NBSIR 75-644 (R)

Focusing the Fresnel Lens Optical

Landing System

W. F. Mullis, I. Nimeroff, and E. L. Walters

Illuminating Engineering Group

Optical Radiation Section

Heat Division

Institute for Basic Standards

April 1975

Prepared for Department of the Navy

Naval Air Engineering Center

Philadelphia, Pa. 19112

NBSIR 75-644

FOCUSING THE FRESNEL LENS OPTICAL

LANDING SYSTEM

W. F. Mullis, I. Nimeroff. and E. L. Walters

Illuminating Engineering Group

Optical Radiation Section

Heat Division

Institute for Basic Standards

April 1975

Prepared for Department of the Navy

Naval Air Engineering Center

Philadelphia, Pa. 19112

'"•f.u of *

U S. DEPARTMENT OF COMMERCE. Rogers C B. Morton. Secretary

NATIONAL BUREAU OF STANDARDS. R. chard W. Roberts. Director

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, D.C. 20234

May 21, 1975

Commander

Naval Air Systems Command

Washington, D. C. 20360

Attn: AIR 53722

Dear Sir:

Enclosed are four copies of National Bureau of Standards Report

No, 75-644 entitled "Focusing the Fresnel Lens Optical Landing System".

Copies of this report are being sent to the agencies listed below.

Sincerely

,

A. T. Hattenburg

Illuminating Engineering Group

Optical Radiation Section

Heat Division

Institute for Basic Standards

Encl osures

cc: NAEC (NE-716) (2)

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PREFACE

At the request of the Naval Air Engineering Center of the

U. S. Department of the Navy, the National Bureau of Standards

conducted a study of various methods of focusing the Fresnel

Lens Optical Landing System (FLOLS) . This work involves both

visual and instrumental procedures by which the correct focus

position of the light source relative to the lens may be determined.

The opinions, findings, and conclusions in this report are those

of the authors and are not necessarily those of the Naval Air

Engineering Center. Certain commercially available equipment,

instruments, or materials are identified in this report to

specify adequately the experimental procedures used. In no case should such identification be inferred as recommendation or endorsement by the National Bureau of Standards, nor does the

identification imply that the materials or equipment are the

best available for the purpose.
CONTENTS

Page

1. Introduction 1

2. Description of FLOLS Unit and Cells 1

3. Fundamentals for Focusing 1 4. Methods of Focusing . .. 8

5. Description of Focusing Methods 9

5.1 Photometric 9

5.1.1 Photometer with Graphic Recorder 9

5.1.2 Photometer with Meter Readout . 13

5.2 Peep Sight 18

5.3 Parallax 22

5.4 Auxiliary Image- forming lens 22

5.4.1 Observing Face of Cell 22

5.4.2 Observing Beam Pattern Projected on White

Card 26

5.5 Pattern Matching i 28

6. Results 31

6.1 Photometric Procedures 31

6.2 Other Procedures for Focusing 38

7. Uncertainty 38

8. Discussion 43

9. Recommendation . .. 43

Appendix 46

A-l Other Factors 46

A-2 Operational Features 4 ^ A-2.1 Effect of Lenticular Lens on Beam Spread ....

4 ^

A Q A-2. 2 Effect of Lens Temperature on Beam Spread ... A-2. 3 Effect of Slit-Width on Beam Spread 48

A-2. 4 Effect of Slit Alignment on Beam Distribution 48

A-2. 5 Effect of Lamp Dimming and Source Color on Focusing 51

A-3 Mechanical Design Features 53

A-3.1 Lamp Mount Assembly 53

A- 3. 2 Fresnel Lens Mounting 56

A-3. 3 Lens Temperature 56
FIGURES

Figure No. Caption Page

1 Fresnel lens optical landing system 2

2 Image formation by a thin lens

3 Ideal formation of beam distribution

4 Virtual image distance v.s. source distance

5 Focusing by measuring width of light beam 1°

6 Geometric principle of beam spread

7 Geometric principle of beam spread increase

with increase of test distance 14

8 Photograph of equipment for photometric method

with meter readout (front view) I"

9 Photograph of equipment for photometric method

with meter readout (back view) ^ 7 10 Geometry of peep-sight focusing device

^

11 Equipment arrangement for peep sight focusing

method 20

12 Appearance of correct and incorrect settings of

21 source

9-3 13 Principle of parallax ^

J 14 Equipment arrangement for parallax focusing 2 ^

9 s 15 Image-formation by auxiliary lens

9 n 16 Principle of chromatic aberration

17 Equipment arrangement for focusing by matching

cell pattern 29

18 Cell and standard pattern display device

19 Beam spread with lens No. 2117

20 Beam spread with lens No. 331

21 Beam spread with lens No. 247

22 Beam spread with lens No. 704

23 Beam spread with "Bad Lens" 36

24 Virtual image distance and beam spread as a function of the source distance 37

25 Location of virtual sources 42

A-l Effect of lenticular lens on beam spread 47

A-2 Effect of lens temperature on beam spread 49

A-3 Effect of slit width on beam spread 50

A-4 Effect of slit alignment on beam distribution ... 52

- iii -
TABLES

Page

I. Measurement of Source Distance for Focusing Methods 39

II. Computed Beam Spread and Image Distance, Lens 2117 ...... 40

III. Computed Beam Spread and Image Distance, Bad Lens 41

IV. Range of Source Position by Several Procedures 44

A-I Effect of Lamp Dimming on Beam Spread and Virtual

Source Distance 54

A-II Effects of Source Color on Beam Spread and Virtual Source

Distance 55

- iv -
ABSTRACT

The Naval Air Engineering Center requested the National

Bureau of Standards to develop a method of evaluating the performance of lenses of the Fresnel Lens Optical Landing System

(FLOLS) . In the course of this work, it was found that focusing

of the cells of a FLOLS unit varied considerably. This led the

NBS to examine the various focusing techniques used in the past

and to develop a more precise method for focusing a cell. The

focusing methods studied include the following:

1 . Photometric

2. Peep Sight

3. Parallax

4. Auxiliary Image-Forming Lens

5. Pattern Matching

In addition, in the study of these focusing methods, several

other factors that affect the light-beam-forming characteristics

of a cell were investigated. These include the effects of,

1. Lenticular Lens

2 . Lens Temperature 1 3. Slit Width

4. Slit Alignment

5. Lamp Dimming and Source Color

The results of this study indicate that visual methods of

focusing the FLOLS system have inherent difficulties that lead to

improper focusing. Photometric procedures developed at NBS,

however, can give adequate focusing precision.

- v

-

1. Introduction

I

This report covers work completed to date in reference to

various methods of focusing the Fresnel Lens Optical Landing System

(FLOLS) . Also attached, is an appendix relative to other factors

affecting FLOLS performance. The work is a part of a larger effort

to determine quantitatively the optical quality of the Fresnel lenses

used in this system in terms of their acceptability (or unacceptability)

for projecting a light beam pattern that has the appropriate parameters.

Factors affecting the beam pattern in the system include such items as optical and mechanical alignment, slit width, lens distortion, lens

temperature, and focusing. Because focusing of the system is the most

critical parameter, a decision was made to report separately the data

obtained to date by various focusing methods. It is recognized that

proper focusing does not necessarily remove all of the problems

associated with the formation of the light beam. However, when the

system is properly focused, the other problems become less significant.

2. Description of FLOLS Unit and Cells

The Fresnel Lens Optical Landing System is a method for assisting

pilots in establishing and maintaining the proper glide path for safe

landing. The system provides a horizontal bar of light that when

viewed with respect to a set of fixed horizontal datum lights indicates

to the pilot whether he is above, below, or on a correct glide slope.

The bar of light is formed by each of the FLOLS cells. The unit consists

of five cells arranged as shown in Figure la. The individual FLOLS

cells in the unit are placed so that these optical axes intersect at a point 150 feet behind the Fresnel lenses.

A FLOLS cell, schematically shown in Figure lb, consists of a lenticular lens, a Fresnel lens, a light source, light-source mount,

and mount-support rods. The lenticular lens, which spreads the beam

horizontally, is placed outside an egg-crate type grille and the

Fresnel lens. The grille consists of two sections between which the

Fresnel lens is placed. The source consists of three lamps, each of

which has a ground-glass covered slit. Focusing, as used in this

report, consists of positioning the light source along the support

rods of the cell until the proper beam spread (beam divergence or

angle) is achieved. The support rods are parallel to the lens axis,

so that the source can be moved along the lens axis.

3. Fundamentals for Focusing

The Gaussian lens equation quantitatively relates the distances

between the lens and the source, the image, and the foci, F and F'

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of any simple thin lens system. See Figure 2. If p is the distance

from the source to the lens, q the distance from the image to the lens,

and f the distance from the foci to the lens, then the equation is

written,

p q f

In addition, it can be shown from Figure 2(a) that the ratio

of the source size, S, to the image size, i, is equal to the ratio

of their respective distances from the lens.' This relationship is

stated thus,

i q

(2)

With this minimal background in the lens equations, the lightbeam characteristics of the FLOLS can be examined. It can be seen

from equation (1) that if the source is placed at the focus, then p

is equal to f, the image distance q is infinite, and the emerging beam

is collimated. This is shown in Figure 2(b). If the source is placed

between the lens and the principal focus, a diverging beam will emerge

and a virtual image is formed behind the lens. See Figure 2(c). As

the source is moved closer to the lens the virtual image moves toward

the source, and the closer the source is to the lens, the wider will

be the diverging light beam.

It can be seen from Figure 2 that a lens will form either a real

or a virtual image. To judge whether an image formed by the FLOLS

cell is real or virtual, a diffusing screen in placed at the position

from which the light appears to originate. If light falls on the screen the image is real, otherwise it is virtual. In either situation

this image, so far as the optics are concerned, acts as the source for

the optical system and light may be considered as if it were effectively

diverging or spreading from that location. The virtual image is called

a virtual source.

The spread of the beam from the image may be described in terms

of the angular divergence 6 about the optical axes or in terms of

extent X together with the distance d along the axis from the image.

In this report, if beam spread is given in angular units the former

is meant, if beam spread is given in distance units the latter is

intended.

The term beam spread is here defined as the divergence where the

intensity distribution of the beam decreases to 50% of the maximum

illuminance on either side of the optical axes. The decrease to 50%

- 3 -
Co])

BEAM SPREAD

ANGLE, 9

LENS

SOURCE

OPTICAL S

REAL

IMAGE

BEAM SPREAD

ANGLE, 9

SOURCE i

> «

VIRTUALIMAGE

(Virtual

Source)

IMAGE AT

INFINITY

BEAM SPREAD

BEAM SPREAD (INCHES)

AT d

APERTURE STOP

Figure 2 a Image formation by a thin lens: a} Source S at distance p from

lens, outside focus F forms real image i at distance q from lens,

outside focus F'; b} Source at p = F, image at q = * ; c) Source

at p between lens and F, image at -q, outside F.

MBS I R 75-544

- 4

-
occurs when the light from one half of the virtual image is occluded

by the aperture stop. The beam illuminance drops to zero when the

image is totally occluded by the aperture stop. This effect is shown

in Figure 3 for an ideal lens and its virtual image.

Although the virtual image distance can not be measured directly,

the FLOLS has been described in terms of this distance. It can,

however, be determined indirectly by computation from the thin lens

equation if the light source position and the focal length of the

Fresnel lens are precisely known. The virtual image distance can

also be determined from measurements of the beam spread.

A curve plotted to show the relation between the beam spread angle

(angular width of the beam) of a particular lens and the source position

inside the focus, can be used to find a position of the light source relative to the focus to obtain the exact beam spread angle desired.

Since the beam spread is inversely proportional to the distance from

the lens to the virtual image (the narrower the beam the greater the

distance from the lens to the virtual image) a position of the light

source can be found that will produce the required distance from the

lens to the virtual image. This latter relation is shown in Figure 4

for a lens with a 24-inch focal length. It must be pointed out at

this time, however, that with a lens of a given focal length and cell

aperture, only one of the parameters needs to be specified (beam

spread or virtual image distance) as one is inversely proportional

to the other. If the distance from a lens to the virtual image is

specified, the beam spread is thereby determined for that lens.

Conversely, if the beam spread is specified, the distance to the

virtual image is thereby determined. Measurements indicate that for

a cell of the Fresnel lens optical landing system (FLOLS) equipped

with an egg-crate grille having an effective aperture of 8.8 inches

and a lens having a focal length of 24 inches the following values

of beam spread angle and virtual image distance will be obtained:

1. for a virtual image distance of 150 feet, the maximum

beam spread angle is 16.8 minutes of arc, and

2. conversely, if the beam spread angle is 18 minutes of

arc, the lens to image distance is about 140 feet, where

maximum beam spread angle is the angle between the

points at which the beam illuminance drops to 50% on

either side of the peak illuminance, with the virtual

image at the vertex of the angle. Also, if a cell is

used without the egg-crate grille (9-inch aperture) the

corresponding values for the cell would be 17.2 minutes

of arc for 150-foot image distance. With a 9-inch

aperture and an 18-minute beam spread, the image distance

would be 143 feet.

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Since the beam spread angle can be measured photometrically to an accuracy of a few seconds of arc (method explained later) and

since the position of the source can be measured mechanically to a couple of thousandths of an inch, the problem of focusing becomes

primarily a matter of how much precision is required. The precision

required, in turn, determines which of the available focusing methods

should be employed. Computations indicate that a 0.010 inch movement

of the light source for a nominal lens, in the region of proper beam

spread, changes the distance to the image about 5 feet. Correspondingly, the 0.010 inch movement changes the beam spread in this

region about 1 minute of arc. These effects reduce the problem of

focusing to two considerations: (1) what focusing precision is

required and (2) which parameter, virtual image distance or beam

spread, "should be specified.

The overall FLOLS unit is mounted in the rack with five individual

cells stacked in such a way as to project a total beam spread of 1.5

degrees or 90 minutes of arc, This arrangement suggests that each

cell is intended to cover an angle of 18 minutes. To cover this

angle with a smooth transition of light between cells, an 18-minute

beam spread (measured to the one-half maximum illuminance level) is

required so that there be no illuminance decrease between cells. The

cells are placed so that their optical axes intersect at a point

150 feet behind the Fresnel lenses. However, as seen from the

discussion above, the beam spread angle is 16.8 minutes to the 50%

points for a virtual image distance of 150"feet. It is probable

that an angle smaller than 18 minutes per cell (0.3 degree) has been

used in practice, as a comparison of the various focusing methods

described below will demonstrate.

4 . Methods of Focusing

Listed below are the various methods of focusing the Fresnel

Lens Optical Landing System included in this study.

1. Photometric (photometer and either a graphic recording

or a meter readout of beam spread)

2. Peep sight

3; Parallax

4. Auxilliary image-forming lens (observing face of cell

or observing beam pattern as projected onto white card)

5. Pattern matching (matching cell beam pattern to a standard film pattern)

- 8 -
Five lenses were used in focusing by the photometric method.

In the intercomparison of the other methods only two lenses were

used, one designated No. 2117 and the other (furnished by NAEC)

designated "Bad Lens".

Repetitive measurements were made with each method to determine

the precision obtainable. Normally, 10 such measurements were made

(five for each of two operators or observers) with each method.

However, if the settings by a particular method were very widely

dispersed, work with this method was stopped with fewer than 10

measurements. A description of the test procedure for each method

utilized is given below.

5 . Description of Focusing Methods

5.1 Photometric

The photometric focusing method is based on the principle

of measuring photometrically the illuminance in a vertical

plane at a stated distance from the cell and finding the

width of the beam at which one-half peak illuminance occurs.

5.1.1 Photometer with Graphic Recorder . These measurements can be made by directly illuminating the photodetector of a photometer, used as an illuminance

meter. Alternatively, with the photometer used as

a luminance meter, measurements can be made by scanning

the light beam pattern as projected onto a diffusing

screen (ground glass or milky plastic) . The photometer

scans the pattern formed after the light from the cell

has passed through the screen. The alternate arrangements of cell and photometer, with and without

diffusing screen are shown in Figure 5. The scanning

is accomplished by traversing the beam pattern with

the photometer rather than by rotating the cell so that its beam traverses the photometer. Rotation

of the cell results in a small angular error in that

the virtual light source is not at the axis of rotation but rather 150 feet beyond it. Because the

travel involved is small and would result in an extremely small error, rotation of the cell could

have been used. In these measurements, however, the

traversing procedure was used for convenience.

In photometric focusing the light source (lamps,

slit and ground glass of the cell) is moved along

the optical axis until the correct beam spread is

obtained for the particular distance selected for the

measurements. The geometric principle is shown in

Figure 6 where

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X^ is the cell 1 aperture or opening

X^ is the beam spread in inches at the 50%

luminance level

is the distance to the virtual image

from the cell aperture

d^ is the distance from the cell aperture

to the plane selected for the measurement.

From Figure 6, because the source can be considered

a point in the plane of the figure, it is apparent, that

the beam spread in inches at a given distance from cell

aperture can be computed from the relation

, or

X„ = x

1 (d

1 + d

2

)

(3)

It can be seen from Figure 3 that the beam does not cut

off sharply, but gradually tapers off, hence the 50%

illuminance points are used to define the beam spread.

Use of a diffusing screen is a means that permits visual

estimate of the beam spread. As an example of the beam

spread at a diffusing screen distance of 20 feet from a

a cell that has an effective aperture of 8.8 inches (cell

with egg-crate grille) and a virtual image distance of

150 "feet would be,

. 8.8,15^ 20,- . 9 . 97 inches .

For a cell aperture of 9 inches (as for a cell without

the egg-crate) the beam spread under the same distances

as the example above would be 10.2 inches. Thus increasing the aperture increases the beam spread if the

distances are not changed. Furthermore, the accuracy

of the computed beam spread depends on how accurately

the cell aperture is measured, on the assumption that no

- 12 -
lens effect in the vertical plane is caused by the

lenticular lens which covers the Fresnel lens. Actually,

a lens effect is introduced by the lenticular lens which

spreads the beam slightly. (This effect is discussed

in the Appendix.)

A variation of this photometric technique involves

measuring the beam spread at two different distances

instead of at one distance from the virtual source. The

results obtained by this procedure are independent of

cell aperture or the effects of the lenticular lens.

The procedure is simply to measure the distance, d,

between the two positions of the ground glass, the

second beam spread, the beam spread increase, x, and the distance from the virtual source to the

second ground glass position, D'2, because ratios of

corresponding parts of similar triangles are equal,

X

= X^2 < 4 ) d D' 2

"

The geometry demonstrating the principle is shown in

Figure 7. With this procedure it has been found expedient

to make two or more measurements at each distance and from

these data calculate the position of the source that gives

the proper beam spread. With this technique, a set of

measurements, one at each distance, is made with the

light source positioned to give a beam spread wider than

normal followed by a second set of measurements with the source positioned to give a beam spread narrower than

normal. A graph is then made from the data for source position versus beam spread for the two different distances.

Then from a line connecting the data points on the graph,

the position of the source can be read which will give the

exact beam spread desired. When the light source is

positioned to produce a virtual image of 150 feet behind

the cell lens, the unit is properly focused.

5.1.2 Photometer with Meter Readout . This method is essentially

the same as the photometer with graphic recorder,

described above , in which the beam spread from the cell

is measured with a scanning photometer and recorded

manually. It differs primarily in that the scanner, photodetector, and readout equipment are simpler. The method

is rapid, accurate, and adaptable to laboratory and field

use and the results obtained are independent of the cell

aperture. The system consists of a simple barrier-layer

type photocell, a manually operated scanning device, and

a suitable operational amplifier feeding a meter readout.

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A diagram of the equipment is depicted in Figure 5. Photographs of the equipment are shown in Figures 8 and

9. The equipment used in this method was as follows:

Photocell — Selenium, Weston Model 856, Type 1*

Scanner — NBS constructed, having a total scan

of 18 inches with graduations in

l/10th of an inch

Amplifier — Pacific photometer, Model 17, with

integral meter readout.

There is nothing unique about the equipment except that

the sensitive surface of the photocell is masked down to

a horizontal slit 1/16 inch or smaller. This is done so that a more accurate measure of the beam spread limits can be obtained. In operation, the photocell is scanned across

the light beam projected onto a diffusing plastic material

until the position of peak intensity is found. The sensitivity of the meter is then adjusted so that the meter

reads 100% at this position. The photocell is first moved

toward one edge of the light beam (vertical plane) , and

then toward the other edge until readings of 50% of the

peak intensity are obtained on either side of the optical

axis. The positions of the two edges are noted. The

distance between the two positions is the beam spread. For

a cell equipped with an egg-crate grille and measured at a distance of 20 feet from the lens, the beam spread should

be 9.97 inches, when a virtual image is produced at a distance of 150 feet behind the Fresnel lens. If the cell

is not equipped with the egg-crate grille, the beam spread

is 10.2 inches when a virtual image is produced at a distance of 150 feet. A suitable linear scale mounted on

the slider-type scanner with graduations of 0.1 inch is

convenient for reading the beam spread in inches. Hundredths

of an inch can be estimated from the scale.

The data obtained by this method are used to place the

lamp in its proper position. From the focal length of the

Fresnel lens the proper source position is computed to

give the expected beam spread (50% of maximum illuminance

at a specified distance) . With the source set near or at

the proper position the illuminance distribution is measured.

If the beam spread is not that which the computations indicate

it should be, the source is moved until the expected spread

is obtained.

*Use of a silicon photodetector is preferable.

- 15 -
Figure 8. Manual scanning photometer with diffussing screen and

meter readout.

NBSIR 7 5-644

~ 16 -

5.2 Peep Sight

The peep sight method measures, at a given test distance

from the Fresnel lens, the vertical displacement, from the

optical axis of the cell, of the bar of light ("meat ball")

emanating from the virtual source. ' When the displacement

is such as to indicate that the virtual source is at a distance

of 150 feet from the Fresnel lens, the unit is properly focused.

The geometry of the method is as shown in Figure 10. The

procedure for focusing by this method is described in the

Naval Air Engineering publication, NAEC-MISC-08668 , Revision

A, "Acceptance Requirements for Fresnel Lens Cell Assembly,

N2562/42534.

This method was executed by means of a hole-alignment kit

furnished by NAEC, Philadelphia, Pa. The kit consists of a "focus plate" with three tiny holes through which the light

emanating from the. cell is viewed and a "lens-alignment plate",

consisting of a vertical plate and three horizontal bars, which

fastens to the face of the cell, simultaneously. The vertical

spacing of the holes in the plate together with the spacing

of the bars of the lens alignment plate are chosen to position

the virtual image at a distance of 150 feet when the light bar

of the cell is successively aligned with the proper peep hole

and bar of the focusing device. The device used was designed

for a test distance of 10 feet. Thus, the outside hole

spacing in the focus plate was 6.4 inches while the outside bar

spacing of the lens alignment plate was 6 inches.

In using the equipment, the height of the focus plate is

adjusted so that its center hole is at the same height as the

center grid of the egg-crate of the cell face. Then while

alternately viewing through the other two holes (upper and

lower) of the plate, the source is moved along the optical

axis of the cell until the correct setting, in the judgement

of the observer, is achieved. Figure 11 depicts the equipment

set-up for the measurements while Figure 12 shows the appearance of correct and incorrect settings.

For the measurements reported here two persons were used

to make the settings; one served as operator while the other

served as observer. While the operator moved the source, the

observer sighted through the holes of the focus plate and

made the judgements as to the correct focus. Then they

exchanged roles until a total of ten such measurements (five

by each observer) were made.

- 18 -
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5.3 Parallax

This method is based on an established principle of optics

in which relative positions of an object closer to an observer

appears to shift relative to an object further from the

observer as the observer moves across the line joining the

two objects. This is shown in Figure 13. Although this principle was used on the first feasibility models, it later came

to be known as the Xerox method.

In this method an observer visually compares the motion

of the light bar of the eel] with that of a stationary object

positioned 150 feet behind the cell as he moves his eye

vertically across the projected beam from the cell. In the

present measurements, a light source with a slit type aperture

1/4 inch in height and approximately 1 1/2 inch in the horizontal plane was used to help the observer judge the proper

alignment. Two small rods are mounted from the cell face and

to the side of the unit. A diagram of the set-up is shown in

Figure 14. As had been done with the preceding method, two

persons were used for the measurements, one operator and one

observer. The operator varied the position of the cell source

until in the judgement of the cbserver the correct rate of

movement, was observed and alignment was achieved. The operator

and observer then exchanged roles until each observer made

five such judgements of the coincidence of the light bar and

extension rods near the extreme movement for a total of ten

readings

5.4 Auxiliary I mage-forming Lens

5.4.1 Observing Face of Cell. Focusing by the auxiliary lens

method employs the quantitative law of lenses that relates

distances of the source and the image to the focal length

as was explained in Section 3. This method is accomplished

with the use of an auxiliary hang-on lens. The lens

used in this investigation, furnished by NAEC, Philadelphia,

is plano-convex with a diameter of approximately 15 inches,

and a focal length of about 32.35 feet. The virtual

image formed by the Fresnel lens acts as the virtual

source of the auxiliary lens and forms an image at a distance q as is shown in Figure 15. This distance can be computed from eg. 1

1 1 1 q

32. 35 150.2

0.030912 - 0.006658

0.024254

or

q 41.23 feet

- 22 -
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- 25 -
Therefore, when the auxiliary lens is mounted over the

face of the cell, 0.2 foot from the Fresnel lens, the

image can be observed at a distance of 41.23 feet from

the auxiliary lens. To make the focus determinations,

an observer looks into the cell while an operator

adjusts the source position.

In this study the yellow lenticular lens was in

place in front, of the Fresnel lens. The lenticular lens

is intended to spread the beam horizontally without

affecting the vertical spread. A yellow lenticular lens

additionally absorbs the blue light and part of the green

light yet passes enough of the green light so that

longitudinal chromatic aberration may be used in

focusing as described below. The principle of chromatic

abei ration, shown in Figure 16, is used to determine the

proper position of the source. With improper settings

of the source, colored fringes (green, red, or red and

green, depending on setting) are obtained at the upper

and lower edges of the beam. F roper focus occurs when

all colored fringes are eliminated and a uniform yelloworange image is obtained. Ten such determinations of source position were made with one lens and seven such

determinations were made with a second lens.

5.4.2 Observing Beam Pattern Projected on White Card . This

procedure employs a slight variation of the method

described in Section 5.4.1. With this method, instead

of looking at the cell face, the observer looks at the

beam pattern projected onto an opaque white card placed

at a distance of 41.23 feet from the cell. The source

is then moved until the colored fringes disappear from

the edges of the pattern. If the white card is moved

closer to the cell than the proper focus requires, red

fringes will appear. If the white card is moved further

from the cell than proper focus requires, green fringes

will appear. For the measurements reported here, a second

white card was utilized. The first white card was placed

at the 41.23-foot distance, while the second white card

was held in hand and moved back and forth until the

distance was found at. which "no colored fringes" were

observed. If this distance differed from the card at

41.23 feet, the cell source was moved until the two

cards coincided. As with the other methods, 10 such source position determinations were obtained in this manner.

- 26 -
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- 27 -
Pattern Matching (Matching cell beam pattern to a standard

pattern)

\ \ The principle of the pattern matching method is to judge

when, at a given distance from the cell being focused, the

beam pattern formed on a translucent screen matches a "standard

pattern" placed at that distance. The standard pattern is a photographic color transparency of a cell beam pattern

properly focused on a transmitting diffuser. The vertical

and horizontal bars of the grille cause shadows in the beam and

form the pattern. The lenticular lens tends to diffuse light

into the shadows of the vertical bars and leaves only distinct

horizontal shadows in the pattern. To obtain the transparency

a photograph is made of the beam pattern after being transmitted

by the diffuser. For the measurements here reported, the

photograph was made at a distance of 20 feet from the face of

the cell. Similarly, the pattern-matching determinations were

made at the 20- foot distance. At this distance the beam spread

(at 50% points) of a properly focused cell is about 9.97 inches

in the vertical plane. The transparency for the standard pattern

is made to actual size by using appropriate dimension markers

in the initial photograph. To display the transparency for the

matching determinations, a box, painted white inside, is constructed

and equipped with a translucent plastic window and an incandescent

lamp mounted in the box. The window is made of the same milky

plastic material as the initial photograph. The plastic material

not only covers the window but extends about 12 inches to one side

of the box. The purpose of the extension is to receive the light

beam from the cell for the pattern matching determinations. This

arrangement places the beam pattern and the standard pattern

side by side for convenient pattern matching. Only a very thin

dark strip separates the two patterns. The window and the

extension plastic material are made in two sections. Their

abutting edges are painted black to prevent light spillage

from one section to the other by diffusion. The arrangement of

the equipment is depicted in Figure 17. Figure 18 is a photograph

of the pattern display device.

In use, the equipment is placed in front of the test cell

at the distance prescribed for the pattern. The light source of the cell is then moved along the optical axis of the Fresnel

lens as was done for the other focusing procedures until, in

the judgement of an observer, an exact match of the beam

pattern and standard pattern is obtained. Particular attention

is paid to matching the respective beam widths as this

parameter determines the proper focus of the cell. This

- 28 -
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- 29 -
- 30 -
procedure was used for the measurements reported here. Ten

such determinations were made with approximately half starting

with the pattern wider and half starting with the pattern

narrower than the standard pattern.

6 . Results

6.1 Photometric Procedures

The results of the focusing measurements of the five

lenses utilizing the photometric method of measurement are given in Figures 19, 20, 21, 22 and 23. These figures give

the beam spread, in inches, as measured at photometric distances

of 20 feet and 50 feet from the cell lens, for a cell equipped

in turn with each of the lenses as a function of the source distance from an arbitrarily fixed reference point. All the curves were made with respect to the same reference. These

data for the lenses and the FLOLS cell aperture were used to

compute the virtual image distance in feet and the beam spread

in degrees. Figure 24 shows the relation of source position to

virtual image distance and to the beam spread in degrees. Lenses

247 and 331 are sufficiently similar that their curves coincide.

It is apparent from curves of the type shown in Figures

19 to 23 that a source setting can be obtained to provide any

desired beam spread and that the accuracy of the setting can be

made to within about two thousandths of an inch. This uncertainty

in source position results in a virtual image distance un- certainty of less than two feet. Only one of the two curves

relating beam spread to source distance is needed if the

cell aperture is utilized in calculating the normal beam

spread at a given distance. Both curves are required, however, in cases where the aperture size is unknown, or uncertain.

The differences in focal length of the lenses are also

apparent from the curves as indicated by the point of intersection of the two curves in Figures 19, 20, 21, 22 and 23.

When the source is at this intersection point the light beam

from the cell emerges collimated and a near-zero beam spread

is obtained. Figure 2b shows that if the source is placed

exactly at the focal point of a lens, a collimated beam will

emerge and the virtual image will be at infinity. Notice that

the curves for lenses 704 (Figure 22)' and the "Bad Lens"

(Figure 23) furnished by NAEC were extended to the point of

intersection.

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- 37 -
6. 2 Other Procedures for Focusing

The results of the comparison focusing measurements of

other procedures for focusing the FLOLS System are given in

Tables I, II, and III. Only two of the lenses (No. 2117

and the Bad Lens) listed above were used for these measurements. Table I gives the individual readings as obtained

for the various focusing methods on lens No. 2117, while

Tables II and III separate the data into the maximum,

minimum and average readings, the reading variation, and the

calculated beam spread and image distance represented by

these readings for each lens. Note that one setting is given

in the tables for the photometric methods using a recorder or

a meter readout as these settings were based not on visual

judgements but on readings from spatial distribution graphs

for the lenses. The source was thus set to the known image

distance (150 feet) , as calculated for the cell without the

egg-crate grille.

7 . Uncertainty

7 . 1 Photometric Procedure Uncertainty

An estimate of the uncertainty in the location of the

virtual image position by means of the photometric method was obtained for lens 2117 and "Bad" lens. The beam spreads observed

at 20 feet and at 50 feet for the condition of the virtual image

computed to be at 150 feet from the lens and the beam spreads

obtained for these distances from readings, fitted the curves

shown in Figure 19 for lens 2117 and in Figure 23 for "Bad Lens".

The beam spread , in inches, at 20 feet from the lens

and the beam spread X^> in inches, were used to locate the

virtual source position q, in feet, following equation, based

on ratios of corresponding parts of similar triangles shown

in Figure 25.

20 + q X

1

(5)

50 + q X

2

or

50 X - 20 X

2 q

x - x

2 1

- 38 -
TABLE I. MEASUREMENTS OF SOURCE DISTANCE FOR

THE INDICATED FOCUSING METHODS TO

PRODUCE VIRTUAL IMAGE AT 150 FEET

Readings of Source Distances to Reference Point (inches)

(a) Lens No. 2117 (Required setting, 1.440 inches)*

huxi lidry Lens

[ \ n J—i

i i y

T.T wiLii T 4~ h f*\ uDbci V"\ C d VTver T {~\ V" Lens With Looking P?ttern

Peep Sight Target Screen

i Paral lax Matching

1.474 1. 509 1. 567 1.465 1.453

1.490 1.490 1. 536 1.490 1. 412

X . _) J—1.460

1. 518 1.500 1.496 1. 535 1 . 467

1. 559 1.520 1. 560 1.564 1.475

1.538 1.495 1. 519 1.689 1.450

1.545 1. 506 1. 531 1.640 1.428

1. 535 1.513 1. 520 1.450 1.446

1. 555 1. 514 1. 504 1 . 444

1.550 1. 506 1.552 1.430

Average 1.525 1. 504 1.534 1.544 1.447

Std. Deviation, a 0.030 0.011 0. 024 0.079 0.018

(b) "Bad Lens" from NAEC (Required setting, 1.795 inches)*

1.827 1. 860 1.803 Not used 1.742

1.744 1. 838 1.864 1.755

1.800 1.817 1.800 1. 774

1.800 1.826 1.786 1.740

1.747 1.857 1.894 1.786

1.758 1. 809 1.905 1.813

1.800 1. 818 1.715 1.812

1.806 1. 817 1.809

1. 790 1.829 1.766

1. 788 1.818 1.779

Average 1.785 1.829 1.824 1. 778

td. Deviation, a 0.025 0.017 0.063

I 0.026

*From photometric measurements.

- 39 -
TABLE II. COMPUTED BEAM SPREAD AND VIRTUAL SOURCE

DISTANCE AT THE MAXIMUM, MINIMUM, AND AVERAGE

SOURCE POSITION READING AS OBTAINED BY THE

INDICATED FOCUSING METHODS FOR LENS 2117

Focusing Method

Source Position

i

| Computed

) Virtual

Beam Spread

at 50%

Intensity

Level

(Degrees)

Readings

Max. Min . Ave.

( Inches')

Reading Range

Max-Min.

( Inches

)

Source

Distance

(Feet) 1

Peep Sight

1 . 559

1.482

1. 525

0. 077

218 0.196

168 0.255

199 0.215

Auxiliary Lens

With Target Screen

1 1.520

1.484

1. 504

0.036

193 0.221

169 0.253

177 0.240

Auxiliary Lens

When Looking Into

Cell Face

1. 567

1.496

1. 534

0.071

238 0.180

174 0.245

203 0.204

Parallax

1.689

1.450

1. 544

. 0.239

584 0.073

151 0.283

220 0.194

Photometric with

Recorder Set at 1.440* 150* 0.287

Photometric with

Meter Readout

Set at 1.440*

1

150* 0.287

Pattern

Matching

1.475

1.412

1.447

0.063

164

134

148

0.261

0.318

0.288

*See Section 6.1

- 40 -
TABLE III. COMPUTED BEAM SPREAD AND VIRTUAL SOURCE

DISTANCE AT THE MAXIMUM, MINIMUM, AND AVERAGE

SOURCE POSITION READING AS OBTAINED BY THE

INDICATED FOCUSING METHODS FOR "BAD LENS"

V

Focusing Method

FROM NAEC

Source Position Computed

Virtual

Source

Distance

(Feet)

Beam Spread

at 50%

Intensity

Level

(Degrees)

Readings

Max . Min . Ave

(Inches)

Reading Range

Max-Min

( Inches

)

Peep Sight

1.817

1.744

1.785

0.073

-

160

133

147

0.263

0.318

0.286

Auxiliary Lens

With Target Screen

1. 860

1,809

1,829

0. 051

184

158

167

0.228

0.268

0.253

Auxiliary Lens

When Looking Into

PpI 1 Fa rp

1.905

1.715

1.827

0. 190

215

118

165

0.196

0. 357

0.255

Parallax NOT MEASURED

Photometric with

Recorder

Set at 1.795* 150* 0.287

Photometric with

Meter Readout

Set at 1.795* 150* 0.287

Pattern

Matching

1.813

1.742

1.778

0.C71

158

131

144

0.268

0. 323

0.293

*See Section 6.1

- 41 -
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spreads by direct observation and by the fitted curves are considered to be an estimate of the range of measurements

by the photometric methods. These ranges are located in Table

IV for lens 2117 and the "Bad Lens".

7 . 2 Uncertainty in Other Procedures

The maximum and minimum virtual source positions obtained

by means of the other procedures are listed in Tables II and

III and give the range for those procedures. These ranges

are also listed in Table IV for comparison with the photometric

method

) 8 . Discussion

Generally, results obtained with all focusing methods where visual

judgements were required differed greatly from the results obtained

photometrically. Large variations in individual readings were obtained

also; although some readings within a group were close, they tended to

give much longer image distances than the specified 150 feet. For

example, for Fresnel lens No. 2117, the peep sight method gave image

distances from 168 feet to 218 feet. Similarly, the two procedures

that use an auxiliary lens gave virtual image distances much too long

for this Fresnel lens. The only visual method that gave reasonable

results for this lens was the pattern matching method. Here again, however, the spread between individual readings is considered too great.

The same trend is seen for the measurements of the "Bad Lens", except that

the set of measurements with the peep sight method gave much closer

tolerances. An image distance spread from 133 feet to 160 feet was obtained during the ten individual readings. This again, is considered

greater than desired but does show what results this method might

achieve from lens to lens. Again, the other methods (except the

pattern matching method) gave results that tend to place the image at

too great a distance. The pattern matching variation was the same order

of magnitude for this lens as for the previous lens.

9. Recommendation

In conclusion it is recommended that the precision of a focusing

method be sufficient to determine the virtual image distance to

5 feet, consistently. To achieve this precision the source needs to

be positioned within 0.010 inch. Of the focusing methods tested only

the photometric method appears to meet this criterion. It is

recommended also that no method that requires visual judgements be

used.

- 43 -
TABLE IV. RANGE OF VIRTUAL SOURCE POSITION FOR

LENS 2117 AND "BAD LENS" BY MEANS OF

SEVERAL PROCEDURES

Procedure

Range, in feet, of Virtual Source Position

2117 "Bad Lens"

Peep Sight

Auxiliary Lens:

1) With target screen

2) Looking into cell face

Parallax

Photometric

1) With recorder

2) With meter readout

Pattern Matching

50

24

64

433

55 30

27

26

97

27
Of the two photometric methods, the procedure that uses a meter readout would appear to lend itself better to field use. It is

simple, precise and will give fairly rapid results. Further, this

method would require only a short period of training in its use.

- 45 -
APPENDIX

A-l OTHER FACTORS AFFECTING CELL PERFORMANCE

In conducting the focusing investigations, certain other

factors were observed relative to the FLOLS system that are

of interest. These include:

1) Effect of the lenticular lens on beam spread.

2) Effect of lens temperature on beam spread.

3) Effect of slit-width on beam spread.

4) Effect of slit alignment on beam distribution.

5) Effect of lamp dimming and source color on beam

spread and image distance.

In addition, certain operational and mechanical design

features of the FLOLS cell were noted where improvements are considered desirable. The first section below treats the

operational features while the second section treats the

design features.

A-2 OPERATIONAL FEATURES

A-2.1 Effect of the Lenticular Lens on Beam Spread.

Beam spread measurements were made of a cell with and

without the lenticular lens installed. The measurements were made with a 1980 Pritchard recording photometer by scanning the

light beam pattern as projected from the cell onto a diffusing

plastic plate at a test distance of 20 feet. The temperature

of the cell was allowed-to stabilize at approximately 35°C prior

to the measurements. The results of the measurements are given

in Figure A-l. As will be noted, the lenticular lens increases

the beam spread of the cell by about 0.21 inch at this distance.

The increase amounts to approximately 0.06 degree or 3.6

minutes of arc in the vertical plane. Instead of being

neutral, the lenticular lens exhibits some "lens effect" in

the vertical plane as well as its intended function of

spreading the beam in the horizontal plane.

- 46 -
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A-2. 2 Effect of Lens Temperature on Beam Spread.

A series of measurements were made of the beam spread at

lens temperatures of 22°C and 35°C as a function of source distance from a fixed reference point. The cell was equipped

in its usual configuration with a lenticular lens and eggcrate grille. The measurements were made photometrically in a manner similar to that described in paragraph 5.1, except that

for these measurements the heater circuit of the cell was turned

off during the first set of measurements and turned on during

the second set of measurements. Air temperature inside of the

unit adjacent to the lens was monitored with a thermocouple

arrangement. The temperature was allowed to stabilize for

about 30 minutes prior to the measurements.

Results of the two sets of measurements are shown in

Figure A-2. The beam spread of the cell increases with lens

temperature. The increase has the effect of decreasing the

distance to the virtual source. Conversely, if the heaters

become disabled during operation and the lens cools, the

distance to the virtual source increases. For example, at a given point near proper lens setting (source setting 1.71 inch)

the distance to the virtual source changes approximately 12.8

feet for a temperature change from 35°C to 22 °C. This amounts

to an approximate change of 1 foot per degree Celsius.

A-2. 3 Effect of Slit-Width on Beam Spread.

Beam spread measurements were made of a cell equipped in

turn with two slit-widths, namely: 0.055 and 0.070 inch. All

other parameters were held constant during these measurements.

Again, the beam spread measurements were made photometrically

at a test distance of 20 feet with a recording photometer

similar to the one described in paragraph 5.1.

The results of the measurements are presented in Figure

A-3. As indicated, the slit width had no appreciable effect

on the beam spread.* Although the wider slit gave a curve which appears to be slightly wider, its relative peak intensity

is also higher and when the two curves are read at 50% of their

respective peaks, their beam spreads are essentially the same.

A-2. 4 Effect of Slit Alignment on Beam Distribution.

Beam distribution measurements were made of the cell as

a function of the slit alignment relative to the optical axis

*Note the width of the "meat ball" is not directly related to the beam

spread, but is increased with increase in slit v/idth.

- 48 -
I
- 50 -
of the lens. For this measurement, only the 0.055 inch slit was

used. For the cell in use, the mounting holes of the slit

plate were oversized so that the slit plate is not pinned in a fixed position. The extreme movement of the slit plate

amounts to approximately 3/32 inch up and down or sideto-side. Because of the slit dimensions and the effects of

the lenticular lens, the up and down movement is critical

and significant while the horizontal movement is not

significant. Therefore, measurements were made of the cell

only as the slit movement affects the projected beam distribution in the vertical plane. The .first measurements were

made with the slit shifted to the extreme upward position

while the second measurements were made with the slit

shifted to its extreme bottom position. In positioning the

slit, attempts were made to maintain a horizontal position

although the slit can be tilted slightly at the two extremes.

The results of the measurements are shown in Figure A-4

It is evident that the two distributions differ considerably.

The need for pinning the slit in a fixed position once the

cell is properly aligned is evident. The pinning should be

accomplished at the factory during the initial alignment

to prevent any likelihood of slippage or maladjustment at

a later time.

A-2.5 Effect of Lamp Dimming and Source Color on Focusing.

Measurements of beam spread as a function of lamp dimming

and of source color were made of a FLOLS cell equipped in turn

with a yellow and a clear lenticular lens. Measurements were

also made of the cell equipped with the clear lenticular lens

and sources of several colors. The beam spread measurements

were made photometrically as described under the section 5.1.

During the measurements, the three lamps of the cell

were operated in series and the cell temperature was allowed

to stabilize prior to obtaining the successive readings. For

the dimming measurements, the lamps were operated at 54, 20

and 12.5 volts when the cell was equipped with the yellow

lenticular lens and operated at 54, 20, 12.5, and 10 volts

when equipped with the clear lenticular lens. The latter

voltage range corresponds to an intensity range from approximately 100% to 0.3%. The voltage was held constant at 54

volts when making the measurements of the effects of source color on the beam spread. For the latter measurements, the

color was changed by inserting colored glass filters in turn

between the source (lamps) and the frosted glass covering the

slit of the cell.

- 51 -
1 - 52 -
The results of the lamp dimming measurements are given in

Table A-I and the results for the source color measurements

are given in Table A—II. For both types of measurements the

distance to the virtual source was calculated from the beam

spreads obtained and the aperture of the cell egg-crate grille.

The geometrical theory for the calculations is as explained in

Section 5.1 of the report under the title "Description of

Focusing Methods". As will be noted, dimming of the lamps

has very little effect on beam spread, and hence, distance to

the virtual source. Notice that a very small change was obtained at extremely low levels of intensity. Chromatic

aberration of the Fresnel lens as the lamp becomes redder

probably accounts for the small change. This effect is more

pronounced in the source color measurements discussed below.

Referring to Table A-II, it will be noted that source

color changes do affect the beam spread and image distance

significantly. For example, with a red filter in front of

the source the beam spread was 10.3 3 inches and with a blue filter the beam spread was 9.53 inches under otherwise

identical conditions. The corresponding image distances for

these two beam spreads are 114 feet and 237 feet, respectively. The effect then is the redder the source, the

shorter the image distance, for a given position of trie source. Consequently, it becomes necessary to focus a cell for the color

with which it will be used. If the color is changed, the unit

must be refocused. (Similar differences in image distance would

be expected when cells of various colors are focused by means

of any of the visual focusing methods, although these were

not performed during this series of measurements.)

A- 3 MECHANICAL DESIGN FEATURES

A- 3.1 Lamp Mount Assembly.

Focusing of the FLOLS cell requires that the assembly

which carries the lamps, frosted glass and slit be moved along

the two support rods in the focal plane of the lens until the

proper beam spread and/or distance to the virtual image is

achieved. In moving the assembly, it was difficult to keep

the two sides equi-distant from a given reference point on the

support rods. One side of the assembly was usually moved

first along one support rod. The other side was then brought

abreast of the first and so on until proper adjustment was

obtained. A feeler gage or calipers was used to determine

when both sides of the assembly were moved the same distance from a given reference point. Movement of both

sides an equal distance cannot be achieved visually. Since

- 53 -
TABLE A

EFFECTS OF LAMP DIMMING ON EEAM SPREAD

AND VIRTUAI SOURCE DISTANCE

Virtual

Lenticular Color Lamp Volts Beam Spread*

( Inches

)

Beam Spread

(Degrees)

Source Distance

(Feet)

Yellow 54 10. 13 0 . 318 131

o nu in l ^ n no 131

12. 5 10.20 0. 334 126

Clear 54 9.73 0.222 187

20 9.76 0.229 183

12.5 9.81 0.241 173

10.0 8.82 0.241 173

*At 20 feet.
TABLE A- I

EFFECTS OF SOURCE COLOR ON BEAM SPREAD

AND VIRTUAL SOURCE DISTANCE

.(Clear Lenticular)

Source Color

(Filter between

CAlivnp ST. Q "I -j +-

1 Lamp Volts Beam Spread*

( Inches

)

Beam Spread

( Degrees

)

Virtual

Source Distance

(Feet)

Clear 54 9.73 0.222 187

Blue 54 9.53 0.174 238

Yellow 54 10.05 0.298 140

Red 54 10.33 0.365 114

*At 20 feet.

- 55 -
satisfactory results were obtained by using the method

described, accurate calipers should be available at each repair

or alignment facility. Further, because this procedure complicates the focusing of a cell, consideration should be given to

redesigning the lamp mount so that the two sides of the assembly

can be moved simultaneously on the mount.

A-3.2 Fresnel Lens Mounting.

The Fresnel lens is held in place against its seating

studs by four spacer-type 90° angle clamps. The clamps are padded with a rubberized sponge material on the surface that

bears on the lens. When first installed, the material is

resilient and provides sufficient pressure to hold the lens

securely in place. After prolonged use, however, the material

becomes compressed or packed and no longer holds the lens

securely. With the clamp padding compacted, slight movement

of the lens can occur. The effect of the lens movement, of

course, is to change focus position with respect to the slit

and, hence change the location of the virtual source. Consequently, consideration should be given to replacing the present

rubber pads by a more stable material.

A-3.3 Lens Temperature.

The temperature of the lens of a FLOLS cell is normally

held constant to within a few degrees by circulating the internal

air over thermostatically-controlled heating elements. When

the system is operating normally, temperature control appears

adequate. However, of the four cells on hand, three were found

to have wiring failures at the point where the power terminal

lugs are connected to the heater element. A form of

corrosion develops at this location that may cause electrical

failure over a long period of time. It appears advisable,

therefore, that a different type of heater be used or at

least a more permanent type of connection to the heater be

substituted for the one currently in use. Cognizant Navy

officials indicated orally that changes to avoid heater

failures are currently being implemented. The defects are mentioned here solely to report the failures noted in the cells

at the National Bureau of Standards and to confirm that similar

type failures had been found.

USCOMM NHS -DC - 56 -
NSS-W4A 'Fit..'. 7-73)

U.S. DEPT. OF COMM. I PUBLICATION OK REPORT NO.

BIBLIOGRAPHIC DATA NBSIR 75-644

SHEET

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Focusing the Fresnel Lens Optical Landing System

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

W. F. Mullis, I. Nimeroff, and E. L. Walters

8. PerformingOrgan. Report No. NBSIR 75-644

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 (Streel, City, State, ZIP)

Department of the Navy

Naval Air Engineering Center

Philadelphia, Pa, 19112

13. Type of Report & Period

Covered

Final, April 1975

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information, if document includes ti significant

bibliography or literature survey, mention it here.)

The Naval Air Engineering Center requested the National Bureau of Standards to

develop a method of evaluating the performance of lenses of the Fresnel Lens Optical

Landing System (FLOLS) . In the course of this work, it was found that focusing of the

cells of a FLOLS unit varied considerably. This led the NBS to examine the various

focusing techniques used in the past and to develop a more precise method for focusing

a cell. The focusing methods studied include the following: 1. Photometric,

2. Peep Sight, 3. Parallax, 4. Auxiliary Image-Forming Lens, 5. Pattern Matching.

In addition, in the study of these focusing methods, several other factors that

affect the light-beam- forming characteristics of a cell were investigated. These include

the effects of, 1. Lenticular Lens, 2. Lens Temperature, 3. Slit Width, 4. Slit

Alignment, 5. Lamp Dimming and Source Color.

The results of this study indicate that visual methods of focusing the FLOLS system

have inherent difficulties that lead to improper focusing. Photometric procedures

developed at NBS, however, can give adequate focusing precision.

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

)

Eeam spread; Fresnel lens; focusing methods; photometric focusing; visual landing

system.

18. AVAILABILITY Unlimited

[x| For Official Distribution. Do Not Release to NTIS

Q^j Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. CM

[7 ] Order From National Technical Information Service (NTIS)

Springfield, Virginia 221M

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21. NO. OF PAGES

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USCOMM-OC 29042-P74