REFERENCE

NBS

PUBLICATIONS

R 85-3251

A 1 1 ID 5 SA3M41

oKy Luminance and

Illuminance

Beam

S. Treado

W. Remmert

J. Bean

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

February 1986

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

*QC

100

• U56

85-3251

1986

C.3

NBSIR 85-3251

SKY LUMINANCE AND DIRECT BEAM

ILLUMINANCE

S. Treado

W. Remmert

J. Bean

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

February 1986

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

ABSTRACT

Measurements of sky illuminance, sky luminance, direct beam illuminance and

direct beam irradiance are analyzed and discussed. The database consisted of

an annual set of averaged hourly measurements made at the National Bureau of

Standards, Gaithersburg, Maryland (77° west longitude, 39,1° north latitude).

The relationship between diffuse sky illuminance and luminance of selected

portions of the sky dome is examined. Measured sky luminances are compared

to luminances calculated using equations for three standard sky types - clear,

partly cloudy and overcast. The percentages of diffuse and direct beam to

total illuminance and irradiance incident upon a surface are presented for four

vertical orientations and a horizontal orientation. This analysis enables

evaluation of the relative daylighting potential of diffuse and direct daylighting and solar heating strategies.

The results indicate that the luminance distribution of actual skies varies

considerably from the standard skies. The percentage of global illuminance

which was direct beam was greatest for a south-facing surface ( —51 percent)

and ranged from approximately 49 percent for a horizontal surface to less than

6 percent for a north-facing surface.

Keywords: Daylighting, diffuse radiation, direct beam radiation, illuminance,

irradiance, luminance, sky condition

iii
TABLE OF CONTENTS

Page

ABSTRACT iii

LIST OF TABLES v

LIST OF FIGURES vi

1 . INTRODUCTION 1

2. DESCRIPTION OF MEASUREMENTS 2

3. RESULTS 4

3.1 Sky Luminance 4

3.1.1 Zenith Luminance 5 3.1.2 Luminance Distribution 7

3.2 Direct Beam Illuminance and Irradiance 9

4. CONCLUSIONS 12

5. REFERENCES 13

iv
LIST OF TABLES

Page

Table 1. Determination of Sky Condition 6

Table 2. Zenith Luminance (y) versus diffuse horizontal illuminance

(x) as a function of sky type and solar altitude 7

Table 3. Point luminance factor (y) as a function of cloud ratio

(x) - best fit regression equations 9

Table 4 . Totals and percentages of direct and diffuse illuminance

and irradiance 11

v
LIST OF FIGURES

Page

Figure 1. Luminance sensor array 15

Figure 2. Zenith luminance factor as a function of cloud ratio (CR) 16

Figure 3. Zenith luminance factor as a function of atmospheric

extinction coefficient (El) 17

Figure 4. Zenith luminance (LZ) as a function of diffuse horizontal

illuminance (ID), for all sky types 18

Figure 5. Zenith luminance (LZ) as a function of diffuse horizontal

illuminance (ID) for apparent clear skies 19

Figure 6. Zenith luminance (LZ) as a function of diffuse horizontal

illuminance (ID) for apparent partly cloudy skies 20

Figure 7. Zenith luminance (LZ) as a function of diffuse horizontal

illuminance (ID) for apparent overcast skies 21

Figure 8a. Zenith luminance (LZ) versus diffuse horizontal illuminance for apparent clear skies, low solar altitude 22

Figure 8b. Zenith luminance (LZ) versus diffuse horizontal illuminance for apparent clear skies, medium solar altitude 23

Figure 8c. Zenith luminance (LZ) versus diffuse horizontal illuminance for apparent clear skies, high solar altitude 24

Figure 9a. Zenith luminance (LZ) versus diffuse horizontal illuminance for apparent partly cloudy skies, low solar altitude 25

Figure 9b. Zenith luminance (LZ) versus diffuse horizontal illuminance for apparent partly cloudy skies, medium solar

altitude 26

Figure 9c. Zenith luminance (LZ) versus diffuse horizontal illuminance for apparent partly cloudy skies, high solar altitude 27

Figure 10a. Zenith luminance (LZ) versus diffuse horizontal illuminance for apparent overcast skies, low solar altitude 28

Figure 10b. Zenith luminance (LZ) versus diffuse horizontal illuminance for apparent overcast skies, medium solar altitude .. 29

Figure 10c. Zenith luminance (LZ) versus diffuse horizontal illuminance for apparent overcast skies, high solar altitude .... 30

vi
LIST OF FIGURES (Con't.)

Page

Figure 11. North luminance ratio as a function of cloud ratio ....... 31

Figure 12. North luminance ratio as a function of atmospheric

extinction coefficient 32

Figure 13. East luminance ratio as a function of cloud ratio 33

Figure 14. East luminance ratio as a function of atmospheric

extinction coefficient 34

Figure 15. South luminance ratio as a function of cloud ratio 35

Figure 16. South luminance ratio as a function of atmospheric

extinction coefficient 36

Figure 17. West luminance ratio as a function of cloud ratio 37

Figure 18. West luminance ratio as a function of atmospheric

extinction coefficient 38

Figure 19. North point luminance factor as a function of cloud ratio. 39

Figure 20. East point luminance factor as a function of cloud ratio . 40

Figure 21. South point luminance factor as a function of cloud ratio. 41

Figure 22. West point luminance factor as a function of cloud ratio . 42

vii

1 . INTRODUCTION

Many studies have demonstrated the effectiveness of daylight utilization in

minimizing the energy requirements for building lighting and space cooling,

and in many cases, total annual building energy requirements [1-5]. Effective

daylighting design requires consideration of many factors, including illumination and visibility requirements, thermal and solar control strategies,

occupant comfort, and integration of the fenestration with the building envelope

and HVAC system. Procedures have been developed for calculating interior

illuminance levels and incorporating the effect of daylighting on building

energy requirements into the overall building design procedure. Some of these

procedures are single-point, design day type techniques, while others attempt

to determine seasonally adjusted daylighting effects [6-8] . The computer

simulation of annual building energy use on an hourly basis has been used

extensively to evaluate the energy impact of daylighting [9, 10].

The common starting point for almost all daylighting calculation procedures is

the specification of the sky and solar conditions. These include some combination of irradiances, illuminances and luminances. Various correlations are used to assemble the conditions needed for the daylighting calculation from

the available information. While most daylighting predictions require sky

luminance or illuminance data, this information is usually only available

for some standard sky conditions or limited time sequence. The weather information available to hourly building energy simulation computer programs consists largely of solar radiation levels, either direct beam and diffuse on a horizontal surface or some other related combination [11]. Luminous efficacy

values are used to convert from irradiance to illuminance, and luminance

distribution equations are used to predict the sky luminance distribution

associated with the actual weather conditions [12]. Luminance distribution

equations are a means to extrapolate from a concise weather description to

a complete sky luminance distribution. Once the sky luminance distribution is

known, it is relatively easy to complete the interior illuminance calculation,

since that requires standard radiation exchange computations. The major difficulty lies in determining the actual sky luminance distribution.

As a step towards examining the luminance distribution of actual skies, the

measured luminances of five portions of the sky are analyzed. The luminance of

each 15 degree f ield-of-view of the sky is compared to zenith luminance and

horizontal diffuse illuminance. The ratio of zenith luminance to horizontal

diffuse luminance is known as the zenith luminance factor, while the ratio of

the luminance at some sky point to the zenith luminance is known as the point

luminance factor, which is comparable to the solution of a luminance distribution

equation for some instant in time. The influence of sky condition and solar

position on these ratios is examined, and the ratios are compared to predicted

values generated from equations for standard skies.

Another important daylighting issue is the potential usefulness of direct bean

illumination in providing interior daylight. In some calculation procedures,

direct beam daylight is assumed to be blocked by a shading device; in other

1
cases the direct beam daylight is included in the calculations. While daylighting with direct beam illumination usually requires a special fenestration

design (light shelves, light pipes, reflectors), the daylighting potential of

direct beam utilization for a particular orientation can be examined by comparing

the relative magnitudes of the direct beam and diffuse components of the illuminance incident upon a surface for that orientation. Examination of the available

direct beam illumination levels enables the determination of the maximum potential

daylighting benefits of direct beam utilization.

2
2. DESCRIPTION OF MEASUREMENTS

The measurement data were collected at the NBS Daylighting Laboratory in

Gaithersburg, Maryland (suburb of Washington, D.C., 77° west longitude, 39.1°

north latitude). As part of a larger data collection program, the following

luminance, illuminance and irradiance measurements were made.

a) Sky Luminance - Using photovoltaic silicon cells with photometric filters

and a 15° f ield-of-view, measurements were made for five areas of the sky,

centered on zenith and 42° elevation due north, east, south and west. A

photograph of the luminance sensor array is shown in Figure 1.

b) Illuminance - Using photovoltaic silicon cells with photometric filters and

cosine diffusers with hemispherical f ield-of-view, measurements were made

for five orientations, namely horizontal and north, east, south and westfacing vertical surfaces. Diffuse horizontal illuminance was measured

using a shading band.

c) Irradiance - Using photovoltaic silicon cells with radiometric filters and

cosine diffusers with hemispherical f ield-of-view for vertical orientations,

and thermopile-type pyranometers for horizontal orientations, measurements

were made for five orientations, namely irradiance for horizontal and north

east, south and west-facing vertical surfaces. Diffuse horizontal irradiance

was measured using a shading band.

A complete description of the measurement system and uncertainty analysis can

be found in a previous report [13]. The data analyzed in this report consist

of average hourly values, based on multiple readings during each hour, for a composite, one-year period, assembled from measurements made in 1982, 1983 and

1984, with the bulk of the data being from the first two years. The estimated

maximum measurement uncertainty is ±3% for luminance, ±8% for vertical

irradiance, ±3% for vertical and global horizontal illuminance, ±5% for

diffuse horizontal illuminance, ±4% for global horizontal irradiance and ±6%

for diffuse horizontal irradiance.

3
3. RESULTS

3.1 SKY LUMINANCE

The illuminance at a particular point on a surface is determined by the

luminance pattern of the surrounding environment. Thus, for an exposed exterior

horizontal surface, the luminance of the sky dome controls the illuminance at

the surface. Since, in most instances, the sky luminance is non-uniform, the

contribution of each portion of the sky must be separately determined and the

results combined to obtain the illuminance on the surface. This type of

calculation involves the luminance (or related existance) of the particular

portion of the sky, and an area integral involving the geometry relating the

portion of the sky and the point on the surface.

Formulas have been developed to model the luminance distribution of standard

skies. A standard clear sky distribution function was developed by Kittler and

adopted by the CIE in 1973 [14]. Moon and Spencer developed an empirical

relation for a standard overcast sky, which was adopted by the CIE in 1955

[15], Pierpoint developed a luminance distribution equation for partly cloudy

skies in a form similar to the clear sky distribution [16]. For the standard

overcast sky the ratio of point luminance to zenith luminance is assumed to be

a function of the elevation of the sky point only. The partly cloudy and clear

equations also include the angle between the sun and the sky points. The

important fact is that if the sky is assumed to conform to one of the standard

luminance distributions, a unique relationship exists between zenith luminance

(LZ) and diffuse horizontal illuminance (Ed), and between point luminance and

zenith luminance, for any fixed set of solar and point angles. The ratio of

zenith luminance to diffuse horizontal illuminance is frequently called the

zenith luminance factor (ZL), while the ratio of point luminance to zenith

luminance is equivalent to the solution of the luminance distribution equation

for that set of sun and point angles (point luminance factor PL). Thus, for

standard skies, luminance at a sky point is related to diffuse horizontal

illuminance by the product of the zenith luminance factor and the point luminance

factor. For convenience, this product has been labeled the combined luminance

factor (CL).

Actual skies rarely correspond to standard skies, due to random cloud patterns

and mixed sky conditions. However, the exact prediction of luminance distributions of dynamic real skies is significantly beyond the reach of current

measurement and modeling capabilities. If sky luminance values are being used

to evaluate the daylighting performance of fenestration systems, what is required

is a ’fair* method of comparing design options on a consistent basis. Thus, if

it is not possible to predict the exact sky luminance distributions over a time

interval such as a year, an acceptable substitute would be a set of sky luminance

values which are representative of the actual luminances in the manner with

which they influence the daylighting performance of fenestration systems.

To address the issue of the relationship between the illuminance and sky

luminance distribution of actual skies, and their comparison with standard sky

luminance distributions, the measured sky luminance and illuminance data were analyzed. The measured zenith luminance factors were determined by the ratio

4
of zenith luminance to diffuse horizontal illuminance; the measured point

luminance factors were determined by the ratio of each of the point luminances

to the zenith luminance and the combined luminance factors were determined from

the ratio of point luminance to diffuse horizontal illuminance. Although the

individual luminance measurements are described as being point luminances, they

actually represent a finite area encompaged by a 15° field of view.

3.1.1 Zenith Luminance

Figure 2 presents zenith luminance factor as a function of cloud ratio (defined

as diffuse horizontal irradiance divided by global horizontal irradiance)

.

Figure 3 is a similar plot with atmospheric extinction coefficient as the

abscissa. These figures demonstrate the range in zenith luminance factor

values which occurred during the year. By comparison, for solar altitudes less

than 75 degrees, the zenith luminance factors for the standard skies range

from 0.144 to 0.540 for the clear sky, to 0.201 to 0.456 for the partly cloudy

sky, to a constant .409 for the overcast sky. (These zenith luminance factors

are multiplied by tt for conventional units of footcandle and foot lambert) . In figures 2 and 3 apparent overcast skies are represented by cloud ratios

near one or extinction coefficients near six. The term apparent is used

because extinction coefficient is determined by the level of direct beam irradiance and thus gives no information about the remainder of the sky dome, and

cloud ratio is determined by the horizontal diffuse and direct irradiances,

which are not explicit indicators of sky condition.

While extinction coefficient and cloud ratio are not explicit indicators of sky

condition, they are frequently the only parameters related to sky condition

which are available when attempting to reconstruct the luminance distributions

of the sky vault from standard weather data. In lieu of direct measurements of

sky luminance distribution, an estimate of sky condition followed by a prediction of the associated sky luminance distribution are the only avenues available for the daylighting designer.

A considerable range in zenith luminance factor values is seen for the apparent

overcast skies as compared to the standard overcast sky.

For the clear and partly cloudy sky conditions, represented by the low and

moderate cloud ratios and extinction coefficients, considerable variation in

zenith luminance factors would be expected, due to the dependence on solar

altitude. This expected result is evident in the figures.

In order to examine in greater detail the relationship between zenith luminance

and diffuse horizontal illuminance, and the influence of sky condition and

solar altitude on the zenith luminance factor, a series of plots was generated

displaying zenith luminance as a function of diffuse horizontal illuminance for

various ranges of solar altitude and atmospheric extinction coefficient. Figure

4 presents zenith luminance as a function of diffuse illuminance for all of the

data points. The slope of the line connecting each data point with the origin

is equal to the zenith luminance factor. Figures 5, 6 and 7 show the data from

figure 4 subdivided into apparent sky condition types clear, partly cloudy and

overcast, respectively. The determination of apparent sky condition was based

5
on the measured extinction coefficient according to the following table.

Table 1 Determination of Sky Condition

Range of Extinction

Coefficient (El) Assumed Sky Type

El 0.4

0 .4 _< El _< 1 .6

1.6 < El

"

Clear

Partly Cloudy

Overcast

These figures illustrate several points. For apparent clear skies, figure 5,

zenith luminance tends to follow two bands. One band has a low slope, meaning

low zenith luminance factor, while the second band has a steep slope and a corresponding large zenith luminance factor. The probable explanation for this

behavior is associated with the difficulty in determining the sky condition

simply on the basis of the direct beam illuminance, which is used to compute

the atmospheric extinction coefficient. The restriction of low extinction

coefficient allows partly cloudy sky data to be included with the actual clear

sky data, if the sun is not obstructed by clouds. Thus the high zenith luminance

values in figure 5 are probably due to overhead clouds occuring in an otherwise

clear sky. Isolated overhead clouds would be brighter than clear blue sky.

The low zenith luminance values are probably more uniformly clear.

Figure 6 presents similar data for partly cloudy skies. A significant amount

of spread is seen in the data, with generally larger zenith luminance factors

than for clear skies. The overcast sky data are shown in figure 7. This figure

shows a clear trend of high zenith luminance factors, with the typical values

for the apparent overcast skies being approximately 50 percent greater than for

the standard overcast sky. A comparison of figures 5, 6 and 7 shows that the

data for the partly cloudy skies are bounded by the clear and overcast sky data.

Some of the scatter in figures 6 and 7 can be explained by considering the

effect of solar altitude in addition to the effect of sky condition. When the

solar disk is at least partly visible, as is the case for clear and partly

cloudy skies, for a fixed level of diffuse illuminance the zenith luminance is

greater if the solar altitude is greater. Examination of the zenith luminance

factors for the standard clear and partly cloudy sky shows that for solar

altitudes less than 30 degress the zenith luminance factors are fairly constant.

Three ranges of solar altitude were chosen, namely 20 to 30 degrees, 30 to 50

degrees and greater than 50. Solar altitudes less than 20 degrees were excluded

to control measurement uncertainty associated with the determination of diffuse

horizontal illuminance and atmospheric extinction coefficient. Very few solar

altitudes exceeded 70 degrees.

The relationship between zenith luminance and diffuse horizontal illuminance as

a function of solar altitude and sky condition is shown in figures 8a, b and c for clear skies, figures 9a, b and c for partly cloudy skies and figures 10a,

b and c for overcast skies. The effect of solar altitude is particularly

6
apparent for clear skies In figure 8c. Also plotted on the figures are lines

corresponding to the standard skies at the extremes of the solar altitude range,

the linear best fit through the measured data points and the linear best fit

for the measured data forced through zero. In actuality, zenith luminance must

be zero when diffuse horizontal illuminance is zero. The measured and standard

sky zenith luminances are in fairly good agreement for low solar altitudes,

figure 8a, although a significant number of measured values are less than the

minimum expected according to the standard sky relations. In figure 8b, more

scatter is seen in the measurements, and the best-fit-through-zero line is

bracketed by the standard sky lines. However, the majority of the measurements

fall below the standard sky ranges, with a smaller number of high luminance

readings causing the slope of the best-fit-through-zero line to be greater than

otherwise would have occurred. The high luminance measurements are probably due

to non-clear sky conditions near zenith. (i.e., white clouds instead of blue

sky). The same effects are seen in figure 8c for the high solar altitude range.

The greatest variations are expected in this range, and the best-fit- throughzero line is less than the standard sky range.

The partly cloudy skies, figures 9a, b and c, exhibit the greatest variation,

both in the magnitudes of the zenith luminances, and the comparison of the

measurements to the standard skies. However, the best-fit- through-zero lines

do fall within or near the standard sky ranges. The large variations are probably due to the uncertain nature of the partly cloudy sky, where random

cloud patterns produce unpredictable luminance distributions.

The overcast skies, figures 10a, b and c, are the most predictable, and exhibit

the least scatter of the three sky types. By definition, the ratio of zenith

luminance to diffuse horizontal illuminance is a constant (0.409 for SI units).

The measured zenith luminances are almost always less than the standard sky

values, with the slope of the best-fit-through-zero lines being 44 to 26 percent

lower than the standard sky lines.

The equations for the standard sky and regression fit lines are listed in

Table 2.

Table 2 Zenith Luminance (y) versus diffuse horizontal

illuminance (x) as a function of sky type

and solar altitude

Atmospheric

Extinction

Coefficient

Solar Altitude

Range

Standard Sky

Low Range High Range

Best Fit

through Zero unforced

<0.4 20 to 30° 0.142x 0.156x 0.118x 305+0. 0943x

<0.4 30 to 50° 0 . 156x 0.234x 0.190x 3040-0. 0268x

<0.4 >50° 0.234x 0.540x 0.400x 4012+0. 162x

0 .4 to 1.6 20 to 30° 0.214x 0.230x 0 . 186x 1352+0. 115x

0.4 to 1.6 30 to 50° 0.230x 0.292x 0.213x 2112+0. 134x

0.4 to 1 .6 >50° 0.292x 0.456x 0.297x 5681+0. 117x

>1.6 20 to 30° 0.409x 0 .409x 0.232x 1033+0. 168x

>1.6 30 to 50° 0.409x 0.409x 0.260x 1279+0. 206x

>1.6 >50 ° 0.409x 0.409x 0 .301x 2028+ ,237x

7
3.1.2 Luminance Distribution

Some of the aspects of sky luminance distribution were investigated by analyzing

luminance measurements of five 15 degree f ield-of-view portions of the sky dome.

These five locations consisted of zenith and north, east, west and south-facing

luminances, although the measurement represents an average luminance over a discrete portion of the sky). The ratio of point luminance to zenith luminance

(the equivalent of the solution to a luminance distribution equation at some instant in time) is examined as a function of sky condition by plotting the

measured luminance ratios against cloud ratio and atmospheric extinction coefficient. Luminance ratios are plotted only when no direct beam radiation was incident upon a vertical surface facing the same direction as the luminance

sensor.

The ratio of north luminance to zenith luminance is presented as a function of

cloud ratio in figure 11, and as a function of atmospheric extinction coefficient in figure 12. The vast majority of the luminance ratios are less than

two, with no observable variation with cloud ratio. The variations in luminance

ratio values are due to differences in solar position and sky condition. There

is no convenient comparison of predicted versus measured luminance ratios,

since the standard luminance distribution equations would have to be solved for

the exact solar position and sky type to compare against each measured value.

However, it is clear that the measured luminance ratios do not vary widely. In

figure 12, the luminance ratios exhibit the greatest variation for low atmospheric

extinction coefficient values, and remain within a fairly narrow band for

extinction coefficients between two and five, before showing greater variation

at an extinction coefficient of six. It should be noted that due to the nature

of the exponential relationship betweeen direct beam irradiance and atmospheric

extinction coefficient, values can extend out to infinity. All extinction

coefficients greater than six were set to that value, so the luminance ratios

at extinction coefficient of six actually include values which would have been

plotted off of the graph.

The luminance ratios for the other three orientations are very similar to north.

The ratio of east point luminance to zenith luminance is plotted in figure 13 as a function of cloud ratio, and in figure 14 as a function of atmospheric

extinction coefficient. South luminance ratios are presented in figures 15 and

16, and west luminance ratios in figures 17 and 18.

The possibility of linking the luminance of a sky point directly to the diffuse

horizontal illuminance was investigated by plotting the ratio of point luminance

to diffuse illuminance as a function of cloud ratio. If the ratio of point

luminance to diffuse illuminance could be predicted based on solar and sky

conditions, the separate determination of zenith luminance and luminance

distribution would not be required. Tbe ratio of point luminance to diffuse

horizontal illuminance can be termed the point luminance factor. Point luminance

factors are plotted in figures 19, 20, 21 and 22 for north, east, south and

west sky points, respectively. A linear least-squares regression line is also

plotted for each figure. Again, this data set excludes measurements taken when

direct beam solar radiation was incident upon a vertical surface facing the

8
same direction as the sensor. Thus, the data presented represent solar

positions that are not near sky points, the closest allowable sky point to sun angle being 90 degrees, or overcast sky conditions. Additional analysis would

be required to allow inclusion of measurements taken when the solar disk was visible within 90 degrees of the sky point, but not closer than 15 degrees. If

the solar disk is visible within the field of view of the luminance sensor, the

measurement would not be indicative of sky luminance, but an average sun and

sky luminance.

Figures 19 through 22 are similar in appearance, as are the linear best-fit

lines. In general, the point luminance factors remain in a narrow range between

0.1 and 0.25, increasing with higher cloud ratios. The greatest variation

occurs for the highest cloud ratios. A linear expression may not be the best

way to relate point luminance factor to cloud ratio, since regression lines

fall above the measurements in the 0.6 to 0.9 cloud ratio range. Other relations

are being pursued. Table 3 lists the equations for the linear best-fit lines

from each figure.

Table 3

Point luminance factor (y) as a function

of cloud ratio (x) best-fit linear regression equation

Sky Point Best Fit to Bata

North

East

South

West

y

= 0.0938 + 0.262x

y

= 0.0970 + 0.264x

y

= 0.0876 + 0.220x

y

= 0.0904 + 0.245x

3.2 DIRECT BEAM ILLUMINANCE AND IRRADIANCE

The potential benefits of direct beam daylighting or direct gain passive solar

strategies were examined by evaluating the available direct beam illuminance and

irradiance for horizontal and vertical surfaces. Direct horizontal illuminance

and irradiance were determined from measurements of the diffuse and global

horizontal components, by subtracting diffuse from global. Direct beam illuminances and irradiances on north, east, south and west-facing surfaces were determined by computing direct normal from direct horizontal and adjusting for

the angles of incidence on the vertical surfaces using the known solar position.

Based on the measurements, annual totals of global, direct and diffuse illuminance

(lux»hr) and irradiance (wm“2»hr) were computed. The annual total ratios of

direct to global and diffuse to global illuminance and irradiance were also

computed. In addition, several hourly averages were computed. For each surface

orientation the following ratios were computed, where:

D = direct

d = diffuse

G = global

D/G = direct/global

D/A = direct/available

G/A = global/available

9
A = available = d+DN

DN = direct normal

d/A = dif fuse/available

d/G = diffuse/global

D/DN = direct/direct normal

Table 4 presents the totals and percentages of direct beam and diffuse illumi- nance and irradiance for horizontal and vertical surfaces. Of the annual

totals, direct illuminance ranged from 41 percent for west vertical to 51 percent

for sourth vertical, except for the north vertical value of less than 6 percent.

The similar irradiance percentages were slightly higher. In general, except

for the south and north-facing surfaces, the percentage of diffuse illuminance

was slightly greater than direct, while the percentage of direct irradiance was slightly greater than diffuse. The south-facing surface direct percentage was

51 for illuminance and 53 for irradiance.

Examination of the annual percentages of direct beam and diffuse sky illumi- nances and irradiances for Gaithersburg, Maryland area indicates that the

daylighting and solar gain potential from direct beam utilization is approximately

equal to the potential of diffuse utilization except for a north-facing surface.

That is, the total amount of available daylight and sunlight is about half

direct beam and half diffuse sky. While it is more difficult to make use of

direct beam daylight than diffuse daylight, due to the need to reflect and

diffuse the direct beam enabling tbe daylight to be distributed throughout the

interior space, significant potential daylighting benefits from direct beam

utilization are apparent. Conversely, half of the available solar energy is

diffuse, indicating the importance of designing a solar gain facility to capture

both diffuse and direct solar radiation.

Another set of ratios was also calculated for each surface orientation. The

annual average hourly ratios, as described above, are shown in Table 4. These

differ from the total annual ratios in the following manner. The total annual

ratios were determined by summing the hourly values over the year and computing

the quotient of the appropriate quantities. The average hourly ratios were determined by computing the appropriate ratio each hour and averaging over the

year. The average hourly percentages of direct illuminance and irradiance are

less than the corresponding annual total percentages, ranging from 20 to 34

percent, except for the north-facing value of about 6 percent. The direct

percent of available illuminance ranges from 12 to 22 percent, except for the

north-facing value of about one percent. The diffuse percentage are much

greater, ranging from 60 to 96 percent. The reason for the significant differences between the annual total ratios and the annual average hourly ratios is

that high direct beam levels occur only a small percentage of the time. Thus

the hourly average direct beam contribution is less than the total direct beam

contribution, since each hourly ratio is given equal weight in computing the

average hourly percentage. The hourly ratios provide a lower bound for direct

beam daylighting benefits, since very high daylight levels might cause over- illumination, reducing the efficiency of daylight as a light source.

The ratios of direct to direct normal indicate the ability of a particular

surface to capture direct normal illuminance and irradiance, by virtue of its

orientation. The horizontal surface captures over 50 percent of the direct

normal energy while the remaining surfaces range from three percent (north)

to 35 percent (south). For the year, direct normal illuminance was 45 percent

of available, and direct normal irradiance was 46 percent of available.

10
Total

Total

Total

Total

Total

Annual

Average

Hourly

Ratios

Annual

Annual

Annual

Annual

Annual

Illuminance

Global

(G)

Direct

(D)

Diffuse

(d)

D/G

d/G

D/G

D/A

G/A

d/A

d/G

D/DN

—

ro

m ao

m CM Onm «—«

co vO LTV

O ro I1"-.

m

m o CM ro CM

m o CM co CM • • • • • • • • • • o o o O O o o o o O o CM in CO sO 00 3 CO •d- 00 sO .

* CO m in >3 CM CO

sD On 00 r^ 00 so O' ao 00 • • • • • • • • • • o o O O o o o o O o O 00 00 sO 00 —4 m m CO m CO r^ o <• in CO o m CM CM CO CM m CM CM CO • • • • • • • • • • o o O o O o o o o o O ao nO ON nO m m vD vO

r^* >d On

m 00 nO CMm 00 ON

r-. CM CO CO CM CO 'd' • • • • • • • • • • o O O o o o o O o O O CM o m m o CM

CM —i COm CM CM —1 CM v£> CM

CM o 9—4 CM o «—i • • • • • • • • • • O o o o o o o o o o o m vO m ON m

CM

m CM o o sO o CM

CO o CM CM CM CO o CM CM CM • • • • • • • • • • o o o O O o o o o o o m vO o m r- m <3* CM ao

-d CM Os 00 so CM

m ao

m o m •3

m <3 ON •d *d «3 • • • • • • • • • • o o O O o O o o O O o m ^d o m cn m NO ao CM o m r^ cn CM <r o *d in >3 in O m m m • • • • • • • • • • o o O o O o o o o o u£ £ • • CM K 1 3 B

fH 5 m r** rH O 00 *d CO sO f- r- o a

r- o r^ CM -3 •3 co CM *i ao 00 nO ao ao m vO m e

sO CO 00 <r Os O' On <-H co

O o

in CM CM co CM *3 CM CO CM

MHu

U o £ £ £ • • CM 4) K 1 CD 3 P 9 5 U-t £ <4-1 m vO On H

in <r On

O OS so vO 00 ao T3

© ao •H O' >3 ao co o On

CO

O o Os

m o m NO

®

>3 ** nO vO sO

—H

NO 3 n CM CO CM in CM co CM f-Ha u »—i £ <0 £ • B • CM c K 1 o 3 P z

•H 5 X 4J Os o 3 CM CO CO vO m CM f^s ao 0)

r-- On ON CO Os ^d NO in

u

in

O 00 nO

O r-- NO CM t4

m <r o sO vO o a <«) CM m m O 9—4 CM

CM <r r>» -d* /—s <s— »—s

0) *Ht a u CO a)

4-1 c 4-1 s—4 c CO c £ o -H o a N x X •o Is) X X H •*-1 AJ 4-> •u 4-» CO *4 AJ A-> i-i 4-» ^4

l-i U4 ao 3 CO l-i

u CD 3 CD <0 o o CO O 0) »-t o o CO O 0) >

sc 2:

W CO :*

M

X 2 w C/D <

11
4. CONCLUSIONS

Analysis of measure sky and solar illuminance, luminance and irradiance values

over a typical annual period for Washington, D.C. yielded several interesting

results. Comparisons of measured sky zenith luminance to standard sky zenith

luminance, and their relation to horizontal diffuse sky illuminance, indicate

significant differences between real skies and the standard skies. Difficulty

in determining sky condition from measurements contributed to some data scatter,

but significant variations in the ratio of zenith luminance to diffuse horizontal

illuminance occurred in any case. Solar altitude has a strong influence on clear

sky zenith luminance. The data for the partly cloudy skies were bounded by the

clear and overcast sky data. The overcast sky zenith luminance factors were consistently less than the standard sky value, being 26 to 44 percent lower

The ratios of luminance at four points to zenith luminance (i.e. corresponding

to the solutions to a sky luminance distribution equation) did not vary widely,

with most values being less than two. A strong correlation with sky type, using

cloud ratio as the indicator of sky condition, was observed. Measurements

taken when the solar disk was visible near the sky points were excluded from

the analysis.

Direct beam illuminance and irradiance constituted approximately one half of

the total annual available solar energy incident upon horizontal and south,

east and west-facing surfaces. The average hourly direct beam percentages were between 20 and 30 percent. These numbers indicate the potential of direct beam

daylighting systems, and the importance of diffuse solar energy to passive

solar designs.

12
5. REFERENCES

1. Kusuda, T. , Collins, B. , Simplified Analysis of Thermal and Lighting

Characteristics of Windows - Two Case Studies , National Bureau of

Standards, Building Science Series 109, February 1978.

2. Johnson, R. , Selkowitz, S., Winkelraan, F., Zentner, M. , Glazing Optimization Study for Energy Efficiency in Commerical Office Buildings , Third

International Symposium on Energy Conservation in the Built Environment,

Dublin, Ireland, March 1982.

3. Jurovics, S., Daylight, Glazing and Building Energy Minimization , IBM

Scientific Center publication, Los Angeles, CA, 1981.

4. Treado, S. and Kusuda, T. , Daylighting, Window Management Systems and

Lighting Controls , NBSIR 802147, December 1980.

5. Treado, S., Gillette, G., and Kusuda, T., Evaluation of the Daylighting

and Energy Performance of Windows, Skylights and Clerestories , NBSIR

83-2726, June 1983.

6. Bryan, H., Clear, R. , Rosen, J., and Seklowitz, S., Quicklite 1: New

Procedure for Daylighting Design , Solar Age, August 1981.

7. DiLaura, D. I. and Hauser, G. A., On Calculating the Effects of Daylighting

in Interior Spaces , Journal of the Illuminating Engineering Society,

Vol. 7, pp. 2-14, 1978.

8. CEL-1 Lighting Computer Program - Users Guide , CR 81.026 Naval Civil

Engineering Laboratory, Port Hueneme, CA, 1981.

9. Place, W. , Fontoynant, M. , Bauman, F., Andersson, B. , Howard T. , Commercial

Building Building Daylighting , LBL0-14348, Lawrence Berkeley Laboratory,

Berkeley, CA 94720, 1982.

10. Gillette, G., A Daylighting Model for Building Energy Simulation , National

Bureau of Standards, Building Science Series 152, March 1983.

11. National Climatic Center, 1976, Tape Reference Manual " Test Reference

Year," National Climatic Center, Asheville, N.C., Also published in the

ASHRAE Journal, February 1977, p. 47.

12. Gillette, G., Pierpoint, W. , and Treado S., A general illuminance model

for daylight availability," presented at the 1982 IES Technical Conference,

Atlanta, GA, August 8-12, 1982.

13. Treado, S., Gillette, G., Remmert, W. , and Beau, J., The NBS Daylight

Availability Database , NBSIR 84-2859, March 1984.

14. "Standardization of luminance distribution on clear skies," CIE Publication

No. 22, Commission Internationale de L'Eclaire, Paris, 1973.

13
15. "International recommendations for the calculation of natural daylight,"

CIE Publication No. 16, Commission Internationale de L'Eclaire, Paris,

1970.

16. Pierpoint, W. , "A simple sky model for daylighting calculations," presented

at the 1983 International Daylighting Conference, Phoenix, February 16-18,

1983.

14
Figure 1. Luminance sensor array

15
om^mouo eo»- • «=>

16

Figure

2.

Zenith

luminance

factor

as

a

function

of

cloud

ratio

(CR)
17

Figure

3.

Zenith

luminance

factor

as

a

function

of

atmospheric

extinction

coefficient

(El)
with

full

range

18

Figure

A.

Zenith

luminance

(LjO

as

a

function

of

diffuse

horizontal

illuminance

(ID),

for

all

sky

types
WITH

El

LESS

<uac

cfl C

•Hea

19
WITH

El

GREATER

THAN

OR

EOOAL 4

AND

LESS

THAN

20

Figure

6.

Zenith

luminance

(LZ)

asa

function

of

diffuse

horizontal

illuminance
WITH

El

GREATER

THAN

OR

EQUAL

TO

21

Figure

7.

Zenith

luminance

(LZ)

asa

function

of

diffuse

horizontal

illuminance

(ID)

for

apparent

overcast

skies
THOUSANDS

ZENITH

LUMINANCE

US

DIFFUSE

HORIZONTAL

ILLUMINANCE

ATMOS

EXTINCTION

COEF.

LE.

0.4,

SOLAR

ALT.

20-30

Deg

T-r

iDC 10 O -P

•H fO -P T)

(0> O

i- -P 0

10 P>

XI

H

O w.

(0 -P

-P 10

(0 0 Q 00 . . P> -P

ro fO

tn tx> 0 0

~a T3 © 
 oj m 0i,PP 0 (0 10 N

!T» p: x: tn io io p

O'O'O

in L, i- -C (0 ro -P TO “O C C

-P (0 (0 -• -p -p

U_ CO CO 1 I I \ \\

\\

W \ »•

\> \1

\?\\

<\\

w

\\

w

\\

V ^

\' $<J'. V. 

wj, I I

LO

r^- nj

LO

ru

LO

ruru


 ru

LO

r-

_ LOH

LO

rutH

_ 
 r4

LOr- - LO

LO

ru

coQ

z:

<rco3OX

t-

—

r

ru 
 00 CO ru

©




JN

22

Figure

8a.

Zenith

luminance

(LZ)

versus

diffuse

horizontal

illuminance

for
THOUSANDS

ZENITH

LUMINANCE

US.

DIFFUSE

HORIZONTAL

ILLUMINANCE

ATMOS.

EXTINCTION

COEF.

.LE.

0.4,

SOLAR

ALT.

30-50

Deg.

O') Q

z:<rIDO

23

Figure

8b.

Zenith

luminance

(LZ)

versus

diffuse

horizontal

illuminance

for

apparent

clear

skies,

medium

solar

altitude
LxJ O

CD

0

\

O

IN]

©

CD

I ©

ID

$r- X

<xo

cd

o

a®

, LxJ t—i Q

*

O

Ul°

is~2 *-•

~rJuj

»—

i

-7- CD LU

t\]

CDQ

z:

<xCD

ZDOX

t~

\ <

\

\

\

\

\

v <

\ \ < ^

\ \ \

\ < <\

\ *

1\ \ < <

\

<\

\

\

\

\*<H „ A

^ „«<3

V

%

<a

3

&> 07

0 0

*73 *0

© LD

ld r-

<3<

<3 <

\

<K' »

' \<

m

c 0 o

o -p

-<0 0 0 0

N

0 5)3)

> 0X^3^

i.-P 0)10 10 0 3

<0 -P 0*73*13

ja -» l, s« Sh O <~« 0 0

-P *13 *0

0 p> c. d

•P io+> io n) 0 0 •*H «P »P

QCGU.UDC/) * ’ I I I

i !

\ \ N * i$*

\ *

< <

<5

» W \ < \ \ \ <i <\

« \ \ \ \

<

\' \\ \\ \V

1 \ ' '

’ \ ' v \

\ '

' \ \

' \ \ \ W \*

\\ \

\\\V

—

r

OJ —

r

© oo —

r

CO —

r

OJ

L0

dcu

LDOJ

LD

CUcu©

cu

LD

_ LDH

IDCUH

_ ©

tH

LD

r*-

h LD

LD

CU

©

CDQ

Z

<r

O') 3

O

X

©

INI

24

Figure

8c.

Zenith

luminance

(LZ)

versus

diffuse

horizontal

illuminance

for

apparent

clear

skies,

high

solar

altitude
LdO

C\] *—

(H

o

CD3

Lien

0) Q

©

n

I ©

OJ

(—3

<ro

<r—

i

;o

cn

* LD

rHV

oQ

'T

LD3

LlI O

z:<x

u_LdOO

E—O

X

Ld

LdM LD

<S) Q

z:

<L

CD3

O

T

\

\ \

\ \

\

\ \' \\

\ \ \

\ \ \

\ \ \ w \

\\ \ \\ \ W \

<

<

-p .p

(0 ft)

Cn cn 0 0

©© •n cun

c it> o

o -P s- .p p>

•H fd 0 ITJ (0 < PT3 N

itj cn cn

:>i*.p> tn*n in 0 3

*0 -P O T5 T3

-Q h L L L

O <h- itj (0 TO T3

(T3^> tf C

•P 0) .p id fd

rtJ H) ~i .p .p

QtfjU.Cn CD

« ; i « i

. I i —

r

—

r

OJ ©

\

\\ \<3

1 V ' .< <f

\>) \<J,

<w

^ ^

'< < <

^ ^ <^<3<}!^A <

$

<

< <

‘v \\

L0

L0

©

ID

LDT

<S> ^r

LHm

©

m

LD

0J

©

OJ

_ ID

_ ©

- LD

\

00 CO —

r

^r OJ ©

cnQ

3

<r

cn3

OX

©

3 M

25

Figure

9a.

Zenith

luminance

(LZ)

versus

diffuse

horizontal

illuminance

for

apparent

partly

cloudy

skies,

low

solar

altitude
THOUSANDS

ZENITH

LUMINANCE

US.

DIFFUSE

HORIZONTAL

ILLUMINANCE

ATMOS.

EXTINCTION

COEF.

>.4

&

<1.6,

SOLAR

ALT.

30-50

Deg. \ \

\

V \

\

\

\

\

<\

\

X \

\ \

\ \

\ \

\ \

\ \<

$

< <

<

<

<SmS> m cn ld

c id o OP L. P P

h io m (0 (0 P*t5 n

fd 3D5)

P tJMO U) ID 3

iO P OtITI X -h $« Sm o <—> x id <d P*o *TJ id p c. c P ud p id id id 0 -h p p Q 00 Ll_ LD LD $

I

< <%. <

<

<

<

\< < \ \ <J \ %

*

* 'Vs**. *

4 C* ' JS* ^

<-

<< V3 < \q A

< y ...

< <

^

<3<

4 <\ V

4 <

< <

* &'

< «3j<

< ^ - |V

<Vj « < O '\ \\

<•<3^ < < '<tK ^

< «<«'• ^:\ 4

a « \ \

$<U

}

<

<J<J —I

ld

LD


 LD

LD

<S» TT

LD

n


 n

LD

ai


 OJ

_ LD

_ 


- LD

\_

LDO

z:<E

LD

IDO

X

t-




OJ 
 00 LO ru 


PM

26

Figure

9b.

Zenith

luminance

(LZ)

versus

diffuse

horizontal

illuminance

for
if) ZD

oX

cu © 00 (D ai ©

CDQ

z:

<L

if) IDO

X

-J N

27

Figure

9c.

Zenith

luminance

(LZ)

versus

diffuse

horizontal

illuminance

for

apparent

partly

cloudy

skies,

high

solar

altitude
THOUSANDS

ZENITH

LUMINANCE

US.

DIFFUSE

HORIZONTAL

ILLUMINANCE

ATMOS.

EXTINCTION

COEF.

.GE.

1.6

,

SOLAR

ALT.

E0-30

Deg.

Lf) LD

<S> LD

LO

<5>

•'T

LDn

<S» CD

LDCU

<S> ru

LD

<S>

LD

<S>

LDO

z:

<rCD3O

X

frQ

i—

i

OJ 
 00 CD 'T nj 


JN

28

Figure

10a.

Zenith

luminance

(LZ)

versus

diffuse

horizontal

illuminance

for

apparent

overcast

skies,

low

solar

altitude
CO

ZD

o

cu 
 00 CO OJ 


toQ

z:

<rto

IDO

X

-J N

29

Figure

10b.

Zenith

luminance

(LZ)

versus

diffuse

horizontal

illuminance

for

apparent

overcast

skies,

medium

solar

altitude
THOUSANDS

ZENITH

LUMINANCE

US.

DIFFUSE

HORIZONTAL

ILLUMINANCE

ATMOS.

EXTINCTION

COEF.

.GE.

1.6,

SOLAR

ALT.

50-90

Deg. o

ZL

<r301

f- 3 N

30

Figure

10c.

Zenith

luminance

(LZ)

versus

diffuse

horizontal

illuminance

for
j 1 z

31

Figure

11.

North

luminance

ratio

as

a

function

of

cloud

ratio
J n z

32

Figure

12.

North

luminance

ratio

as

a

function

of

atmospheric

extinction

coefficient
o o — u— < ~- OL C

•f s > o c

-l o u o

V* • > o

-J o V •

•43 4*4 <

a.a2o

<0

33

Figure

13.

East

luminance

ratio

asa

function

of

cloud

ratio
I

E/12

VS

ATMOS

.

EXTINCTION

coef

no

direct

beo*

on

verticol

J K U

34

Figure

14.

East

luminance

ratio

as

a

function

of

atmospheric

extinction

coefficient
LS/LZ

VS

CLOUD

RATIO

_i a </>

35

Figure

15.

South

luminance

ratio

as

a

function

of

cloud

ratio
LS/LZ

VS

ATMOS

EXTINCTION

COEF

.

no

direct

baon

or*

vert

«

col

_» at

36

Figure

16.

South

luminance

ratio

as

a

function

of

atmospheric

extinction

coefficient
L

V/l l

VS

CLOUD

RATIO

J t >

37

Figure

17.

West

luminance

ratio

asa

function

of

cloud

ratio
_i a. »

38

Figure

18.

West

luminance

ratio

as

a

function

of

atmospheric

extinction
•0C

CDD

~a


 ru o

Qjo

•Q

ft1 ( O —

*~i ro

X

t-

<XC

C£ id

... > c. o

ID '0 O

_J *> o

(0 »o o

ru

CDru+

GOro

CD




II >

<3 I Q C *—i 0 X Z> —1 CoE

ro

QJ XI

10c ro o x>

•h ro x> *rs

ro> o

Sh X> d

>0

x

XJ -h o <-*-• x>u

0)

•H TJ

ro x>

X> v0

ro a) Q 0Q

Oc o—t

t-

<LO'Q3Oo i T

--

T -j

r 1 r

ru ir> LD LT) «-( Lf> Lfi LO <s» r- QJ r- QJ

<rH <S> rH ^rH 
 <S>

_)2T TJ

h > -4 ri

D SX CD *—' Q

39

Figure

19.

North

point

luminance

factor

asa

function

of

cloud

ratio
LE7

ID

vs.

CLOUD

RATIO

no

direct

beam

on

vertical,

solar

alt.

above

50

deg.

AO

Figure

20.

East

point

luminance

factor

asa

function

of

cloud

ratio
LS/ID

vs.

CLOUD

RATIO

CD

4 <S>

03

<S>

r-




(JD




LO

4

<S>

'T

<5> n




OJ

<S>







o

<xQL

Q

o

o

ru LO LO Lf) •*—t 10 L0 LO ru r- 4 OJ

o 4

rH tH 
 <S>

_) iO t) h > HT3 ID J3 !J) t-H Q




41

Figure

21.

South

point

luminance

factor

as

a

function

of

cloud

ratio
LU/ID

vs.

CLOUD

RATIO

42

Figure

22.

West

point

luminance

factor

asa

function

of

cloud

ratio
MBS -1 IU REV. 2-«0> U.». DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 85-3251

2. Performing Organ. Report No, 3. Publication Date

MARCH 1986

4.

TITLE AND SUBTITLE

SKY LUMINANCE AND DIRECT BEAM ILLUMINANCE

5. AUTHOR(S)

Stephen J. Treado, William E. Remmert and John W. Bean

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

10.

SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Measurement of sky illuminance, sky luminance, direct beam illuminance and direct

beam irradiance are analyzed and discussed. The database consisted of an annual

set of integrated hourly measurements made at the National Bureau of Standards,

Gaithersburg, Maryland (77° west longitude, 39.5° north latitude). The relationship

between diffuse sky illuminance and luminance of selected portions of the sky

dome is examined. Measured sky luminances are compared to luminance calculated using

equations for three standard sky types-clear, partly cloudy and overcast. The

percentages of diffuse and direct beam to total illuminance and irradiance incident

upon a surface are presented for four vertical orientations and a horizontal

orientation. This analysis enables evaluation of the relative daylighting potential

of diffuse and direct daylighting and solar heating strategies.

The results indicate that the luminance distribution of actual skies varies

considerably from the standard skies. The percentage of global illuminance which

was direct beam was greatest for a south-facing surface (~ 5 1 percent) and ranged

from approximately 49 percent for a horizontal surface to less than 6 percent for

a north-facing surface.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Daylighting, diffuse radiation, direct beam radiation, illuminance, irradiance,

luminance, sky condition.

13. AVAILABILITY

[ XI Unlimited

| | For Official Distribution. Do Not Release to NTIS

| | Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

fjjr~| Order From National Technical Information Service (NTIS), Springfield, VA. 22161 *

14. NO. OF

PRINTED PAGES

50

15. Price

$9.95

USCOMM-OC «0«j-p»0