NATIONAL BUREAU OF STANDARDS REPORT

1x659

VISUAL RANGES OF FOUR OBSTRUCTION LIGHTS

by

F. C. Breckenridge

Robert T. Vaughan

NBS Test 21A-2/S6

to

Airways Sigineering Division

Office of Federal Airways

Civil Aeronautics Administration

Department of Commerce

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0201-30-2328 April 1956 Ii659

VISUAL RANGES OF FOUR OBSTRUCTION LIGHTS

by

F. C. Breckenridge

Robert T. Vaughan

NBS Test 21A-2/56

to

Airways Engineering Division

Office of Federal Airways

Civil Aeronautics Administration

Department of Commerce

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, reprinting, or reproduction of this report in any form, either in whole or in part, is prohibited

unless permission Is obtained in writing from theOffice of theDirector, National Bureau of Standards, Washington

25, D, C. Such permission is not needed, however, by a Government agency for which a report has been specifically prepared if that agency wishes to reproduce additional copies of that particular report for its own use,

Visual Ranges of Four Obstruction Lights

F. Co Breckenridge

Robert To Tfeughan

SCOPE

This report gives the results of a comparative study of the visual

ranges of two catenary neon obstruction lights, a neon light operated by

a transformer, and a conventional incandescent obstruction light. The

candlepower values used for this study are those presented in two previous

reports

NBS Report hhh1, Performance Characteristics of Three Washington

State College Neon Obstruction Lights.

NBS Report h5h2, Candlepower and Color Characteristics of a Double

Obstruction Light.

The study has been limited to nighttime use because it has already been

shown in Report khhl that the large catenary unit even at its full intensity is seen no better than the transmission line in the daytime.

COMPUTATION OF VISUAL RANGES

The conversion of the candlepower values into visual range values has

been carried out in accordance with the conventionally accepted formula

known as Allard* s law:

e0

= ii^/d2

( 1)

E0

= Threshold illuminance

I = Candlepower toward observer

T = Transmissivity of the atmosphere

D = Distance of light from observer

The righthand member of this equation, if all its constituent

qualities are known, gives the density of light flux, that is, the illuminance, at the observer* s position. This member may be derived mathematically from the inverse square law, the definition of transmissivity,

and the assumption that the atmosphere is uniform. While the last is

never strictly true, it is the only assumption practicable and it leads

to calculated values which approximate conditions in service. Allard

found that the illuminance, E, at which lights became just visible at

night was substantially constant. Later studies have found it to increase

with background brightness and decrease as the dark adaptation of the

observer improves. For the case of an airplane pilot, allowance must

also be made for the limited attention a pilot can concentrate on his

lookout for obstruction lights. The value used for these computations is:

E0

= 0.5 mile candles for night conditions.
>
-2-

This value was proposed by Toulmin-Smith and Green (Aircraft Engineering

3, 12, 1931) as a practical threshold for the background brightness and

dark adaptation representative of a pilot*s situation. The tests made by

this Bureau for the C.A.A. at Nantucket indicated this value was reasonable (illuminating Engineering UO, 8l6, 19h$)» Although the value varies

greatly with the conditions prevailing, the exact value used for -the computations does not seriously affect the relative distances to which the

several obstruction lights can be seen.

To carry out the computations it is also necessary to select values

of T but since it is customary in regulations to express the transparency

of the atmosphere in terms of meteorological visibility, rather than

transmissivity, it is desirable to base our selection of T on typical

values of meteorological visibility. The values of T used in this study

have been calculated from (1) with the following as sump ticns

D-j_ = 10 miles, typical of clear weather.

D2 = 3 miles, minimum visibility for visual flight rules.

« 1 mile, a somewhat restricted visibility.

I = 2$ candles.

E0 = 0.1 mile candles, threshold for meteorological observer.

It will be noted that the value of Eq the weather observer is

lower than the value used for the pilot. This is because of the better

dark adaptation and opportunity for concentration of the meteorologist.

When the values given above are substituted in (1), the following

values of T are obtained:

T

x

= 0.9121*

t

2 = 0.3302

T3 = 0.00U0

The laboratory measurements of candlepower give the values of I for

different directions so that with the values of T and E0 selected as stated

above the values of D for each direction are determined. There is, however,

no simple method of solving (1) for D. This difficulty is overcome by preparing curves showing I as a function of D for the values of T listed above

since the value of I may be readily computed for assumed values of D and T.

These curves are then used to read the values of D corresponding to the

values of I taken from the previous reports. The curves used for this

conversion are given in Figures 1 and 2.
H|B
-3

-

SCOPE OF COMPARISONS

A number of -variants affect the visual ranges of both the neon and

incandescent lights. The ones listed below are included in the present

comparison

Variation in candlepower in both azimuth and vertical angles

;

affects all units.

Variation of atmospheric transmissivity; affects all units.

Variation with transmission line current; affects both catenary units

but measured values are available for the large catenary unit only.

Variation from lamp to lamp; affects all units, information available

for two large catenary lamps and three incandescent lamps.

Variation with type of lens; affects incandescent unit only, values

available for two -types of lenses.

Variation with color; affects incandescent unit only, limits indic-ated

by the color specifications; see discussion in Report NBS h5k2.

It is not feasible to prepare visual -range curves for all combinations

of these variants. It has, therefore, been necessary to select cases in such

a way as to give as good a comparison as practicable of the average visual

ranges of the neon and incandescent lights. Since different variants affect

the four -types of lights, it has been necessary to be somewhat arbitrary in

the selection of variants to be included in each figure.

The general plan has been to compare first the azimuthal and vertical

variations of the visual ranges for all the lights for favorable conditions

such as 10-mile visibility with the maximum line currents for the catenary

units and pale-limit glassware for the incandescent unit. Average curves

have then been developed from the same candlepower values to serve as a

basis for comparing the visual ranges of the lights for less favorable

conditions.

AZIMUTHAL LIMITS OF VISUAL RANGE

The first comparison (Figure 3) shows how the visual ranges of each of

the lights varies with azimuth under optimum conditions. These assume, as

stated above, an atmosphere having 10-mile visibility, a current of 1000

amperes In the transmission line for the large catenary unit and 1?0 amperes

for the small catenary unit, and pale-limit glassware on the incandescent

light. The curves far the two catenary units have been oriented as if the

transmission line extended horizontally across the center of the figure.

It happened in the case of the large catenary unit that lamp A has longer

ranges than lamp B when seen from the unobstructed side and shorter ranges

than B when seen from the obstructed side. The two half curves for lamp A

have, therefore, been used to represent the variations of visual range of

this unit as seen from different directions under optimum conditions and

these have been shown in the figure on the near and far sides of the axis.

In the case of the incandescent light, which has two lenses, lens A gave
, = 1 1 I I

,

.

I

.
-Itthe better values but even these intensities do not represent optimm

operation. It has been necessary to multiply them by the ratio 5/3* to

obtain the intensity to be expected with pale-limit glassware which has

been taken as the nearest equivalent to operation with a maximum transmission line current. Since this unit uses two lamps and two lenses,

each of which is blocked off by the other at certain angles, the distribution of visual ranges for this light has been based upon a candlepower

curve obtained by adding the measured distribution values to the values

for the same curve rotated 180®. This rotation simulates the second lens

which has its shadow in the opposite direction from the first.

Figure 3 also shows the horizontal limits of visual range for the

small catenary light and the Model 300 neon light which is the transformer

operated neon unit. The curve for the Model 300 unit corresponds to operation at 117 volts. In the case of the small catenary unit the design is

symmetrical, neither side being obstructed more than the other. The candlepower of the Model 300 unit is uniform except for a small periodic variation caused by shadows of the rods in the guard. When the candlepower values

were converted to visual ranges, the departure of the shadows from the

average was approximately 1% at its maximum. This was considered unimportant.

VERTICAL LIMITS OF VISUAL RANGE

For the determination of the vertical limits of visual range, only

those angles between -20* and +50° were considered of importance. Except

for helicopters there can be little utility in seeing obstruction lights

outside of these angles since airplanes under control do not glide down

at angles greater than 50* nor climb at angles greater than 20°.

The second comparison. Figure U, shows the vertical limits of visual

range under the same conditions as were assumed in preparing Figure 3* As

in the case of the horizontal limits, the minimum and maximum curves for

the large catenary light are both based on the measurements for lamp A

and represent the obstructed and unobstructed sides respectively. For the

incandescent light, the vertical minimum and maximum curves are based on

interior and exterior envelopes of the set of vertical curves shown in

Figure 17 of Report NBS U5U2. In this case no attempt was made to allow

for the shadow but the candlepower curves were corrected to correspond to

pale-limit glassware and doubled to take account of the two lenses. Since

the large variations in candlepower with azimuth which necessitate the two

curves for the large catenary and incandescent unit are not present in the

case of the smaller neon lights, these units are each represented by a

single curve and these two curves have been turned away from those for the

large catenary light to avoid confusion with the minimum curve for this

unit.

* See Report NBS h5h2
.
-5-

OPERATIONAL VARIATIONS

Figure 5 has been prepared to show the effects of operational variations# It is based upon the candlepower "values used for Figure k but shows

the effect of -varying the current in the case of the large catenary light

and varying the glassware transmittance in the case of the incandescent

light# The curves for the most favorable operating conditions in this

figure are based on an average of the minimum and maximum candlepower values

used for the same angles in the curves of Figure U# These represent the

operation of the large catenary unit on a transmission line carrying 1000

amperes and the incandescent unit fitted with pale-limit glassware. To

represent the other extreme of operating conditions, the candlepower values

underlying the curves for favorable operating conditions have been multiplied

by suitable factors# In the case of the catenary unit, the factor was

selected to give candlepawers corresponding to operation of the catenary

light on a transmission line carrying only 100 amperes# For the incandescent light the minimum operating condition has been based on lens (B), which

was considered as typical of minimum transmittance glassware. The values for

lens B were multiplied by 2 to allow for a second lens# The intersection

of this curve with the curve for lens A at +5>0° is caused by differences

in their distributions. The curves included in this figure for the small

catenary light and the Model 300 unit are identical with those in Figure U

since no measurements are available as to how these are affected by transmission line current.

EFFECTS OF HEATHER

The final comparison. Figure 6, shows the effects of the weather.

These curves again are based on average operating conditions. For the large

catenary unit this was considered to be operation on a transmission line

carrying U00 amperes# For the incandescent light an arithmetical average

of the candlepower used for the minimum and maximum range curves in Figure 5

was taken as the basis for the visual ranges# The maximum curves in this

figure represent weather characterized by 10-mile meteorological visibility,

as do all the curves of Figures 3, U* and 5# The other curves of Figure 6

are derived from the curves for the 10-mile visibility by multiplying the

candlepower values by factors to allow for the difference in transmissivity

between 10-mile visibility and 3-mile and 1-mLle visibility, respectively.

In the case of the large catenary light, it was also necessary to make a

small reduction amounting to approximately .1 mile in the visual range

values for 1-mile visibility in order to allow for the size of the unit

which is large enough so that the light is not as effective as if it came

from a point source#

COMPARISON OF VISUAL RANGES

The azimuthal distribution of visual ranges shown in Figure 3 should
I; £f_

t,'i taw e«U r
*6“

not be unduly weighted in comparing the performance of the incandescent

and catenary units o It must be borne in mind that these curves represent

a con^arison at the mo^t favorable vertical angles which improves the

performance of the incandescent light more than it does that of the neon

units o These curves, however* do bring out the fact that in one direction

along the transmission line the large catenary light has a much reduced

visual range o This is caused by the shadow of the transformer on the end

of that unit. In the opposite direction along the transmission line there

is a gap in this curve which is caused by the shadow of the goniometer.

A comparison of the construction of the two ends of the unit shows that

there is a fairly large gear at this end, and although this gear will not

reduce the visual range as much as the transformer, it will cause a

definite dimple in the curve, indicating that the visual range along the

transmission line in this direction also is considerably reduced. Similar

dimples must also occur in the corresponding sectors of the curve representing the visual range of the small catenary light, but the Model 300

unit is free from these dimples.

Figure U, which shows the vertical variations in visual range, gives

a more significant comparison than Figure 3 of the relative performance

of the large catenary unit and the incandescent obstruction light under

optimum operating conditions and clear weather. In this case the obscured

variation is the reduction of the visual range for the catenary units in

both directions along the transmission line. In these directions, the

comparison would favor the incandescent light even more than the curves

of Figure The Model 300 unit has a shadow cone downward but it is not

large enough to cut off any necessary light.

Figure 5 gives a more comprehensive comparison than Figures 3 and b

since Figure 5 shows how the visual range falls off when the operating

conditions are less favorable. The effect of the transmittance of the

glassware on the visual range of the incandescent light is seen to be much

less than that of the current in the transmission line on the distances

to which the large catenary light can be seen, and even with the poorest

acceptable glassware the incandescent light provides a longer visual range

than the large catenary gives with maximum line current between the angles

of k° and lU° above the horizontal. These are angles at which the obstruction lights are frequently needed. If we make a comparison on the basis

of the minimum transmission line current, 100 amperes, the incandescent

light is superior at all vertical angles between -20* and +50*. While

the visual range of the small catenary unit at its minimum line current

is not shown, we can make an estimate of its range for this current if

we assume that its candlepower is proportional to the line current as is

nearly the case with the large catenary unit. This assumption indicates

a visual range slightly under U. miles for the small catenary at 60

amperes

.

-7-

The final comparisons of Figure 6, which are based upon as nearly

normal operating conditions as we could select, show that in all weathers

the incandescent light is superior at all angles from -lli® to +50®. As

already pointed out, except for helicopters, aircraft under control do

not approach obstructions at angles outside of these limits. Figure 6

also shows how rapidly the visual ranges are reduced as the weather deteriorates •All the comparisons indicate that, if there is a choice as regards

the obstruction lights to be used for airplanes, the incandescent units

will afford a greater protection than any of the neon units. Since, however,

an incandescent light cannot be used on the catenary of a transmission line,

the use of one of the catenary neon lights will give some protection even

though it is somewhat less than that provided by the incandescent lights.

Their use for this purpose, therefore, appears to warrant consideration

from the standpoint of visibility. The Model 300 unit appears to have

merit for use in locations where helicopters are landing and taking off.

The small catenary light gives better candlepowers in proportion to

its size and line current than the large one. This is partly due to the

smaller range of currents for which it is designed. However, this suggests

that a larger unit similar to the small one might give high candlepower

values than either of them. Since a helical tube could not be installed

concentric with a transmission line without opening the line, such a unit

might be designed with the transformer concentric with the line and the

tube supported below it. This construction would also have the advantage

that the ends could be less obscured than they are in either of the present

designs. This type of unit could also be designed for two or three concentric tubes with separate secondary windings.

-&1K



340= 3509

170°

/ 90® 16015 200°

60® 300®

80® 280®

no®

, 230 ®

\ 1 20c /N 240=

140® /1 220 ®

230®

200= 160

190° 170

150° 210

210 ’ 150° 180

’ 170° 160°

1 90r> 2000

!40 3 2 20 3

130°

2 3 O'

5

120’ 240 ’

no5 250°

1 00° 280 ’

90’ 2703

80° 280°

70 ’ 290’

60’ 300°

50° 310’

210 ’ 200 = 190°

, 170 160° 150* J50'‘ 100 170 loU 190° 200c 210•

I40 3

130= 230’

120° 240°

100° 2603

330w340’ ?n° 350c in2 2O'

1 Ift? 30

'

I 5 O

a 200

'

! 60

!f.0

3 200° 150 210

X 50‘ >

310‘

3303 340° 350° n /0° 20° 300 30° 20- IQ

3 v 330’ 340° 3X0°

THE NATIONAL BUREAU OF STANDARDS

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