NBSIR 76-1098

DAYLIGHTING OF / BUILDINGS

A Compendium and Study of its

Introduction and Control

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
v
NBSIR 76-1098

DAYLIGHTING OF BUILDINGS

A COMPENDIUM AND STUDY OF ITS

INTRODUCTION AND CONTROL

John K. Holton

Center for Building Technology, IAT

National Bureau of Standards

Washington, D C. 20234

October 1 976

Joseph R. Harris Building, Gaithersburg, Md.

courtesy of DANAC Corp.

U S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Acting Director

ABSTRACT

We can no longer ignore daylight as a valuable natural resource for building

illumination. Significant reductions in the' energy consumption of buildings

are possible by decreasing the dependence on artificial illumination and

decreasing air conditioning loads by employing methods which bring in cool

light. In order to design buildings utilizing daylight effectively there must

be an understanding of the design principles of daylighting. This requires a knowledge of illumination to meet the needs of the building users, an understanding of characteristics of daylight at the location of the building, and

imagination in developing ways to introduce and control daylight. This paper

is directed toward the identification of innovative techniques for the introduction and control of daylight. It is arranged in three sections: a Compendium, a Study section, and a Reference section. The Compendium presents

a number of state-of-the-art methods to assist the designer in successfully

employing daylight more extensively. The Study section provides more detailed

information on these methods so an increased understanding can be developed of

those which appear to have suitability in a given situation. Finally, the

References provide background for further investigation.

The Study organizes daylighting methods as they relate to three zones of a building, the perimeter, the intermediate and the deep zone. Each has different

characteristics and is suited to daylighting by different methods.

Key Words: Daylight gathering; daylighting of buildings; energy conservation;

lenses; light conduits; light control; reflectors; skylights; windows.

ACKNOWLEDGMENTS

The author wishes to thank William Allen for his counsel and extensive information on British daylighting experience, Robert Glass and Belinda Collins of

the Sensory Environment Section for valuable information on optical systems and

window research, John Jerke of the Institute for Basic Standards for information

on fiber optics, and Porter Driscoll, Chief of the Technical Evaluation and

Application Division for continued encouragement and counsel.

This study was conducted by the author during his tenure as the first AIA/NBS

Architect-in-Residence. This program, which seeks to promote improved linkage

between the architectural profession and the research community is coordinated

by the Technical Evaluation and Application Division and is jointly sponsored

by the American Institute of Architects and the National Bureau of Standards.

1
I

SI CONVERSION UNITS

In view of the present accepted practice in this country for building technology,

common U.S. units of measurement have been used throughout this document. In rec- ognition of the position of the United States as a signatory to the General

Conference on Weights and Measures, which gave official status to the metric SI

system of units in 1960, assistance is given to the reader interested in making

use of the coherent system of SI units by giving conversion factors applicable to

U.S. used in this document.

Length

1 in = 0.0254* meter

1 ft = 0.3048* meter

Area

1 in*

- = 6.4516* x 10"^ meter

2

1 ft

2 = 0.09290 meter

2

Volume

1 in2 = 1.638 x 10

-2

meter

2

1 gal (U.S. liquid) = 3.785 x 10

2 meter

2

1 liter = 1.000* x 10

-2

meter

2

Mass

_2

1 ounce-mass (avoirdupois) = 2.834 x 10 kilogram

1 pound-mass (avoirdupois) = 0.4535 kilogram

Pressure or Stress (Force/Area)

] inch of mercury (60°F) = 3.376 x 10

2 pascal

3 3

1 pound-force/inch (psi) = 6,894 x 10 pascal

Energy

1 foot-pound-force (ft-lbf) = 1.355 joule

3

1 Btu (International Table) = 1.055 x 10 joule

Power

1 watt = 1.000* x 10

7 erg/second

1 Btu/hr = 0.2930 watt

Temperature

to

c = 5/9 (to F

- 32)

Illumination

1 ft candle = 10.6 lux

*Exactly

li
CONTENTS Page

Preface 1

Compendium of Methods 2

1. Perimeter Zone 3 1.1 Windows 3 1.2 Direct Skylights 7 1.3 Reflective Skylights, Light Monitors, Clerestories. . • 9 1.4 Diffusing Panels 10

2. Intermediate Zone 11

2.1 Linear Lenses 11

2.2 Reflectors 12

3. Deep Zone 13

3.1 Tracking Reflectors 13

3.2 Concentrating Reflectors 14

3.3 Light Conduits 15

Study of Methods 16

Problem 16

Preface 16

Study

1. Perimeter Zone 17

1.1 Windows 17

1.2 Direct Skylights 18

1.3 Reflective Skylights, Light Monitors, Clerestories* • • 18

1.4 Diffusing Panels 19

2. Intermediate Zone 20

2.1 Linear Lenses 20

2.2 Reflectors 21

3. Deep Zone, 23

3.1 Tracking Reflectors 23

3.2 Concentrating Reflectors 23

3.3 Light Conduits 27

Conclusions and Recommendations 28

References 30

iii

PREFACE

Daylight is a natural resource that is readily available to provide illumination for the interior of buildings and has been used in this manner for

centuries. As buildings have grown larger -in size, the ability to constructively employ daylight has suffered from the problems of drawing it deep into

the interior. Furthermore, in an economy of inexpensive electric energy,

artificial illumination has often been less costly than equivalent daylighting.

More recently, the ever-increasing demand for environmental control has brought

a lessened use of daylight because of the seeming difficulty in its control

and the heat gain and loss through large glazed areas.

We can no longer ignore daylight as a valuable natural resource for building

illumination. Significant reductions in the energy consumption of buildings

are possible by decreasing the dependence on artificial illumination and

decreasing air conditioning loads by employing methods which bring in cool

light. In order to design buildings utilizing daylight effectively there must

be an understanding of the design principles of daylighting. This requires a knowledge of illumination to meet the needs of the building users, an understanding of characteristics of daylight at the location of the building, and

imagination in developing ways to introduce and control daylight. This paper

is directed toward the identification of innovative techniques for the introduction and control of daylight. It is arranged in three sections: a Compendium, a Study section, and a Reference section. The Compendium presents

a number of state-of-the-art methods to assist the designer in successfully

employing daylight more extensively. The Study section provides more detailed

information on these methods so an increased understanding can be developed of

those which appear to have suitability in a given situation. Finally, the

References provide background for further investigation.

This paper is concerned with the introduction and control of daylight. In

many cases the apertures used for this purpose would also normally be used

to provide a view. Though no consideration of the requirements for view have

been included here, they will of course modify the design as developed for

daylighting. Also, since the purpose is to introduce concepts and reference

sources of detailed information, no attempt is made to provide quantitative

data on daylight availability, requirements for task lighting, or computational

methods

.

A word should be added about the nature of daylight. There are two types, the

direct rays of sunlight and diffused sky light (from either a clear or cloudy

sky) . The type of light influences the effectiveness of the various methods

of introduction and control. All methods will work with sunlight but some are

not particularly effective with diffused light (notably polished reflectors

and lenses). Where a system is not well suited to diffused light, it is so noted.

1
COMPENDIUM OF METHODS

1. Perimeter Zone

- The zone where direct daylight penetration is effective.

2. Intermediate Zone

- The zone beyond the perimeter where daylight can be directed

utilizing simple, stationary, non-concentrating means.

3. Deep Zone

- The core area or areas where daylight is diminished by distance or obstructed by partitions or floors. Concentrating and beaming methods

are required in this zone.

2
1.0 PERIMETER ZONE

1.1 Windows

Windows are the commonest of all introductory methods. There are several characteristics that are of significance from a daylighting

standpoint

1. The proportion of the wall area that is opened up influences the

sheer volume of light admitted.

100% maximum

20% minimum

(

suggested

by studies

(Note: In current codes, minimums are often expressed as a % of the room floor area.)

3
1.1 Windows continued

2. The position of the window relative to the internal room surfaces

influences internal light distribution.

- high window allows

deep penetration

- low sill can allow

floor reflected light

to help balance near ceiling and wall areas

- near wall emphasizes

side reflected light

4
1.1 Windows continued

3. The position of the window relative to external surfaces

influences the ammount of light that can be admitted by the

window.

- adjacent surfaces can obstruct portions of

the sky and reduce the

amount of daylight

- adjacent surface can reflect light into

windows and increase

the amount of daylight

- some examples of adjacent surface reflective action

5
1.1 Windows continued

t

4. The number of room surfaces that contain windows will affect the

ability to provide more than mono-directional light. (True only

for diffused sky light).

- windows on adjacent

walls will light their

adjacent walls

- windows on opposite walls

give good bi-lateral

light and can light their

adjacent walls

- windows on multiple sur- faces provide opportunity

to reflect light in

several directions

6
1.2 Direct Skylights and Clerestories (non-ref lective)

- direct downlight skylights

depend on reflectivity of room surfaces (particularly

floor) for dispersion

- single side skylights or clerestories give mono- directional light unless room surfaces reflect well

- opposite side skylight

or clerestories give

two-directional light

(with diffused skylight)

- light can be more uniform

than with single side

installation
1.2 Direct Skylights and Clerestories (non-ref lective) continued

- saw tooth skylights or clerestories give mono- directional light unless room surfaces reflect

well

- light can be more uniform

than with single unit

installations

8
1.3 Reflective Skylights, Monitors, Clerestories

can gather light into

spaces otherwise difficult to illuminate

depends on reflectivity

of surface

eliminates problems of

direct sunlight if not

highly reflective

curved shape engineered

for uniform vault

illumination

Kimball Art Museum

9
1.4 Diffusing Panels

- panels of diffusing glass

or plastic give diffused

light on inside

- may be single or double

layer and can provide thermal

insulation

- glare must be controlled

- may be used on wall or roof

- prismatic glass block can re-direct light

- also can accept light from

selected directions thereby

cutting out light at undesired

times

- can be used on wall or roof

- most effective with sunlight

10
2.0 INTERMEDIATE ZONE

2.1 Linear Lenses

lenses similar to those employed

in lighting fixtures can angle

daylight upward and thus deeper

into buildings than normal ray

pattern would allow

most effective with sunlight

11
2.2 Reflectors

- reflecting surfaces can direct

daylight into the upper levels

of a room and thus to greater

depths

- may be fixed or movable

- may reduce portions of window

view

- most effective with sunlight

- multiple reflector surfaces

can be movable like blinds to

allow adjustment to track the sun or control the amount of

daylight entering

- may reduce portions of window

view

- most effective with sunlight

12
3.0 DEEP ZONE

3.1 Tracking Reflectors (Heliostat)

Useful only with sunlight

13
3.2 Concentrating Reflectors

Host effective with sunlight

The CPC is a reflector composed of parabolic elements which can concentrate

sunlight severalfold. To do this, the sun's rays must strike it directly

or within an angular range (9°) to either side. Because of this angular

acceptance, it can operate through much of the range of the sun's daily

movement. The sunlight, concentrated by reflection within the parabolic

surfaces, may be introduced into a light pipe or fibre optics bundle for

transmission into the building.

14
3.3 Light Conduits

fibre optic filaments and

bundles of glass or plastic

accept light through an angle

of approximately 39°. High

transmission losses are the

main drawback to their use. New development may reduce

this loss but currently they are not effective for daylighting use.

Light pipes of acrylic

plastic can conduct light

in many directions. They

can be larger than fibre

optics. High transmission

losses are the main drawback to use. Currently

they are not well suited

to daylighting use.

15
STUDY OF METHODS

Problem

To utilize daylight effectively for building illumination purposes the

designer must understand interior illumination requirements, the characteristics of daylight, and how daylight can be introduced and controlled.

This study section examines in greater detail the methods by which daylight

may be introduced and controlled to provide the designer with an understanding

of the performance of these methods.

Preface

The study of daylight introduction and control has been organized into

building zones (perimeter, intermediate and deep) because each presents a different set of basic geometric and spatial daylighting design conditions.

The perimeter zone, where direct daylight penetration is effective, allows

the greatest freedom because the resource can be used in its raw state. Its

very abundance, however, may introduce other problems such as glare, brightness

and thermal control.

The intermediate zone cannot be served by direct daylight and requires some means of manipulation of the light for reasonable effectiveness as illumination. However, simple, stationary, non-concentrating methods may be employed.

Consequently diffused light as well as sunlight may be utilized.

In the deep zone the problems are more severe. It is so removed from the

perimeter by distance or obstructions (by floors or partitions) that only

concentrating and beaming methods are effective. Here the principal issue is

simply introducing sufficient natural light to be effective.

As previously noted, daylight is composed of sunlight and diffused light.

The two are quite different in their behavior and hence in effective methods

for introduction and control. Sunlight is collimated light with essentially

parallel rays. It can therefore be manipulated by optical means. With diffused

light, on the other hand, the light rays that may be coming from any direction,

cannot be as effectively manipulated by optical means such as lenses and mirrors.

When considering the use of a method for introduction or control it is important to determine what type of light is to be dealt with. It is also worth

noting that diffused light is present along with sunlight on clear days and

is also present on overcast days when sunlight is not. Therefore, methods

which are useful for both types of light offer more flexibility.

Orientation is another factor to be considered as any methods depending on sunlight alone will be most effective with a maximum exposure to the sun's

path during the day. Sunlight availability should be keyed to space use recognizing the sun's morning, noon and afternoon positions. The vertical

position of the sun also varies over the year and must be considered in the

design since at certain times it may be desirable to encourage deep penetration by the sun while at other times this may be undesirable.

16
1 . 0 PERIMETER ZONE

1.1 Windows

Extensive research has been conducted on the introduction of daylight

through windows. Some very good work was done early in the century in

relation to industrial applications where light was needed for critical

tasks inside factory buildings [1.1-8], Daylight was a normal design

requirement in most buildings prior to the introduction of fluorescent

lighting in the late 30' s and 40* s. Thereafter it became less and less

critical as the provision of high level artificial illumination became

more "practical". Nevertheless, studies and analysis continued on daylighting, particularly in relation to school classrooms — probably

because their moderate room sizes and predominant occupancy during daylight hours continued to make it practical to consider the use of daylight

[1.1 - 10, 11, 12]. After the 50's, however, most daylight research

took place in England and Europe and it is from these sources that much

of our current research information is obtained [1.1 - 1.3, 4, 5, 7, 16].

A few sources from the U.S. are also useful [1.1 - 6, 15, 2.].

In order to evaluate the usefulness of windows as a light source, a number of factors must be considered. The proportion of the wall area

that is occupied by windows will influence the volume of light that it

is possible to introduce. The upper limit, of course, is the complete

window wall. This will provide maximum light volume in a given situation

but may well create other problems such as over-illumination, over-heating and psychological problems such as exposure and insecurity [1.1 - 14],

At the other end of the range are the suggested minimum window areas. Building standards and codes such as the FHA Minimum Property Standards

will set limits such as 10% of the room floor area which give a simple

recognition of the need for daylight as a function of the area of the

room. Other minimum standards have been suggested [1.1 - 9] such as 20% of the wall area based on the psychological value of a view. Light

related and view related standards are probably both useful minima, as window performance relates to both requirements.

The position of the window in the exterior wall will clearly have an effect on the nature of penetration of sunlight and to a lesser extent,

diffused daylight as well. A window located high on the wall will allow

the deepest penetration of sunlight. A low sill height allows the floor

surface to play an important role in light reflection. A window located

adjacent to a side wall improves illumination by reflection from the

adjacent wall [1.1 - 1]. Therefore, window position and related interior

surface characteristics can influence the design of windows where daylighting is an important criteria.

Conditions outside the window have received considerable attention in

the development of methods to predict daylight illumination of buildings

[1.1 - 1, 3, 15, 16], Methods to calculate the effect of light blockage

17
1.1 Windows continued

by adjacent buildings or landscape features can be used to determine the

conditions at a specific window [1.1 - 3], The light reflected from

light colored ground surfaces, roof or soffit elements or adjacent walls,

can appreciably increase the amount of daylight that enters windows

[1.1 - 15], Both the decrease of light by blockage and the increase of

light by reflection can be caused by forms located beyond as well as within the site. The effectiveness of these elements is influenced by

their orientation which can be designed to maximize the light directed

into the window.

In a given room, the number of walls in which it is possible to provide

windows will determine the nature of the light admitted. With windows

in only one wall the light will be mono-directional, and task layout in

the room must be based on this condition. If it is possible to provide

windows on two opposing walls bi-directional light will be provided,

task layout will have more flexibility, and a space twice as wide can

be illuminated [1.1 - 15]. (Note that sunlight, if it enters, will

be mono-directional.) Windows on several sides of a room can accommodate

the movement of the sun and allow good illumination over a large portion

of the day.

1.2

Direct Skylights and Clerestories (Non-ref lective)

Direct downlight skylights are quite efficient for introducing diffused

light which is incident from all directions of the sky. With sunlight

they are subject to angular cut-off problems similar to windows, but

because roof construction is frequently thick, the cut-off may be more

severe. Calculation methods for determining the amount of daylight introduced by these skylights are well developed [1.2 - 1, 2].

Single and double side skylights and clerestories are treated more like

windows located at an elevated position as they are frequently subject

to obstruction of some portions of the sky which illuminate them [1.2 -

1, 2]. Opposite side skylights or clerestories, when oriented to east

and west, allow the introduction of sunlight (at varying levels) through

almost the entire day or a balanced bi-directional diffused light which

gives greater uniformity of room illumination.

Studies also have been made which assess the potential for energy saving

through the use of skylights for daylight introduction and control

[1.2 - 3, 4, 5].

1.3

Reflective Skylights, Monitors, Clerestories

Reflective skylights, monitors and clerestories offer added degrees of

light control. By their configuration they can introduce light into

spaces that might otherwise be difficult to illuminate with daylight.

Shape design can give selective admission or cut-off of sunlight. Color

treatment of the reflective surfaces can change the nature of the re- flected light, cooling or warming it as desired. Added screens and

18
1.3 Reflective Skylights, Monitors, Clerestories continued

louvers can provide very precise control as in the Kimball Art Museum

installation where a reflector of mathematically calculated shape made

of perforated reflective metal produces almost totally uniform illumination of the curved vault [1.3 - 1, 2],

1.4 Diffusing Panels

Diffusing panels offer a range of light manipulation capabilities. In

a very fine grained diffusing panel such as transluscent plexiglass,

light which enters the panel will be diffused over a wide area. This

will convert incident sunlight to a distribution similar to diffused

light, thus providing a more uniform distribution throughout the day.

Because they distort visible rays, such panels offer increased privacy

and visual screening. Glare can be a problem with these materials,

particularly under sunlight conditions when they can become very bright;

thus, the design of their installation must be carefully considered.

Design data for these materials is generally available from plastic,

glass, and assembly (such as transluscent insulating panel) manufacturers.

Installation of prismatic glass block can provide unusual properties.

Constructed with rows of louvers cast into the two internal faces of

the block, sunlight can be selectively introduced into the room. In one installation [1.4 - 1] roof mounted block were designed to transmit

the low angle winter sun, block summer sun above 45°, and admit all

north light. These directional properties are most effective with sunlight, as diffused light can only be partially directed by the louvers.

Since the internal surfaces consist of continuous small ridges (louvers),

these units will obscure vision and are useful in providing visual

privacy.

19
2.0 INTERMEDIATE ZONE

2.1 Linear Lenses

Linear lenses are generally long rectangular lenses based on the Fresnel

principle which accept light from a point source and form it into a beam

of essentially parallel rays [2.1 - 1] (Fig. 1). By reversing this and

using sunlight as the essentially parallel ray light, a converging pattern of light may be projected into a room (Fig. 2).

Lighting Application

Fig. 1 Daylighting ApplicationFig. 2

Current applications employ a long lens, usually in a fluorescent trough

light, with a fluorescent tube mounted either in the center or to one side to provide wall wash light [2.1 - 2, 3]. Using this principle,

incident sunlight coming from an angle above the lens could be directed

deeper into a room to provide illumination on a wall or ceiling. These

lenses are normally of glass or plastic.

20
2.2 Reflectors

The use of reflective surfaces to direct daylight through windows

is a very straightforward approach to increasing the amount of light

gathered and the depth of its penetration into a room. Specular

reflective surfaces can direct a very -controlled beam into the room. They are effective with sunlight which is collimated and work well with

diffused light also. Light matte surfaces are less effective but will

provide a diffused reflected light from incident sunlight or diffused

light. Simply providing light colored surfaces on those building

elements viewed by a window will increase the level of reflective light

but specifically designed reflectors are more effective as they can be

formed to direct light where it is desired in the interior [2.2 - 1].

Keeping the reflectors clean is imperative if they are to be effective.

Design provisions must be made for easy cleaning or self cleaning by

rainfall.

An interesting variation on reflected sunlight is being researched by

a group at the University of Calif ornia, Lawrence Radiation Laboratory

[2.2 - 2], In this system narrow reflecting blinds are located at the

window to direct light deep into the room. These are adjustable and can be set to give maximum light penetration for a given angle of sunlight.

Further, they can be moved to follow the motion of the sun, direct light

to various portions of the room or control the amount of light entering.

The blinds are often used in conjunction with a highly reflective ceiling

surface to extend the depth of light penetration by additional reflection.

The reflecting blinds may be used on east, south, or west walls but different configurations provide optimum performance in each case. For east or west walls the louver system is rotated up to 45° in the plane of the

window (Fig. 3).

east or west south

Fig. 3 Fig. 4

21
2.2 Reflectors continued

On the south wall light can be collected through a much longer portion

of the day if the louver/window system is tilted out at its base up to

20° to intercept more vertical sun's rays (Fig. 4). This is because,

as the sun reaches higher noon positions in the summer months the amount

of sunlight which is incident on a vertical window is reduced. If the

louver/window system is tilted outward at the base it can intercept an appreciably larger portion of this incident light and reflect it back

into the room. Such a configuration introduces numerous practical

design and maintenance problems.

22
3 .

0

DEEP ZONE 3.1

Tracking Reflectors (Heliostat)

The heliostat is a reflecting device which has mirrors mounted on a polar axis mount similar to that of a telescope that can be moved to

track the motion of the sun. In this way a constant beam of light can

be directed into the building from a suitable location on the building

roof or face. The installation at the Hyatt Regency Chicago [3.1 - 1,

2] uses 4' diameter mirrors which provide a 7' diameter pool of light

in the lobby, 360' below the roof line. On clear days (illuminance of

10,000 ft. candles [100,000 lux]), the illuminance of the light pool

will be 1,000 - 2,000 ft. candles (10,000 - 20,000 lux).

The heliostat is motor driven and automatically controlled through a photoelectric guider to track the sun each day at its correct position

and to return to the sunrise position in the morning. A Xenon spotlight

projects onto the mirror system at night or on cloudy days and will

provide a 14' diameter pool of light of 300 ft. candles (3,000 lux).

In this installation four heliostats have been mounted on the roof and

illuminate garden areas in the hotel lobby by beaming down through the

glass roof. The intensity of light is affected by cloud cover, air

haziness and elevation of the sun. The beam is brightest when the sun

is directly overhead and drops off as it lowers in the sky. According

to the operating engineers, the intensity is quite affected by haziness

and air pollution as the blurred (partially diffused) light source does

not form as sharp and intense an image. Intensity of light at the lobby

floor is also affected by cleanliness of the roof skylight through which

it beams.

The light beam can be directed to different locations at the lobby level

by moving the upper mirror and predetermined target locations have been

included in the automatic guidance program.

3.2

Concentrating Reflectors

The Compound Parabolic Concentrator (CPC) is a shape originally designed

for high energy physics to receive faint nuclear particles coming from

widely divergent angles. Its suitability for use as a solar collector

was developed at Argonne National Laboratories [3.2 - 1, 2, 3] in pursuit

of a solar collector for heating water that could concentrate sunlight

and reduce the need for tracking movement (figure 5) . This ability also

appears to make it attractive for use with light conduits to gather and

transmit daylight into a building.

23
»*

I m

Ji J

•:

s 'H

24
AXIS OF

CPC

Fig. 7

25
3.2 Concentrating Reflectors continued

The geometry of the CPC (figure 6) is such that the higher the concentration factor the narrower the acceptance angle for light. This tradeoff is illustrated as follows:

AlsOj the wider the acceptance angle the less the CPC will need to be

moved to track the path of the sun. Thus, with an angle (9) of 7°,

sunlight can be collected for 7.4 hours without movement. With wider

angles this can be increased to take care of some seasonal movement

of the sun as well.

In order to use the capability of the CPC to accept sunlight over a con- siderable angle while itself remaining stationary it is desirable that

it be designed as long trough so that the ends do not interfere at the

lower portion of the range (figure 7).

The concentrating ability is an advantage in stepping up the intensity

of the light that is fed into the end of the light conduit, but it also

increases the heat. In a lighting application (as contrasted to solar

heating) this must be dissipated to a certain degree. Approaches to

this so far have centered on radiation by the metallic surface of the

CPC itself or the passage of air through the CPC trough. In either case

the heat could possibly be extracted for useful purposes.

One of the reasons that keying the CPC to fiber optics and light pipes

is attractive is that much of the light that departs from the base of

the CPC is at an angle that an optical fiber can accept. The acceptance

angles for these range from 32° to 42° (from vertical) for plastics

and 39° for glass.

Research is currently being conducted at Argonne National Laboratories

on the CPC and its application for solar heating and cooling. Investigations have been conducted by the author on its usefulness for daylight

introduction when combined with light conduits. These have not yet reached

a definitive stage and some questions still need to be answered, such as:

he t build-up and dissipation; losses in the light conduit; optimal balance

of concentration vs. acceptance angle; and most effective CPC geometry.

9 max (1/2 total angle) Concentration

19.5°

11.5°

5.7°

3

5

10

26
3.3 Light Conduits

Light conduits are of three basic types: (1) glass fibers, (2) plastic

fibers and, (3) plastic shapes. Glass and plastic fiber optics are

are very small strands usually gathered together to form a bundle.

Currently the maximum diameter of such bundles is 1". Light shining

down the fiber is reflected back and forth off its internal surfaces

and stays completely within the fiber as long as it does not hit the

sides at too steep an angle (called the critical angle) . Reflective

losses at the ends and losses in the fiber itself reduce the total

possible transmission. These materials have approximate properties as tabulated below.

Material Transmission loss Acceptance

(Visible light) Angle (1/2 ) (critical angle)

glass-normal (fib opt) 50% 39°

glass-medium loss (fib opt) 4%

glass-loss (fib opt) less than 1% 8°

plastic-Crof on (fib opt) 80% 32°

plastic-acrylic (light guide) 70% 50°

[Ref. 3.3 - 1, 2, 3, 4, 5, 6, 7, 12, 13]

The acceptance angle influences the care with which the incoming light

must be collimated and aligned with the end of the fiber. These ends

must also be carefully finished to avoid high reflective losses. The

fiber bundles may be curved and bent to almost any shape as long as the

radius of curvature is not too small.

Installations at a scale similar to room illumination have been demonstrated in expressway signage illumination [3.3 - 13].

Plastic light guides are generally made of acrylic plastic and perform

in the same manner as fiber optics but are usually of larger dimension.

Their minimum radius of curvature is greater than that of fiber optics

because the light channel is normally much larger.

For both plastic fiber optics and light guides of plastic there are significant limits to the temperature of operation. This means that

if concentrating reflectors provide the light source cooling may be

necessary.

For any of the light conduit materials there is a trade-off between

directionality of the entering beam and transmission loss. These must

be selected and balanced for a given application. The low and medium

loss glass materials are just being developed for the market. They are currently limited to communications applications. Their use for building

lighting has not been evaluated but it may hold promise if the balance

of properties can be keyed to effective collection methods.

27
Conclusions and Recommendations

In the perimeter zone there are many methods available for daylight introduction and control. Because daylight is so readily available this is the zone where its usefulness can be extended through more hours of the day and

into more variable conditions. For example, color control is a very interesting area of study, for frequently interior lighting is used simply to

combat the gloom of daylight on a gray day and an ability to modify this

(commonly defined) coldness could greatly extend the use of daylight. Though

many types of architectural light control are provided by the methods noted

in the study, there are numerous others that are considered to be in the area

of furnishings and interior work. Methods such as curtains, shutters, shades,

and screens all are potentially very effective methods of modulation and control. The question of light balance is also important as daylight levels

vary greatly and light levels at the rear of a room require a balancing of

daylight and artificial sources.

In the intermediate zone the major problem is to extend the usefulness of

daylight deeper into the building. Sunlight is more usable than diffused

light in this zone because it can be manipulated by optical methods. The

challenge is to simplify the devices used and to make them effective over

the longest portion of daylight hours.

In the deep zone the transmission losses of light through various devices

appears to be the primary problem to overcome. Introducing sufficient

light to be useful is by no means an easy task as current installations

demonstrate. Distribution from gathering points to dispersed use points

is also a problem involving control and losses.

In all methods of introduction and control maintenance is a critical issue.

The methods depend on reflection from light colored surfaces or transmission

through clear or translucent panels. Cleanliness, scratches, denting and

warping are all types of degradation that can have serious effect on the

usefulness of these methods. Designs must consider this fully.

When diffused light and sunlight are introduced at a single point in a room, they have directional characteristics. Even though the diffused light

radiates in many directions, it still displays a principal directionality.

In these cases the planning and arrangement of functions in the interior

spaces so illuminated must take this directionality into consideration. In

many instances this will represent a strong planning constraint.

Based o i this investigation, a number of recommended study areas may be

suggested

1) A study of the methods for daylight color control.

2) Studies of daylight control including curtains, shutters,

shades, and screens.

3) A study of the role of the human occupant of the room as a part of the complete light control circuit.

28
4) A study of light modulating glass and plastics. Chemical

additives and coatings may offer unusual means for varying

light transmission and reflection properties.

5) A study of construction methods as related to the various

light introduction methods. Many offer some unique opportunities for integrating structural and window components

with the light introduction components.

6) A study of the probable costs and the cost effectiveness of

the various methods.

7) A study of the combined energy costs of the various introduction methods. This would include manufacture and installation as well as long term operation and maintenance.

29
REFERENCES

1.1 Windows

1. H. E. Beckett, J. A. Godfrey, "Windows: Performance, Design and

Installation," Van Nostrand Reinhold, 1974.

2. B. Collins, "Windows and People," BSS 70, National Bureau of

Standards, Washington, 1975.

3. J. A. Lyons, "Principles of Natural Lighting," Elsevier, London,

1968.

4. J. W. T. Walsh, "The Science of Daylight," MacDonald, London, 1961.

5. R. G. Hopkinson, "Lighting," Her Majesty’s Stationery Office, 1963.

6. IES, "Lighting Handbook," 5th Edition, Illuminating Engineering

Society, New York, 1972.

7. R. Sheppard, H. Wright, "Building for Daylight," Allen & Unwin,

London, 1948.

8. A. Vogel, et al, "Daylighting in Multi-Story Industrial Buildings:

Transactions of the IES," February 1928.

9. E. C. Keighley, "Visual Requirements and Reduced Fenestration In

Offices," Journal of Building Science , _8, 1973.

10. R. Hammel, L. Johnson, "Manufactured Light vs. Daylight for Schoolrooms," Illuminating Engineering , July 1956.

11. D. B. Harmon, "The Coordinated Classroom," Research Department,

Pittsburgh Corning Corp., February 1950.

12. A. Baas, "Luminous Environments," Educational Facilities Review Series , Educational Resources Information Center, Eugene, Oregon, March 1973.

13. R. G. Hopkinson, "Glare from Daylighting in Buildings," Applied

Ergonomics , December 1972.

14. T. A. Markus, A. Gray, "Windows in Low-Rise, High Density Housing—

The Psychological Significance of Sunshine, Daylight, View, and Visual

Privacy," (Proceedings of CIE Conference on "Windows and Their Function

in Architectural Design," October 1973.)

15. "Recommended Practice of Daylighting," Illuminating Engineering Society,

August 1962.

16. "Planning for Daylight and Sunlight," Ministry of Housing and Local

Government, Her Majesty's Stationery Office, 1964.

30
1.2 Direct Skylight and Clerestories

1. "Recommended Practice of Daylighting," Illuminating Engineering

Society, August 1962.

2. J. A. Lynes, "Principles of Natural Lighting," Elsevier, London,

1968.

3. "Skylight Energy Performance," Center for Industrial and

Institutional Development, University of New Hampshire, October

1975.

4. "Systems Analysis for Skylight Energy Performance," University of

New Hampshire, October 1975.

5. J. B. Murdoch, "A Procedure for Calculating the Saving in Lighting

Energy from the Use of Skylights," Electrical Engineering Dept.,

University of New Hampshire, December 1975.

1.3 Reflective Skylights, Monitors, Clerestories

1. "Lighting Starts with Daylight," Progressive Architecture , September

1973, p. 83-85.

2. "Daylight," Interiors , November 1952.

1.4 Diffusing Panels

1. "Daylight Research House," Architectural Record , November 1956.

2.1 Linear Lenses

1. IES, "Lighting Handbook," 4th Edition, Illuminating Engineering

Society, New York, p. 6-9.

2. Pub'n. ASG 7308, ASG Industries

3. A-Symmetric I, Product News, Plaskolite, Inc.

4. J. W. T. Walsh, "The Science of Daylight," MacDonald, London, 1961,

p. 97.

2.2 Reflectors

1. Interiors , November 1952, p. 97.

2. "Beam Daylighting of Buildings," R. Farrell, E. Arens, A. Rosenfeld,

Summer Study on More Efficient Utilization of Energy in Buildings,

University of California, Berkeley, California, 7/17/75.

31
3.1

Tracking Reflectors (Heliostat)

1. "Skytrack Solar Illuminators for Hyatt Regency Chicago Hotel,"

Technical Release, A. Epstein & Sons, Inc., Engineers, Architects,

Chicago

.

2. Skytrack Illuminator System - Drawings, Specifications; Carson

Astronomical Instruments, Inc., Valencia, California, September 13,

1974.

3.2 Concentrating Reflectors

1. "Solar Concentrators of a Novel Design," R. Winston, Enrico Fermi

Institute, University of Chicago, March 1974, EFI 74-21.

2. "Solar Concentrators of a Novel Design," Talk to International Solar

Energy Society, August 20, 1974, V. J. Sevich, Argonne National

Laboratories

.

3. "Optical and Thermal Properties of Compound Parabolic Concentrators,"

A. Rabl, Argonne National Laboratories, February 1975.

3.3 Light Conduits

1. "Flexible Glass Fiber Optics," Corning Glass Works, Bulletin 3,

July 1969.

2. W. Bart Bielawski, "Low Loss Optical Waveguides Current Status,"

Optical Systems Design, April 1973, p. 22-28.

3. F. L. Thiel, W. B. Bielawski, "Optical Waveguides Look Brighter Than

Ever," Electronics, March 21, 1972.

4. R. D. Compton, "Fiber Optics Puts It All Together," Electro-Optical

Systems Design, June 1975.

5. W. P. Siegmund, "Fiber Optics, Principles, Properties, and Design

Considerations," American Optical Corp., 1970.

6. W. Cerny, Corning Glass Works, private communication.

7. "Designing with DuPont Crofon Fiber Optics," E. I. DuPont de Nemours

publication A-76182.

8. A. M. Blumenfeld, S. E. Jones, "Parts that Glow," Design, October 29,

1959.

9. "Plexiglass Design Fabrication and Molding Data," Bulletin No. PL-177,

Rohm & Haas, November 1967.

10.

"Plexiglass Molding Powder Design Manual," PL-897b, Rohm & Haas,

October 1973.

32
11. "Plexiglass Sheet Physical Properties," PL-229m, Rohm & Haas.

12. "Transmittance and Exposure Stability of Colorless Plexiglass

Cast Sheet," PL-53h, Rohm & Haas, June 1973.

13. "This is Valtec 1974," "Fiberoptics , " Valtec Corp., 1974.

USCOMM-NBS-DC 33
NBS-] 14A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLIC ATION OR RLPORT NO.

NBSIR 76-1098

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTIT

Daylighting of Buildings

A Compendium and Study of its Introduction and Control

5. Publication Date October 1976

6. Performing Organization Code

7. AUTHOR(S)

John K. Holton

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Bureau of Standards

Department of Commerce

Washington, D.C. 20234

13. Type of Report & Period Covered

Final Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary ol most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

We can no longer ignore daylight as a valuable natural resource for building

illumination. Significant reductions in the energy consumption of buildings are possible by decreasing the dependence on artificial illumination and decreasing air

conditioning loads by employing methods which bring in cool light. In order to design

buildings utilizing daylight effectively there must by an understanding of the design

principles of day lighting. This requires a knowledge of illumination to meet the needs

of the building users, an understanding of characteristics of daylight at the location

of the building, and imagination in developing ways to introduce and control daylight.

This paper is directed toward the identification of innovative techniques for the intro

duction and control of daylight. It is arranged in three sections: a Compendium, a Study section, and a Reference section. The Compendium presents a number of state-ofthe-art methods to assist the designer in successfully employing daylight more

extensively. The Study section provides more detailed information on these methods

so an increased understanding can be developed of those which appear to have suitabilit

in a given situation. Finally, the References provide background for further

investigation.

The Study organizes daylighting methods as they relate to three zones of a building,

the perimeter, the intermediate and the deep zone. Each has different characteristics

and is suited to daylighting by different methods.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first tetter of the first key word unless a proper name; separated by semicolons

)

Daylight gathering; Daylighting of buildings; Energy conservation; Lenses; Light

conduits; Light control; Reflectors; Skylights; Windows.

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| Order From Sup. of Doc., U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C13

[4? Ord er From National Technical Information Service (NTIS)

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