficient luminaire is one with a high luminaire efficacy rating (LER). This metric is explained in detail in Section 15.12. It combines photometric efficiency (a high coefficient of utilization [CU]; see Section 15.10) with energy efficiency of the luminaire's components to obtain a high overall efficacy for the luminaire, expressed in lumens per watt.

3. A low-maintenance luminaire remains clean for extended periods and is designed so that all reflecting surfaces can be easily and rapidly cleaned without demounting. Enclosed fixtures should be gasketed.

Non-gasketed units collect and retain dust and cause rapid output depreciation.

Relamping should be simple and rapid to encourage group relamping programs that are energy-efficient and cost-effective. A 20% increase in maintained light is possible if lamps are replaced at the end of their useful life-that is, when output is down to 70% of initial maintained lumens-and if fixtures are cleaned and maintained on a fixed schedule. No cost trade-off is generally involved because periodic maintenance and relamping are normally cheaper than one-at-a-time maintenance and burnout replacement, and yield 20% higher average lumens.

Fixtures in relatively inaccessible locations such as high ceilings must be designed for low maintenance, and maintenance should be on a fixed schedule.

4. A thermally controlled luminaire is one that controls the heat generated by the light source. This item depends to a large extent on the type of HVAC system, the lighting heat load, and the types of fixtures employed. Detailed analysis of this point involves HVAC considerations and the overall impact of lighting energy on the building. In the late 1980s, the thermal problem was principally directed at removing fixture heat. Today, the use of electronic ballasts and low-wattage lamps has reduced the seriousness of this problem (FIG. 2).

C. Use efficient light sources and accessories.

This point is self-explanatory. The ready availability of high-efficacy, high color rendering index (CRI), compact sources has made this item far less problematic than previously. The only cost trade-off involved is between relatively expensive, high-CRI sources and cheaper, low-CRI units. The cost differential is not large, and the choice is frequently made on other than an economic basis. Sources with high lumen maintenance should be given preference.

Spill light and borrowed light are often neglected sources. Glass in upper wall sections can provide sufficient corridor lighting from borrowed office lighting.

D. Select the appropriate lighting system. As detailed in Sections 13.10 to 13.15, there are five general approaches to light delivery, each with its particular distribution characteristics and, therefore, applicability. In addition, all luminaires must be carefully located to provide uniformity of ambient lighting and task lighting, where applicable. When using indirect luminaires, correct spacing and hanging stem length will avoid ceiling "hot spots," which can cause direct and reflected glare. Properly designed indirect lighting allows fully effective use of a space in that users are not restricted to facing in any particular direction to avoid direct or reflected glare from fixtures. The mounting height of suspended fixtures must be coordinated with cavity sizes and finishes to decrease light loss and maximize coefficient of utilization.

E. Use daylight, properly. Daylight must be considered as a normal light source subject to weather variations and time of building use. Obviously, a three-shift industrial plant cannot use daylight on all shifts, but it can for at least one shift, and design should address this fact. Part of proper daylight design is control of window luminance, which can cause severe and even disabling glare. A corollary of excessive luminance is excessive heat gain. Both are manageable with common control devices, both manual and automatic. Window control devices must also be designed to reflect light back into the space at night to avoid light loss via windows.

FIG. 2 Method of fixture installation controls the transfer of a luminaire's heat. (a) Suspended units contribute all their heat to the space while remaining fairly cool. (b) Surface-mounted fixtures also place all their heat in the space but, because of blocked transfer upward, run hot. (c) Completely enclosed recessed units transfer about 50% of their heat to the plenum. (d) Open-louvered, baffled units transfer about 75% of their heat to the plenum. When they are ducted, heat transfer up can be as high as 85%.

F. Use energy-efficient lighting control strategies.

The subject of lighting control, including manual and automatic switching, dimming, sensing, and intensity control, is covered in Sections 15.13 to 15.17 for both new installations and retrofit work. Proper design of controls can reduce energy consumption over a noncontrolled installation by as much as 60% without reducing lighting effectiveness. Appropriate controls are mandated in ANSI/ASHRAE/IESNA Standard 90.1.

G. Use light finishes on ceilings, walls, floors, and furnishings. This point is self-explanatory and is examined in a number of sections. A brief summary of recommended ranges is:

Ceilings 80-92% Walls 49-60% Furniture, office machines, and equipment 25-45% Floors 20-40% In addition to producing higher illumination levels in the room, high reflectances minimize uncomfortable luminance ratios, as between luminaire and upper wall or between task and background (see Table 11). For maximum luminance ratios:

1. Between task and near surround-aim for 3:1.

2. Between task and immediate area-aim for 10:1.

3. Between luminaires and their back ground-aim for 20:1.

4. Anywhere in the normal field of view- aim for 40:1.

Note that these targets themselves are maximums and that values above maximum should be accepted only with excellent justification.

II. Detailed requirements, generally from ANSI/ ASHRAE/IESNA Standard 90.1, include:

A. Mandatory requirements (summarized)

1. All interior and exterior lighting must conform to the stated energy limitations.

Trade-offs between the two are not permitted.

2. All interior and exterior lighting systems in buildings larger than 5000 ft^2 (465 m^2), except emergency and exit lighting, must be equipped with some form of automatic shutoff control.

3. Where applicable, occupancy sensors and automatic daylight compensation control should be used.

4. Separate tasks or spaces must have separate controls.

5. Areas with lighting power densities above 1.1 W/ft^2 (11.8 W/ m^2) should have facilities for at least two lighting levels.

B. Recommendations

1. Where task/ambient lighting is used, the ambient level should not be lower than a third of the task level to avoid uncomfortable luminance ratios.

2. Accent lighting should not exceed five times the ambient level. Therefore, in merchandising areas where the contrast between ambient and task levels is critical, reduce ambient levels as much as is practical.

3. When specifying super-reflective aluminum in fluorescent (or other) luminaires, determine that the material's high reflectance will be maintained in the specific application and that the required luminaire maintenance procedures will be available.

4. Utilize self-luminous exit lights or those utilizing LED displays where permitted by local codes.

The energy-efficiency benchmarks in ANSI/ ASHRAE/IESNA Standard 90.1 are updated on a regular basis. The current version of this standard should always be used for design. The accompanying User's Manual (ASHRAE, 2008) is recommended as a valuable supporting document to assist with interpreting and implementing Standard 90.1.

8. PRELIMINARY DESIGN

Again referring to FIG. 1, the preliminary design phase is the time during which ideas crystallize, but in terms of areas and patterns as well as light and shadow, and not yet in terms of hardware. At this stage the quality of the system is decided on; that is, the luminances, diffuseness, chromaticity, and pro portion of vertical to horizontal lighting are deter mined. The last factor establishes in large measure a room's "mood" or lighting ambience. In preceding sections, these items were discussed in some detail.

In the sections that follow on lighting systems (direct, indirect, etc.), the quality of each is considered and applications are suggested. In the overall view, how ever, the ultimate quality of the lighting system, its visual pleasantness, centers of visual attention, highlights and shadows, as well as texture and forms, are a deft and perhaps artistic combination of the previously mentioned considerations and establish, as the term implies, the quality of the lighting design. A few observations, not covered elsewhere, are mentioned in the following paragraphs.

Planes other than the working plane must always be considered. The ratio of vertical to horizontal illumination of the chosen lighting system determines wall luminance, which in turn greatly influences the overall impression of the space's brightness (see FIG. 12). The floor finish has a pronounced effect on the ceiling illumination for direct lighting systems because in direct systems, ceiling illumination derives only from room inter-reflectances, with floor reflectance being particularly important in large spaces.

The chromaticity of a room's lighting depends primarily on the source but secondarily on the luminaire and surface finishes. A "white" source can be tinted slightly by the use of a colored reflector in the luminaire. Of course, the effect on luminaire output of such a change must be considered. In the case of semi-indirect and indirect lighting, this same effect can be accomplished by the use of colored ceiling and upper wall surfaces, which serve as secondary reflectors and become the actual luminous source for the room. Recommendations in Table 11.12 cover the choice of source as affected by its chromaticity.

9. ILLUMINATION METHODS

The discussion that follows is primarily addressed to electric lighting systems. The descriptions apply directly to lighting fixtures and fixture arrangements. Similar considerations, however, can be seen to apply to daylighting design-and the implications of these considerations on daylighting design should not be overlooked during the design process.

There are three broad methods of illumination: general, local/supplementary, and combined general and local.

(a) General Lighting

This is a system designed to give uniform and generally, although not necessarily, diffuse lighting throughout the area under consideration. The method of accomplishing this result varies from the use of luminous ceiling to properly spaced and chosen downlights, but the resultant lighting on the horizontal working plane must be the same, that is, reasonably uniform. It may be, but is not necessarily, task lighting.

(b) Local/Supplementary Lighting

These are two terms that are used interchangeably. By definition, both local lighting and supplementary lighting provide a restricted area of relatively high intensity. A desk lamp, a high-intensity downlight on a merchandising display, and track lighting illuminating wall displays in practice are all referred to as local, supplementary, or local-supplementary lighting. Typical of this genre are the units illustrated in FIG. 3.

(c) Combined General and Local Lighting

This illumination method is used in spaces where the general visual task requires low illuminance, but supplementary lighting is required in a limited area for a particular task.

These three methods of illumination can be accomplished in many ways by the use of luminaires and luminous sources of different types, because the illumination method is a function of both luminaire arrangement and the luminaire's inherent lighting distribution. The term lighting system is used to describe the combination of a particular fixture type applied in a particular way.

Thus, a reflector-type fixture, when aimed down, gives direct light. The same fixture beamed up at the ceiling gives indirect light. The following section describes the systems that constitute the vast majority of lighting installations.

10. TYPES OF LIGHTING SYSTEMS

No single lighting system can be said to be the only choice in a given instance; on the contrary, the designer normally has a choice of at least two systems that, if utilized properly, yield illumination of adequate quantity and good quality. How ever, other factors, such as harmonization with the architecture and economics, usually tip the balance in favor of one or the other.

The five generic types of lighting systems are indirect, semi-indirect, diffuse or direct-indirect, semi-direct, and direct.

FIG. 3 Typical supplementary lighting units for incandescent, CFL, and linear fluorescent sources.

11. INDIRECT LIGHTING

See FIG. 4a. Between 90% and 100% of the light output of the luminaires is directed to the ceiling and upper walls of the room. The system is called indirect because practically all the light reaches the horizontal working plane indirectly, that is, via reflection from the ceiling and upper walls. There fore, the ceiling and upper walls in effect become the light source, and if these surfaces have a high-reflectance finish, the room illumination is highly diffuse (shadowless). Because the source must be suspended at least 12 in. (300 mm) and preferably 18 in. (450 mm) or more from the ceiling (depending on the unit's output) to avoid ceiling "hot spots," this system requires a minimum ceiling height of 9 ft 6 in. (~3 m). If luminaires are correctly spaced, the resultant illuminance is uniform, and direct and reflected glare potentials are both low.

FIG. 4 Indirect lighting. (a) The luminaires deliver 90-100% of their output above their own horizontal plane. (b) The ceiling and upper wall surfaces of the space are directly illuminated, and by reflection become large secondary sources that illuminate the space below. When properly designed, this type of installation yields a substantially uniformly bright ceiling. (c) Use of architectural coves gives an acceptable luminance gradient on the ceiling and, if properly designed, nearly uniform, glareless illumination in the room. This system of illumination is particularly useful in spaces with VDTs.

To avoid an excessive luminance ratio between the luminaire and its surrounding field, the luminaire can be made translucent on the bottom, the sides, or both. Approximately 750 lux (75 fc) is the maximum horizontal-plane illuminance attain able without exceeding an overall ceiling luminance of about 2500 cd/ m^2 (730 fL) (see Section 11.28). With practically no veiling reflections, this illuminance level is sufficient for all but the most difficult tasks. The lack of shadow, low source brightness, and highly diffuse quality created by indirect lighting give a very quiet, cool ambience to the space, suitable for private offices, lounges, and plush waiting areas.

Areas having specular visual tasks such as an office with visual display terminals (VDTs) use this system to advantage. In such spaces, indirect fixtures with out luminous bottoms or sides should be specified.

When properly designed, particularly when the source of light is architectural coves (see FIG. 4c), the ceiling has a floating sky quality, which is pleas ant and can be used to give an impression of height in a low-ceilinged room. (This system is not to be confused with a trans-illuminated ceiling, which is really a direct lighting system of entirely different quality and effect.) A further characteristic of the indirect lighting system is loss of texture on vertical surfaces, as is common to all fully diffuse lighting.

Indirect lighting is inherently inefficient, as much of the useful light reaches the working plane only after double reflection-within the luminaire and off the ceiling. Although to a considerable extent this inefficiency is offset by the glare-free lighting, applications to difficult seeing tasks frequently require an additional light source. Thus, an indirectly lighted architect's drafting room having tables equipped with supplementary lamps would take advantage of both systems-the local high-intensity lighting at a maximum of 200 fc (~2000 lux) for the restricted area being worked on and overall table lighting of 40 to 50 fc (~400 to 500 lux) of high-quality lighting free of veiling reflections. The latter would also solve any reflected glare problems arising from the many viewing angles required by large tasks such as drawings.

12. SEMI-INDIRECT LIGHTING

Between 60% and 90% of the light is directed upward to the ceiling and upper walls. This distribution is similar to that of indirect lighting, except that it is somewhat more efficient and allows higher levels of illumination without undesirable brightness contrast between the luminaire and its background, along with lower ceiling brightness. A typical fix ture, illustrated in FIG. 5, employs a translucent diffusing element through which the downward light component passes. The ceiling remains the principal radiating source, and the diffuse character of room lighting remains. Direct and reflected glare are both very low, as they are with indirect lighting (see Fig. 11.36). In both indirect and semi-indirect systems, it is often desirable to add accent lighting or downlighting to break the monotony inherent in these systems and establish visual points of interest or to create required modeling shadows.

In both indirect and semi-indirect lighting systems, the light undergoes a number of ceiling and wall reflections before reaching the horizontal working plane. The use of colored paints, particularly on the ceiling, can serve to tint the room illumination slightly by selective absorption.

FIG. 5 Semi-indirect lighting.

13. DIRECT-INDIRECT AND GENERAL DIFFUSE LIGHTING

Direct-indirect lighting provides an approximately equal distribution of light upward and down ward, resulting in a bright ceiling and upper wall (FIG. 6). For this reason, luminance ratios in the upper-vision zone are usually not a problem. As the ceiling is an important although secondary source of room illumination, diffuseness is good, with resultant satisfactory vertical-plane illumination.

General diffuse fixtures (FIG. 7) give light in all directions, whereas direct-indirect fixtures have little horizontal component. Stems for both types should be of sufficient length to avoid excessive ceiling brightness, generally not less than 12 in. (305 mm).

Because the impression of illumination depends to a large extent on wall luminance (as this is the surface we see most often), a space with general diffuse illumination appears lighter than one with direct-indirect illumination due to the darker walls in the latter.

By avoiding excessively bright luminaires and giving attention to the positioning of sources and viewing angles, direct and reflected glare can both be minimized. Furthermore, because the luminaire (like any other luminous source in the field of view whose luminance is higher than that of the aver age scene) draws the eyes' attention, particular care must be taken to limit its luminance and to avoid disturbing fixture patterns (see FIG. 14).

The efficiency of these two systems is good.

Both are well applied in spaces requiring overall uniform lighting at moderate levels such as class rooms, standard office work spaces, and merchandising areas.

14. SEMI-DIRECT LIGHTING

With this type of lighting system, 60% to 90% of the luminaire output is directed downward, and the remaining upward component serves to illuminate the ceiling (FIG. 8). If the ceiling has a high reflectance, this upward component is normally sufficient to minimize direct glare from the luminaires, depending on eye adaptation level. The degree of diffuseness depends largely on the reflectances of room furnishings and of the floor. Shadowing should not be a problem when upward components are at least 25% and ceiling reflectance not less than 70%. With smaller upward components, the system is essentially direct lighting (see Section 13.15). The system is inherently efficient. Reflected glare can be controlled by the methods discussed in Section 11.29. With adequate wall illumination, the quality of the lighting gives a pleasant working atmosphere. It is applicable to offices, classrooms, shops, and other working areas.

FIG. 6 (a) Direct-indirect lighting. Upper and lower room surfaces are luminous (b), but the center of walls is not because of the lack of horizontal light from fixtures (c). Principal light on the working plane comes directly from the luminaire.

FIG. 7 (a) General diffuse lighting. (b) Note that all room surfaces are illuminated and become secondary sources, although those closest to the fixtures (ceiling, upper wall) are the brightest secondary sources. The primary source of illumination is the direct radiation from the fixture. The floor contribution is low due to its normally low reflectance.

FIG. 8 Semi-direct lighting provides its own ceiling brightness (a), with surface-mounted fixtures (b) or pendant/surface units (c). Other characteristics are similar to those of direct lighting.

15. DIRECT LIGHTING

In this system essentially all the light is directed downward. As a result, ceiling illumination is entirely due to light reflected from floor and room furnishings. This system, then, more than any other, requires a light, high-reflectance, diffuse floor unless a dark ceiling is desired from an architectural or decorative viewpoint. Occasionally, the ceilings are deliberately painted a dark color and pendant direct fixtures used to lower the apparent ceiling of a poorly proportioned room or hide unsightly piping, ductwork, and so on.

The effect of direct lighting depends greatly on whether the luminaire light distribution pat terns are spread or concentrating (Figs. 13.9 and 13.10). In the former cases, considerable diffusion of light results from reflections on the floor, furniture, and walls. The result is a working atmosphere with slightly darkened walls and ceiling. This type of lighting, which is most widely represented by the recessed fluorescent troffer in a suspended ceiling, is common for general office lighting. The luminaires themselves form a ceiling surface of light and dark areas, and the quality of the entire system is not unpleasant. Difficulties associated with direct glare and veiling reflections can be controlled by proper use of reflectances, use of low-brightness units, and judicious arrangement of viewing positions. When direct lighting units are used in a uniform pattern, this latter option disappears, and the need for particularly low brightness units and high ceiling reflectivity, or specialty diffusers such as those with a batwing distribution, is increased.

FIG. 10 With concentrated direct distribution (a), the floor is the only luminous surface (b) other than the ceiling fixture. Diffuseness is absent. Walls are dark. Incandescent downlights (c) and, to a lesser extent, CFL downlights are of this type unless equipped with spread-type lenses.

FIG. 9 Spread-type direct lighting (a) illuminates all room surfaces except the ceiling (b), which is only illuminated by reflection from the floor. Some diffuseness is evident. The most common type of unit in this category is the direct fluorescent unit, either surface mounted (c) or troffer type, recessed in the ceiling.

FIG. 11 Large-dimension lighting fixtures may be used in a low-ceilinged room if the apparent size of the unit is reduced. Here at a mounting height of 7 ft 6 in. (2.3 m), 4 × 4-ft (1.2 × 1.2-m) units are acceptable because the lattice on the face of each unit gives the impression of reduced fixture size. Note also that the apparent illumination in (b) is greater than in (a)-although both are exactly equal on the table surface-due to the wall wash in the background. The eye perceives vertical surface illumination more readily than horizontal illumination and retains the impression for the entire space.

Direct lighting gives little vertical surface illumination, requiring the addition of perimeter lighting in business atmospheres (FIG. 11).

Concentrating downlights create sharp shadows and a theatrical atmosphere that are not appropriate to a working commercial space. They can be used in restaurants and other areas where the privacy type of atmosphere generated by limited area horizontal illumination and minimal vertical surface illumination is desired. When a lighting fixture is designed with a black cone or baffle or another device that is nonreflecting at the viewing angle, it appears dark even when lighted. It is our opinion that installations providing high-horizontal surface illumination, with no apparent source of brightness, such as those using black-cone down lights, are disturbing to our normal bright-sun-and sky orientation, and should be used cautiously and only in limited areas. This same comment, but to a lesser extent, is applicable to very-low-brightness diffusers such as the parabolic wedge type. (There, however, the unit has the redeeming characteristic of low reflected glare, which is not the case with downlights.) Stated otherwise, the psychological impression of a space without a visible source of light is gloomy and cavelike. When very low-brightness sources are used, as in VDT areas, this negative impression can be alleviated by the addition of luminous surfaces in nonreflecting areas or points of light sparkle.

In summary, then, spread-type direct lighting is suitable for general lighting, whereas concentrated direct lighting, which reduces vertical illumination, is appropriate for highlights, local and supplementary lighting, and specialized privacy-atmosphere installations.

FIG. 12 (a) Luminous ceiling. When properly designed, piping and ductwork above the suspended diffusing material are not seen and do not affect the installation's light distribution. As a rule of thumb, the strip fluorescent fixture spacing, S, should not exceed 1.5 times the fixture height above the diffusing element for uniform illumination. (b) A luminous ceiling provides low brightness and highly diffuse, uniform illuminance, generally at levels exceeding 500 lux (45 fc). It is particularly useful for specular tasks where supplemental lighting is impractical. To relieve the monotony of large, unbroken expanses of diffusers, as in (a), designers frequently use clearly defined diffuser panels, as shown.

FIG. 13 Coffer-type light sources can be manufactured (a) or architectural (b). Manufactured coffers (a) are large direct lighting fixtures generally available in a 4-ft (1.2-m) width with standard-length 4-ft (1.2-m) fluorescent lamps installed across the width, as shown. Length is variable, as required. Architectural coffers (b) can be constructed in any shape or size using hidden linear fluorescent lamps or CFL units to illuminate a white (plaster) flat or domed surface, which in turn illuminates the space beneath. Both types of coffer can give an illusion of great depth and of a floating illuminated surface. (c) Multiple coffers create a dominant architectural effect and, when designed in conjunction with skylights, can furnish soft, glare-free illumination throughout the day and night.

FIG. 14 (a) Longitudinal lines in the direction of the sight line increase apparent length, direct traffic flow, and decrease direct glare. (b) Lines perpendicular to the line of sight shorten and widen a space but also increase direct glare. (c) Diagonal lines minimize shadows and break rectangular patterns. (d) Rectangular pattern is architecturally dominant.

FIG. 15 Lighting designs for various spaces with high ceilings. The fixtures in (a) and (b) follow structural beams. (b) Floor reflection and daylight provide ceiling and wall illumination. (c) Lighting in this space was handled by recessing fixtures into the lattice ceiling pattern. Metal-halide HID units and tungsten-halogen units were used. (GTE/Sylvania, Inc.)

FIG. 16 Downlights are unobtrusive light sources. They can be spaced evenly throughout a room (a) or unevenly (b).

16. SIZE AND PATTERN OF LUMINAIRES

Because of its luminance, each luminaire or other luminous source is a point of visual attention. To the extent that luminaires are numerous, large, very bright, or arranged in striking patterns, attention is drawn to them and away from other surfaces.

Furthermore, color elements or accent lighting can be added deliberately to draw attention. Rigid rules cannot be established to cover these criteria, but examples can demonstrate the principles involved.

Luminaire size should correlate with room size and ceiling height. Fluorescent fixtures larger than 2 × 4 ft (600 × 1200 mm) should not be used in ceilings lower than 10 ft (3.1 m) unless their size is minimized by some sort of surface pattern (see FIG. 11). Trans-illuminated (luminous) ceilings (FIG. 12)

are totally lighting fixtures and require a minimum of 12 ft (3.7 m) mounting height. When they are installed below this level, particularly in large rooms, the effect is oppressive, as if the sky were lowered on us. To offset this effect, the use of colored, shaped, or dark panels is of some help. In place of a luminous ceiling, large-area, coffer-type fixtures can be utilized that give the impression of depth (FIG. 13).

To achieve the uniformity of illumination desirable for general lighting, regular spacing is required. However, various effects may be obtained within the regularity to accomplish an architectural purpose, as shown in FIG. 14a-d. The pattern of lights must never be at cross-purposes with any dominant architectural pattern; rather, it should either reinforce an architectural form or be neutral.

If a strong architectural element is absent, a dominant lighting pattern may be desirable. Conversely, a strong architectural element can either be reinforced (FIG. 15a) or utilized to carry a neutral lighting pattern (FIG. 15b,c).

FIG. 17 The large, can-shaped, surface-mounted downlights dominate the area's appearance despite the high ceiling.

Continuous row installations eliminate the dominant checkerboard effect of (closely spaced) individual luminaires and are cheaper. Coves and cornices give the ceiling a floating or lightness effect.

Geometric patterns can be used to add interest or break the monotony of large areas, such as those found in department stores. Generally, downlights are not dominant, and regularity of placement is not essential (FIG. 16). However, when down lights are surface-mounted (a generally inadvisable procedure), the result can be very far from visually neutral (FIG. 17). Nonuniform layouts with large sources create a distinct pattern problem inasmuch as they are too large to be neutral, and the nonuniformity can create visual confusion (FIG. 18). The only cure for this problem is to minimize the source brightness by using low-brightness luminaires.

In spaces where circulation is the primary "seeing task," but where isolated areas requiring greater illumination exist (such as waiting areas in transportation terminals), a perimeter lighting system with supplemental lighting as required is a viable solution (FIG. 19). In addition to their illumination function, lighting patterns are often used as directional markers. This is particularly useful in transportation terminals, where signs only partially serve this purpose. Figure 13.20 illustrates this principle, as well as the use of a lighting pattern to emphasize an architectural aspect of the space.

A frequently neglected consideration is the appearance of a luminaire when deenergized. With proper daylight and energy-conserving design, many sources can be unlighted during the normal use hours of a space. (Low-brightness sources will change least in appearance.) Cognizance of the visibility and appearance of luminaires in daylight, regardless of whether they are illuminated, from inside or outside a building can be used to advantage, as shown in Figs. 21 and 22.

FIG. 18 (a) The layout of the lighting fixtures is economical and may provide uniform illuminance. (b) The nonuniform layout lacks integration with the functions below and may not provide the illuminance needed for the task locations.

FIG. 19 Cornices, valances, and coves are luminous ceiling borders. In large rooms, suspended coves achieve a uniform ceiling brightness gradient and, when designed with a downward component or combined with supplemental lighting, as illustrated, create a pleasant, intimate atmosphere.

FIG. 20 The lines of light fixtures in the center converge optically to produce a directional flow of traffic toward the escalator. (Penn Station, New York.)

FIG. 21 (a) Lighting can be utilized as a medium to connect the inside and outside of a building. The simple maneuver of continuing the lighting pattern beyond the window visually connects the inside and outside spaces. Care must be exercised to avoid fixture placement that reflects in the glass. (b) As fixtures are readily visible even when unlit during daylight hours, their outline can be accentuated and the resultant pattern utilized as an architectural motif. (Welton Becket & Assoc.)

FIG. 22 Electric lighting fixtures can be seen through the glazing even during the daytime. (Photo by B. Stein.)

17. OTHER DESIGN CONSIDERATIONS

Because the phenomenon of vision as affected by lighting is the core of our discussion, it is appropriate to present here some miscellaneous yet important observations that are not readily subsumed under any of the major headings in our discussion.

1. As shown in Figs. 14a and 14b, the impression of room length and width can be emphasized by the direction of lines of lighting.

An even wall wash of light can also be used to shorten and widen a hallway or corridor.

2. As can be seen in Fig. 11, a light wall in the line of sight increases the room's apparent size. Therefore, both light and low-chroma, high-value paint hues can be used to expand a space (lighted walls, light colors) or contract it (unlighted walls, dark color).

3. Vertical surface illuminance should be approximately 25% to 35% of horizontal illuminance for a space to appear dimensionally undistorted. Because high luminance attracts the eye, fixtures with sparkle draw the eye away from the walls and thereby shrink the space's horizontal dimensions.

4. As shown in Figs. 13c, 14c and d, 16, and 17, luminaire patterns can be dominant (i.e., can become a focus of attention) by virtue of their size or arrangement. The same is true of wall-lighting patterns to an even greater extent, as walls are always in the direct line of sight.

Therefore, wall lighting that creates meaningless scallops, spots, irregular gradients, and points of sparkle becomes a dominant visual element. In most cases, this is neither intended nor desired.

Unscalloped, even-gradient wall illumination is readily accomplished with linear sources, with elliptical reflector lamps or luminaire reflectors, utilizing proper luminaire spacing.

5. Concentrated pools of light in an overall low-ambient-light space effectively isolate the illuminated areas from each other (i.e., define a limited specific geographic space). This can be used to advantage in restaurants and work or school areas where it is desired to define individual "territories" in a single large space.

References / Resources

ASHRAE. 2007. ANSI/ASHRAE/IESNA Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Atlanta, GA.

ASHRAE. 2008. Standard 90.1-2007 User's Manual. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Atlanta, GA.

ASHRAE. 2004. Advanced Energy Design Guide for Small Office Buildings. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Atlanta, GA.

Lightsearch.com-fixtures: USGBC. Leadership in Energy and Environmental Design (LEED). U.S. Green Building Council: usgbc.org

Prev. | Next

Home  Similar articles   top of page