03 - Holography Technique and Practice

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H O L O G R A P H Y T E C H N I Q U E A N D P R A C T I C E

MATT LEHMANN

Abstract

A discussion of what a hologram is and how it is made is followed by a detailed procedure describing how to determine whether a proposed holographicsystem will be successful. Particular attention is given to the formation of thefringe pattern as it is the delineation of the fringes which determine therequirements and quality in holography. Several holographic systems arediscussed giving the advantages and disadvantages of each as well as adescription of the optical elements required.

Introduction

During its short life, the laser has probably had more misinformation published about it than any other scientific discovery. A few years ago a

San Francisco newspaper published a full page advertisement, depicting.

a laser cannon shooting down missiles, entitled ‘The Incredible Laser’.

Dr Arthur Schawlow, one of the inventors of the laser, has this advertise-

ment posted on the door of his laboratory with the sub-caption, ‘For

CREDIBLE Lasers, See Inside!’ The laser is, however, truly an

‘incredible’ tool in the field of optics. It has made possible the technique

of wave-front reconstruction, more popularly known as holography.The dramatic realism achieved by holographic reconstruction has

captured the imagination of scientist, engineer and layman alike. As

with any new dramatic discovery, reports frequently have scant regard





MATT LEHMANN HOLOGRAPHY-TECHNIQUE AND PRACTOCE

The P I R. O* is the real or conjugate image coming into focus in front

of the holographic plate. The presence of both a real and virtual image is

characteristic of all holographic processes, but in pictorial holography

the recording geometry is usually arranged so that the real image does

not interfere with viewing the virtual image appearing to exist behind

the plate or film.

The reflected light from the object and the light from the reference

must originate from the same highly coherent source to form inter-ference fringes on the film. Coherence means that there must be a

definite continuing relationship between the object and reference illu-

mination. This is accomplished if all the light originates from one point

and is all of the same wavelength (i.e. colour). Fortunately many kinds

of lasers melt this requirement as a source of illumination.

Reference

Emulsionthickness

z ‘ Filmbacking

Fringe spacing

’

The formation on photographic film of a fringe pattern expressing the

position of every point or resolution cell on the object as it relates to the

reference is a hologram. A thorough understanding of how these fringes

are formed is necessary to evaluate the physical limitations in making the

hologram. The stability of the system, the film resolution required, and

even to some extent the degree of coherence required are all dependent

on how these fringes are formed.

F i l m

emulsion

In the formation of the hologram fringes the angle 0 between the

reference and object is all important (Fig. 2a). Each pofnt on the object

bears a distinct relationship to the point reference dictated by the angle

formed between the object ray and reference ray as they meet on the

surface of the film. The spacing of the fringes can be shown by simple

geometrical construction to follow the equation (fig. 2b).

I

Large

Fine fringes

cl

Coarse fringes

where d is the spacing between fringes and λ is the wavelength of the

light used. It is apparent that a single wavelength source must be used,for with multiple wavelengths, each wavelength would produce its own

set of fringes and make reconstruction impossible.

This equation that expresses the relationship between the fringe

spacing, illumination angle and wavelength is known as Bragg’s Law and

is fundamental to hologram reconstruction as well as recording.

Every point on the recording film and on the object will provide a

slight variation to the angular relationship and consequently a variation

in fringe spacing. Since it is the fringe spacing that delineates the object

4

ik

fringes

0

(e)

Fig. 2 Fringe spacing dependence on object/reference angle.

during reconstruction, all of the optical information about the object is

recoverable and the parallax and three-dimensionality of the reconstruc-

tion is assured.5



MATT LEHMANN

Since the spacing of the fringes is inversely proportional tosin40

(assuming, of course, a single wavelength source), the larger the angle 0

the finer the fringes (fig. zc). Conversely, when the angle is small, the

fringes are far apart (fig. 2d ). This is a relative matter, however, since

when 0 = ’1 the fringe spacing is approximately 40 /on or - of a milli-

metre. This is coarse, however, when compared with the thickness of the

film emulsion which is less than 20 /fin. When the fringes are compara-

tively coarse the hologram acts as a diffraction grating and can be viewed

by a point source of partially filtered white light, such as from the sun

or a flashlight through coloured gelatin. As the fringes become finer the

Bragg effect becomes apparent. This requires a fairly precise alignment of

the hologram with the illuminating beam to effect constructive refraction

by the fringe pattern. This angle corresponds to the conjugate of the

reference angle during recording (i.e. the angle the reference beammakes

with the surface of the hologram).

When the reference-object angle θ approaches 180° the fringes are

formed parallel (or nearly so) to the surface of the film emulsion (fig. ae).

The fringe spacing becomes +h or about o-3 ilrn. Holograms made with

this geometry are called Bragg-Lippman type, Denisyuk holograms, or more descriptively, reflection holograms. These holograms are distinc-

tive in that they can be viewed in the reflection of a point source of white

light. Complying with the Bragg condition that constructive interference

is dependent on wavelength, illumination angle and fringe spacing, the

hologram acts as a colour filter and only the colour (i.e. wavelength) thatis a function of the illumination angle and fringe spacing reconstructs to

form the image.

2. Requirements for making a hologram

A hologram is a system of fringes recorded on photographic film or plate.

If the fringes move one-half fringe width during recording no fringes are

recorded and there is no hologram image. When the fringes move less

than one-half fringe width the hologram image may not be destroyed

but the brightness is adversely affected. Unlike conventional photo-

graphy, movement causes loss of image brightness rather than blurring.

It is apparent, therefore, that the finer the fringe spacing the greater the

restriction on movement during recording. This means movement of the

object being recorded as well as movement of the recording film, mirrors,

beam splitters or even the air through which the illuminating beam must pass.

HOLOGRAPHY-TECHNIQUE AND PRACTICE

2.I. Stabili ty r equir ement

The stability of the hologram recording system must be assured before

any attempt is niade to photograph the fringes. This can be accomplished

by setting up what corresponds to a long-legged Michelson interfero-

meter on the table intended for the system, and observing the fringe

stability. The bcnm splitter and first surface mirror no. I, which are to be part of the system, are set up as shown in fig.

3~.

The laser beam is

Laser Beam

a. Stability

Laser Beam

b. Coherence

Fig. 3 Stability and coherence measurement.

directed through these elements and the two beams thus achieved re-

flected back on themselves by two additional first surface mirrors, nos. z

and 3. These mirrors are placed on the work surface down the beam

approximately where the object is to be located during recording. The

distancedshould be approximately equal to dr + . Adjust the two mirrors

nos. 2 and 3 until the back reflections of each mirror are superimposed on

a wall or other matte surface. Placing a short focal length lens in the back

reflected beam near the splitter will then expand the superimposed

beams so that the interference fringes can be clearly seen. Observing the

movement of these fringes will indicate the stability of the proposed

system. If a fringe moves as much as one-quarter of its width during the

7



MATT LEHMANN

time that will be required to expose the hologram film the system stability

must be improved to ensure success.

If the lack of stability is indicated, check the following:

1. Floor vibration transferred through to the work surface.

2. Inadequate rigidity of splitter or mirror supports.

3. Air movement across the light path from air conditioning or venti-

lating system.

4. Temperature changes in any part of the system. (Handling of

mirrors or mirror supports can warm up these elements such that the

shrinkage as they return to the ambient room temperature is sufficient to

cause fringe movement.)

5. Accoustical disturbances such as loud talking or radios playing

which cause vibration of the optical elements.

2.2. Coherence measurement

This same set up for measuring stability can also be used to determinethe coherence length (i.e. axial coherence) of the laser. While it is true

that a laser, especially a HeNe gas laser, is highly coherent, its coherencelength is limited. The depth of an object of which a hologram can be

made is limited by the coherence length of the source. For this test th e

two mirrors nos. 2 and 3 can be placed quite close to the splitter and

mirror no.I

(fig. 3b). This will reduce the stability problems originating

from air movement and accoustical disturbances.. With the distance

from the splitter to the mirrors equal, that is, d = di +da,

both paths are

identical and the fringes projected on the wall or screen will be of maxi-

mum contrast. When mirror no. 2 is moved back the fringe contrast will

be degraded. For most gas lasers axial coherence is a function of laser

cavity length. Note that ∆d is actually a round trip path so that when ∆d

equals one-half the laser cavity length this is a light path difference of

one cavity length. At one cavity length the fringe contrast will be a

minimum increasing again to almost full contrast at two cavity lengths

of path difference. For maximum fringe contrast in the hologram the

maximum path difference must be kept to a reasonable fraction of the - laser cavity length. A safe figure is one-quarter cavity length, but this can

be increased if the coherence length test indicates the axial coherence

is sufficient.

The real image of the transparency can be viewed with a CW laser. A

small diameter beam from the HeNe laser is passed through the pro-

cessed hologram. The real image can be detected on a matte surface.

Scanning the hologram with the small beam should produce an image in

which there are only minor changes in brightness over the image surface

Movement of bright areas in the reconstruction indicate lack of trans-

verse coherence.

Transverse coherence of the laser can be checked by observing the con-

figuration of a cross-section of the laser beam (see plate I a). To observe

Axial coherence measurement requires a more elaborate system. A

the beam it must be expanded with. a lens and spatially filtered with a

hologram is made as for transverse coherence except that the object in

this case is a Fabry-Perot interferometer with specially coated mirrors


pinhole filter. If the laser is operating in theTEAI,

mode the output

from the filterwill beasmooth Gaussian spot (see plate I b). Other modes,which will bc doughnut or double ovals in shape, indicate a deficiency

in transverse coherence (see plates I c and I d ).

Filtering of the laser beam can be accomplished by placing a pinhole

at the focal point of the expanding lens. The size of this pinhole D can be

determined from the equation

whereD = 2 1*22 ;5

7

λ is the wavelength of the laser light,

d is the diameter of the laser beam incident on the lens, and

f is the focal length of the lens.

A I mil o 001 in) pinhole will effectively filter the average laser when

a IO x microscope objective is used to expand the laser beam. It is well

to point out that a I mil pinhole is difficult to position without a three

axis micro-positioner.Coherence measurements of a pulsed laser require a somewhat different

approach. The pulsed nature of the laser output makes it impossible to

observe the fringes visually so a photograph must be made. A hologram of

the laser output will give the necessary information about the coherence.

Transverse coherence can be determined by making a hologram of a

transparency of a letter or number on a diffusing background (fig. 4a).

Prisms arc used as splitters and for beam steering to avoid mirror

damage frequently encountered with high-peak energy lasers. A nega-

tive lens is used to expand the beam to avoid ionization of the air at the

focal point of a positive lens due to high-energy concentrations. Stability

is, of course, no problem with sub-microsecond pulses.

9



MATT LEHMANN HOLOGRAPHY-TECHNIQUE AND PRACTICE

and with one mirror sloped slightly to obtain multiple, spaced, internal

reflections (fig, 4b). The mirrors are coated to be selectively reflective to

the wavelength being examined. For the ruby laser this is694.3

nano-

meters. The multiple reflections in the Fabry-Perot are similar to the

multiple reflections in a barber shop mirror. A hologram made of the

output from the interferometer will reveal a number of equally spaced

Prism splitter Prism

to set up the system and expose the film. However, some discussion of

film response’ is pertinent. There is always a trade-off between film speed and resolution. The high resolution films are invariably very slow. It is also essential that the film selected have adequate sensitivity in the

spectral range of the laser being used. Information on the relative spectral

sensitivity of a film is usually available from the manufacturer; however,

the information on film speed is generally not adequate for holography.

Transmission holograms are reconstructed by illumination of the

film or plate and observing the image by the light transmitted through the emulsion. It is therefore desirable to have the film as transmissive as

possible consistent with enough density to carry the fringe information

essential for image reconstruction. The film curve which gives-the

incident energy requirements is called the T/E curve (i.e. Transmission- Exposure curve). Film manufacturers customarily present their film

characteristics with the H and D curve (Hurter and Driffield), which

a. Transverse coherence

Prism splitter

Special ly coatedmirrors. adjustable

Hologramfilm

b. Axial coherence

Fig. 4. Pulse laser coherence measurement.

is a plot of density against log ex osure and hence improper for holo-

graphy.

The ideal exposure for a hologram is that exposure required to pro-

vidc a transmitted intensity varying uniformly about the mid-point of the straight-line portion of the T/E curve. Furthermore, the variations

in exposure should not extend beyond this straight-line segment of the

curve (fig. 5).

The fringe contrast formed by interference between the object and

the reference beams will produce exposure variations about the mean

measured intensity. When these exposure variations stay within the

straight line portion of the T/E curve the resulting transmission through

the processed film is an amplified replica of this exposing wave. The

resulting image in the hologram is actually brighter than would be anti-

cipated with conventional imaging on the same film. Actual amplification

of the imaging process has been accomplished by holographic recording.It is also apparent that if the maximum or minimum esposure extends

beyond the straight line portion there will be some distortion of the

transmitted information. The result of such distortion in the hologram is

higher order images (ghost images) and cross-correlation. A study of the

T/E curve will also show that when the bias point is placed for low trans-

mission (higher densities), very large variations in the input result in

only small variations in the transmitted output. This means that the

hologram image will be of low brightness and weak contrast.

The ideal bias point for exposure is that which records fringes that

spots.- Dependent on the geometry of the system and spacing of Fabry-Perot mirrors, the second or third spot will define a zero path-length

difference. The other spots will correspond to progressively increasing

path-length differences equal to twice the mirror spacing. Thus the

visibility of the spots in the hologram will indicate the coherence length

of the laser as a function of the Fabry-Perot mirror spacing.

2.3. Film exposure

Having ascertained that the stability of the system and the coherence

of the illuminating source are adequate for holography, it only remains

I0 II





MATT LEHMANN

1 . 0

0 . 8

0 . 6

E a s t m a n 6 4 9 F tia?e I

4

tiIi i

0 2

I

\ i

I

00 IL erJ

t x p o su r e

rp l

cm-1

I I I I I

2 0 4 0 3.

6 0

1.0

c.o

0 . 6

*I

0 . 4

t-

0.2

a

0

ib

0 .20 .40 .60

Exposure pi/cm*

.80

Fig. 6. Characteristic transmission/exposure curves for several filmsavailnble for holography.

A narrow angle hologram such as this is called a lensless Fourier-

transform hologram, or under certain conditions a Fraunhofcr or far

field hologram and is distinguished by the reconstruction of both the

image and its conjugate flanking the bright centre spot of the reference.

The larger film grain inherent in fast films is also a limiting factor, When

the film grain size is a large fraction of a wavelength of light it diffracts

the light passing through the film and causes a bright glow around the

I4


centre spot which may spread over and swamp the reconstruction of the

image. This is called film grain noise and although it varies considerably

over different films it is more prevalent in the faster emulsions.

The best films for holography are the very high-resolution, fine-grain

emulsions such as Eastman Kodak 649F, (a spectroscopic film), or

Agfa-Gcvacrt SE70 which was developed specifically for laser photo-

graphy (fig. 6~2 . Both of these films are capable of resolving better than

3ooo lines per millimctre and can be used to make reflection holograms

which require the maximum in resolution capability. Both EastmanKodak and Agfa-Gevaert make somewhat faster high resolution films

with different characteristics curves (fig. 66).

All four films for which T/E curves are given were exposed by a

HeNe laser, processed for 5 min in Eastman HRP developer, then washed

and fixed for 3 min. Processing solutions and baths were maintained at

2 C (68 °F).

2.5. Fi lm processing

Processing of the film is a straight-forward procedure following the

manufacturers recommended practice. High resolution developers are

preferred. Maintaining precise temperature control is not essential but

would be desirable if consistent results are required. One word of warn-

ing: the Eastman Kodak HRP (High Resolution Process) Developer

will cause darkening of emulsion when the developer is old or nearly

exhausted.

Experimental studies of phase holography can be made by bleaching

the emulsion during or after processing. The potassium dichromate-

acid bleach works very well but will cause a brown stain if it is used after

the emulsion has been fixed. Information on processing and bleaching is

available through most film chemical manufacturers. The phase holo-

gram is interesting in that all the silver is removed from the emulsionleaving a clear film or plate that still permits reconstruction of the

hologram image by diffraction of the transmitted light through the

plate.

Most photographic emulsions tend to shrink about 1 during

fixing apparently due to the removal of unexposed silver halide. This

becomes critical when reflection holograms are made as the fringes are

parallel to the surface of the emulsion so that shrinkage reduces fringe

spacing. As we have seen, the fringe spacing determines the colour with

which these reflection holograms reconstruct; thus a reduction in fringe

I5



MATT LEHMANN

spacing means a downward shift in wavelength. Holograms made with

the red illumination of the NeHe laser reconstruct in the green. Tocorrect this the film emulsion can be expanded back to its original

volume by soaking for 5 min in a 5% solution of triethanolamine. It is

possible, of course, to expand the emulsion to a greater than its original

thickness and produce a hologram that will only reconstruct in the infra-

red. Fortunately the triethanolamine is readily soluble in water and can

be removed by washing the film in a water-bath if it has remained too

long in the expanding solution. It also appears that film emulsions will

continue to shrink over a long time period and may have to be re-

expanded to reconstruct at the proper wavelength.

3. The elements of holographic systems

System layouts for holography range from the very simple system con-

sisting of only an expanding lens and film holder (fig. 7a) to a more

sophisticated layout with beam splitters and mirrors (fig. 8).

We have discussed methods and techniques of checking the facilities

for hologram making and the basic problems that may affect the qualityof the product. The system layout for holographic photography is

HOLOGRAPY-TECHNIQUE AND PRACTICE

tant factor since with higher energy levels larger objects can be recordedwith shorter exposure times. The laser, however, need not be on the

same stable surface with the optical system.

3.2. Mirrors and splitters

Some sort of optical bench placed on the stable surface is desirable

but not essential. The optical bench permits precise location of the

various optical elements and will make it possible to repeat an experi-ment or make accurate measurements if this is essential. The beam

splitter and mirror holders can be firmly supported on the bench. Clean,

best quality, first surface mirrors are necessary. Good first surface

mirrors on in plate glass are inexpensive and readily obtainable.Mountings for mirrors need not be complicated or expensive. They

should be firm and rigid. Mirrors can be cemented to a brass or aluminum

frame or even held securely with edge clip mountings made for ordinary

plate mirrors. Care must be taken to use cements such as epoxy which

will not creep. If clips or similar holders are used they must be firm but

not clinched tight as the strains set up in a mirror will gradually relax and cause creeping of the mirror with consequent fringe movement. Holders for splitters are of necessity a bit more complicated as they

must be edge mounted to permit the laser beam to pass through the

splitter. A metal yoke with the bottom and the two sides rabbeted to

accept the splitter which is held firmly by spring clips is quite satis-

factory. A splitter made with graded densities of Inconel is most useful

as this permits varying the reflected and transmitted beams without

extreme loss of energy. Inconel coated glass wedges are available from

several optics supply firms. These will permit rcflectivities of from 4%

(uncoated glass) to 99% with the unreflected portion passing through

the splitter. Since it is usually necessary to have a much more intense object illuminating beam than a reference beam (particularly with low reflection objects), the ability to vary the reflected percentage without

loss in the transmitted beam is most advantngeou

splitters also reflect from the second face of the splitter. This produces

two beams close together, and when they overlap fringes are formed.

which adversely affect the uniformity of the illumination. A thick glass substrate (p in) for the splitter will separate the two reflections, and elim-

inate this annoyance. In the event that Inconel splitters are not available

and one of the beams is excessively bright is can be attenuated with glass

neutral density filters readily available at all photographic supply houses.I7

limited only by the imagination of the holographer. As we have seen the

hologram is very sensitive to vibration or movement of any kind so the

fewer optical elements that are incorporated the less

them

beam

chance that one of will produce undesirable fringe movement. Furthermore, after the

has been spatially filtered, each additional optical element adds unwanted spatial intensity variations.

3.1. Stable support

The first consideration in any set up to make holograms is a stable

surface to support the optical elements. The degree of stability required

is dependent on the laser power available and the elaborateness of the

holography contemplated. A simple Fourier-transform hologram (fig.

7c), where the reference, object, and film holder are contained in the

same support, can be made on an ordinary workbench! However, a more

general approach is usually desirable. A massive granite slab supported

on efficient vibration dampeners such as air cushioned supports makes

an ideal work surface.

A I in thick marble slab resting on 4 in of foam rubber or even on a

thick stack of newspapers will frequently provide adequate stability for

fairly sophisticated holographic experiments, Laser power is an impor-I6



MATT LEHMANN HOLOGRAPHY-TECHNIQUE AND PRACTICE

3.3. Lenses and spatial filters 4. Types of holography

Since the output beam from the laser is from z to 5 mm in diameter

it must be expanded to fully illuminate the object, or in the case of the- -reference beam, it must cover the film surface. Inexpensive microscope

objectives are eminently successful for this purpose The reference

beam is spatially filtered after passing through the splitter and lens so that

high quality lenses are not necessary.The spatial filter placed at the

focal point of the reference beam expanding lens must be a high quality

pinhole 1 in or less in diameter. Such small precise pinholes are

best made by electroplating processes. Several manufacturers stock

these pinholes, formed in nickel, from o*ooo in diameter upwards.

3.4. Film plate holders

The basic hologram set up (fig. 7a), which is the simplest possible

arrangement to produce a hologram, will not, however, produce the

simplest hologram. The angle between the object and reference is

Laser n.J

Lens Pinhole

filter

BASIC HOLOGRAM SETUP (a)

Film and plate holders are also available for purchase. These, like the

mirrors, must be firmly supported but not stressed. When reflection

holograms are made both faces of the plate must be exposed. Therefore

conventional holders are not usable. A forked structure supporting the

plate on three sides similar to that described for the beam splitter will provide the rigidity required. The plate must be held firmly by a spring-

loaded clip but like the other optical elements must not be stressed. It

must be remembered that handling any of the optical elements or their

supporting structures raises their temperature. They must be allowed to

dissipate their heat and return to the ambient temperature of the surround-

ing air. This is particularly true of the photographic plate as it is handled preparatory to placing it in the film holder, For reflection type holograms,

with their severe fringe stability requirement, a period of 3-5 min may be

required to insure that the film has relaxed to room temperature.

object with

diffusingback

LENSLESS FOURIER TRANSFORM HOLOGRAM (b)

Collimating

Lens Prism

Lens

3.5. Baffles

A somewhat neglected essential to successful holography is actually.

not an optical element; this is the baffle. It can be any kind of an opaque

curtain, preferably non-reflecting black. When all the optical elements

for a holographic experiment are in place the system must be checked to

insure that there is’ only one source of reference illumination. Sighting

through the film holder the relative positions of the object and reference

can be seen. If any other point of light is visible other than the one point .of reference it must be blocked by’some sort of a baffle. Multiple re-

ferences, if not prevented, will cause ghost images. in-reconstruction.

I8

Transparency ’

I

I

object withdiffusing back

f f I

FO URIER TRANSFO RM HO LO G RAM (c)

Fig. 7. Some simple holographic set-ups.

necessarily fairly large with consequent fine fringe spacing and sensitivity

to movement or vibration. Probably the best arrangement for the first

hologram is the lensless Fourier-transform hologram (fig. 7b). This set-

up is also called Fraunhofer or far-field holography. The separation

between the reference and the object should be quite small compared

rPinhole

Film

/ ILTransforming

Lens

I9



MATT LEHMANN

with the distance to the film plane so that the reference/object angle is

small and the fringe spacing is large, reducing the sensitivity of the system

to vibration. This set up is distinguished by the fact that the film plane

is in the far field of the diffuser placed behind the object. The Fourier-

transform set-up uses a transforming lens to produce identical results

(fig. 7c). A prism is used to steer a portion of the beam and provide the

reference. The film and transparency are placed at the focal point of the

lens so that a prism must be selected that will deflect the beam to accom-modate the lens geometry. The Fourier-transform arrangement using a

lens is therefore not as flexible as the lensless system which permits

placing the film plane at any distance which will provide the desired

fringe spacing. The geometry can thus be adapted to the resolution

capability of a particular film.

The geometry shown for making a Fourier-transform hologram with a

lens (fig. 7c) can also be used for reconstruction of a hologram. The

reference is omitted and the hologram-is placed in the position of the

transparency and diffuser. The Fourier-transform hologram will recon-

struct on a matte surface at the focal point of the lens. The image and its

conjugate will appear flanking the bright centre spot. Only if the refer-ence and object both originated in the same plane when the hologram

was made will both images be in focus.

A typical set up for making three-dimensional object holograms is shown in fig. 8a. Looking through the film holder at the illuminated object will indicate how the virtual image will appear on reconstruction.

An iris is shown in the object beam. This can he used in place of a baffle

to control the spread of the beam and cut of light that would fall on the

back of the film or reference beam mirror. Reflection holograms,which reconstruct with a point source of white

light, require a set-up where the film can be placed between the objectand reference (fig. 8b). To view the reconstructed hologram through the

glass side of the photographic plate, the emulsion side must be placed

toward the object during exposure. The reference illumination is

arranged to be incident on the plate at an angle of I0 to 20 degrees from

the perpendicular. Baffles must be used to control the light. A piece of

glass placed in the film holder will reflect the object showing how it will

appear on reconstruction. As this type of holographic recording has the

finest fringe spacing it is the most sensitive to movement or vibration.

Only films of the highest resolution can be used and every precaution

must be taken to insure stability.

20


Making high quality holograms is not difficult but it does require aknowledge of how a hologram is formed in order that the limitations can

be appreciated. It is discouraging to attempt a holographic experiment

and not know why it failed. Checking the stability of the system and

knowing the resolution and exposure limitations on the film will

Splitter l

Pinhole

FRESNEL HO LO G RAM (a)

vSplitter /-Pinhole

Mirror Object

REFLECTION HOLOGRAM W

Fig. 8. Typical holographic set-ups.

assist materially in assuring a successful experiment. It is always advis- .

able to start with a simple format not overly demanding in fringe

requirements.

The encouragement implicit in initial success is an essential ingredient

for confidence in continuing research. Application of the procedures

described in this paper will provide a sound basis for the development of

holographic techniques applicable to diverse fields of engineering

research.

Acknowledgments

Specific references to prior publications of holographic methods have

been omitted from this paper as all successful researchers in this field

21



have experienced and solved these problems. Much of this paper is the

result of experimentation in the Stanford University Electronic Labora-

tories. However, the author wishes to acknowledge the work of Emmett

Lcith and Juris Upatnieks of the University of Michigan, Robert Collier,

Keith Pennington and Lawrence Lin of Bell Laboratories, J. W. Good-

man, D. W. Jackson, and W. H. Huntley of Stanford University

Electronic Laboratories.

MATT LEHMANN

(4

LEHMANN

22

Plate I (a-d). Examples of mode patterns in a CW laser. (a) Typical CW laser beam.(b) Same beam with

’

spatial filter. (c) Multi-mode pattern. (d) T modepattern.

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03 - Holography Technique and Practice

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