This Is From The Digital (Scientific American Amateur Scientist February 1967 issue ) Is This Real ? Does Anyone Have the Paper Original Issue ?
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It is as well the first Hologarphy Article .
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About These Ratings Difficulty: Danger 4: (POSSIBLY LETHAL!!)
Utility:
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How to Make Holograms and Experiment with Them or Ready-Made Holograms
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by C. L. Stong
February, 1967
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A FEW OF THE MANY AMATEURS who made the helium-neon laser described in
this department in September, 1964, and December, 1965, have now taken
up holography: the photographic technique of recording light waves in
mid-flight and in effect reconstructing them later so that they
continue onward unchanged. If the recorded waves are reflected by an
object, the reconstructed waves enable an observer to see the object in
three dimensions, as though he were viewing it through a window. The
reconstructed scene appears in three dimensions, with full perspective
and all the effects of parallax. For example, background details that
may be obscured behind a feature in the foreground can be brought into
view simply by moving the head to one side or the other. Similarly, the
eyes must be refocused when attention is shifted from objects in the
foreground to those in the background [see "Photography by Laser," by
Emmet N. Leith and Juris Upatnieks; SCIENTIFIC AMERICAN, June, 1965].
Figure 1: Reconstruction of entire hologram at top of opposite page
It is now possible for amateurs to buy a hologram and to perform some
interesting experiments with it, a subject to which we shall return.
Sylvain M. Heumann of South San Francisco, Calif., is one of those who
do their own holography. Discussing both the principles and the
procedures he uses for making holograms at home, Heumann writes:
"In principle holograms should be easy to make. In practice they are
not.
"The object to be recorded is placed on a solid platform and flooded
with light from a laser. Light reflected by the object falls on a
photographic plate that faces the object. The plate is simultaneously
flooded by a second set of rays, called the reference beam, that is
reflected by a mirror. The reference waves travel on a path that
bypasses the object. After adequate exposure the plate is developed.
"No lens is used to form an image, and no image appears on the
completed plate. Instead the emulsion records an abstract pattern of
fine lines and whorls that may be roughly likened to a thumbprint. If
rays of colored light are now directed through the hologram along the
path of the reference beam, a new set of rays emerges from the back of
the hologram. The new waves are in every respect identical with those
that were reflected by the object. A viewer who sees them for the first
time is likely to think he is being tricked, because the object looks
so real.
Figure 2: Hologram made with a laser built by an amateur
"If the exposure were made with ordinary light, the photographic plate
would merely blacken. Every part of the plate receives light from every
point on the object and from the mirror. Laser light, however, is
coherent: the waves are identical in length, and they proceed in step.
At some points on the photographic plate the crests of waves reflected
by the object coincide with the crests of waves in the reference beam.
The two waves reinforce each other and expose the photographic emulsion
at that point. At other places the crests of waves reflected by the
object coincide with the valleys of waves in the reference beam. They
cancel, so that the plate receives less exposure. Such interference
effects vary at all points on the plate, depending on the shape and
surface texture of the object.
"The pattern of fine lines in a hologram has the property of
diffracting, or bending, light rays. The diffraction is greater with
close spacing than with narrow spacing. Advantage is taken of this
effect in the hologram to reconstruct the light waves that were
reflected by the object. Rays that enter the hologram from the same
direction as the reference beam are bent and scattered precisely,
enough to match those that were reflected by the object. In effect they
duplicate the object rays. The light used for reconstructing the object
rays should be coherent, but remarkable realism can be achieved with
ordinary colored light emitted from a pinhole source.
Figure 3: Perspectives obtained by reconstructing segments of the
hologram
"The structure of the hologram involves dimensions that are determined
by the wavelength of light and the angle made between the object beam
and the reference beam. Normally the plate must record many thousands
of lines per inch, a resolving power that greatly exceeds that of
ordinary photographic plates. Indeed, the ultrafine structure of the
pattern explains why the hologram can record more information than an
ordinary photograph. It also helps to explain why holograms are
difficult to make at home. During exposure the photographic plate must
remain motionless with respect to the relative positions of the mirror
and the object. Any relative movement between the three in excess of a
few millionths of an inch causes the lines to blur; hence the quality
of the reconstructed waves will be seriously degraded.
"For best results the photographic plate must be capable of recording
about 60,000 lines per inch. Emulsions capable of this high resolution
are comparatively insensitive. Those I use are rated at an ASA speed of
only .003, in contrast with ordinary black and white film, which is
rated from 400 up. The problems of making holograms, then, consist in
devising rigid structures for supporting the apparatus, insulating the
apparatus against vibration and maximizing the available light to
minimize the exposure interval.
Figure 4: Alternative rectifier circuits
"The first requirement for making holograms is a laser. Mine was built
at home. The apparatus described previously in this department will
work splendidly if it is modified to develop somewhat more power and to
generate light waves of a single frequency. The output power can be
increased substantially by operating the laser on direct current, a
requirement that is simple to meet A string of two or more silicon
rectifiers, such as type CR210, can be connected in one lead of the
neon-sign transformer as indicated by the accompanying illustration
[right]. The resulting unidirectional current is smoothed by connecting
a capacitor across the output of the rectifiers. The circuit must also
include two resistors, one for limiting the current in the rectifiers
and the second to compensate for the negative resistance of the laser
tube. High-voltage rectifiers are expensive. The experimenter who has
more time than money can substitute a synchronous rotary switch.
Figure 5: The synchronous-switch rectifier
"The switch consists of an insulating shaft that carries two switch
arms spaced 180 degrees apart. Each arm passes close to but does not
touch an opposing pair of semicircular electrodes [left]. In the case
of 60-cycle operation the switch arm rotates synchronously at 3,600
revolutions per minute. It is driven by a Barber-Coleman synchronous
motor, type KYAJ622-328. The motor is available from the Edmund
Scientific Co., 101 East Gloucester Pike, Barrington, N.J. 08007. The
base of the switch and the supports for the electrodes can be made of
Lucite or any comparable insulating material.
"Alternating current is connected to the switch arms through brushes
made of brass shim stock that ride on brass slip rings. The slip rings
make a snug fit with the shaft. Other essential mechanical details are
evident in the illustration. The inner diameter of the semicircular
electrodes must be at least two inches, and opposing electrodes must be
spaced at least three-quarters of an inch apart. All the other
dimensions can differ from those shown.
"When the synchronous switch is in operation, the blades must stand
midway between the opposing semicircular electrodes at the beginning of
each cycle. They must complete half of a revolution at the end of each
alternation of current. In other words, the switch must operate in
phase with the alternating current.
"To set the switch arms in phase, connect one output lead of the
neon-sign transformer to the brush of one switch arm and connect the
other output lead of the transformer to one of the semicircular
electrodes. Apply power to the neon-sign transformer from a
variable-voltage transformer, such as a Variac. Connect the motor to
the power line. When the motor comes up to full speed and is running
synchronously, gradually apply power to the neon-sign transformer and
observe the gap between the switch arm and the electrode. When the
voltage has been increased sufficiently, sparks will bridge the gap.
Figure 6: Optical train for making holograms
"Note the point on the semicircular gap where the sparks first appear
Perhaps they will begin approximately halfway around the electrode. If
so, shut off the power, stop the motor and rotate the switch arm 90
degrees on its shaft. Reenergize the apparatus and again observe the
gap. Doubtless the sparks will now fill the entire arc of the
semicircular electrode. Should the sparks originate at greater or
lesser angles around the semicircular electrode, adjust the angular
position of the switch arm on its shaft by an appropriate amount.
"After the switch arm has been positioned so that the sparks fill the
complete arc of the electrode, fix it to the shaft with a dab of
quick-drying cement. Then rotate the remaining switch arm 180 degrees
from this position and similarly cement it to the shaft. Adjacent
semicircular electrodes are interconnected. When an alternating-current
source is connected to the rotating switch arms, unidirectional current
can be drawn from leads connected to opposing semicircular electrodes.
"The switch functions as a full-wave rectifier and can replace the
costly diodes. The switch requires no current-limiting resistor. A
capacitor of about .25 microfarad should be connected across the output
of the switch, however, and a resistor must be inserted in one lead
between the capacitor and the laser to compensate for the negative
resistance of the laser tube.
Figure 7: Arrangement of interferometer
"Conduction between the switch arms and the semicircular electrodes is
established through the spark. For this reason the switch unfortunately
acts as a copious generator of electromagnetic noise at frequencies
close to all television channels. In order to prevent the radiation of
this noise, the switch, the neon sign transformer, the capacitor and
the resistor must be installed in a grounded metal cabinet.
"In some cases it may also be necessary to insert choke coils and
bypass capacitors in the alternating-current power line and the
direct-current output leads. The choke coils and bypass capacitors
should be potted in grounded metal containers and installed in the
cabinet. The cost of the complete synchronous rectifier should not
exceed $20. Warning: The high voltage is lethal. Handle it accordingly.
"To make certain that the laser will generate coherent light of a
single frequency (that it will operate in the so-called TEM00 mode),
the resonator should consist of one mirror of spherical figure and one
flat mirror. The flat mirror can be bought from Henry Prescott, 116
Main Street, Northfield, Mass. 02118. The laser described previously in
this department was equipped with a pair of mirrors of spherical
figure. Either one of these can be replaced with the flat mirror.
Figure 8: Details of the beam splitter
"To make the modification, align the two spherical mirrors so that the
laser functions normally. Remove one mirror and replace it with the
flat mirror, which can be aligned by inserting a microscope slide
between it and the adjacent Brewster window at an angle of about 45
degrees, shining a small light on the slide and manipulating the
adjustment screws while looking through, the spherical mirror and down
the capillary tube. When the reflected light reaches maximum intensity,
the flat mirror is in proper adjustment.
"The adjustment can also be made by the method described in this
department in December, 1965. Occasionally a small additional
adjustment is necessary. It is made by applying direct current to the
tube and rocking the adjustment screws back and forth slightly until
the beam appears. Direct the beam onto a white screen and observe the
pattern. If it consists of an array of two or more spots, adjust the
screws until the spots merge into a single disk of uniform intensity.
Incidentally, the laser may not develop maximum intensity when adjusted
for TEM00, mode, but more intense multimode beams cannot be used for
making holograms.
"The desired disk-shaped spot of light may contain a number of
interference fringes and circles. Such spurious effects usually
represent diffraction patterns that are caused by dust or by
imperfections in the mirrors. The beam can be cleaned up by passing the
light through a pinhole about .0005 inch in diameter. The pinhole must
be located at the focus of the two lenses that will be used to spread
the beam into a pair of broad cones. A good pinhole can be made by
pressing a sharp needle into a sheet of aluminum foil backed by a piece
of plate glass. The pierced foil can be mounted on a ring of cardboard
for clamping into position in the optical train. Finally, the power of
the laser can be further increased 10 to 25 percent by placing a series
of reasonably strong horseshoe magnets every inch or so along the laser
tube. The magnetic fields reduce the tendency of the laser to generate
infrared waves and therefore concentrate the output at the desired
wavelength of 6,328 angstrom units.
Figure 9: Laser resonator used in making holograms
"In addition to the laser, the experimenter will require the following
equipment: a heavy table that is insulated against vibration; four
first-surface mirrors; two lenses for spreading the laser beam; two
beam splitters, and a supply of high-resolution photographic plates
together with chemicals for their development. All these materials,
except the table and the chemicals, can be bought from the Edmund
Scientific Co.
"My table consists of a granite surface plate mounted on dense
polyfoam. It weighs 100 pounds. The polyfoam rests on the cement floor
of my basement. Another amateur who goes in for holograms uses a stack
of concrete blocks of the type sold by dealers in gardening supplies.
Each block is two feet square and two inches thick. Six blocks are
cemented together with roofing tar and placed on a foot-thick stack of
old newspapers. The assembled table weighs 500 pounds. The heavier the
table the better. It cannot be insulated too well.
Figure 10: Optical arrangement for reconstructing hologram rays
"To check the stability of the table you will require a small
interferometer consisting of a beam splitter (Edmund catalogue No. 578)
and two first-surface mirrors (Edmund catalogue No. 40,040). These
components can be secured to one corner of the table by wax, blocks of
wood or rigid fixtures such as machinist's vises [see Figure 7]. Direct
the rays of the laser into the beam splitter and adjust the position of
the components until the two beams superpose on a screen that can be
permanently mounted on a distant wall.
"The superposed beams will make a small spot of light on the screen.
Enlarge the spot by inserting a lens with a focal length of 10 to 50
millimeters in the beam at a point within a few inches of the
apparatus. Interference fringes will appear in the enlarged spot. They
must show no perceptible movement. If they do, add mass to your table
and improve the insulation. During the hologram exposure the fringes
must show no movement. Street traffic and other sources of vibration
can present a problem. In some regions exposures can be made only
during the early hours of the morning when traffic is at a minimum.
"Once the table has become stable you can assemble the optical train of
the holograph apparatus [see Figure 6]. You will require a piece of
thick glass for the beam splitter (Edmund catalogue No. 2,183), a large
front-surface mirror (Edmund catalogue No. 40,043), a small
first-surface mirror (Edmund catalogue No. 40,040) and two simple
lenses of good optical quality, any convenient aperture and a focal
length of about 17 millimeters. The lenses need not be achromatic. The
mounting supports can be improvised according to the tastes and
resources of the experimenter. Again, stability is the essential
requirement.
Figure 11: Segment of a hologram enlarged 150 diameters
"The subject to be photographed should consist of small objects that
will stand still. Chessmen are a good example. The available light from
a homemade laser limits the size of the scene to about one square foot
if the exposure is to be kept within a five-minute interval. The
photographic plate should be placed vertically, facing the subject at a
distance of about 10 inches. First, however, place a piece of white
cardboard in the position the plate will occupy. The cardboard should
match the size of the plate.
"Darken the room, direct the rays of the laser into the beam splitter
and adjust the lens of the appropriate beam to floodlight the object.
(The laser does not have to be on the stable table.) Block off this
beam and adjust the lens and mirrors so that the second beam, as
reflected by the small and large mirrors, floods the cardboard screen.
If the diagram [Figure 6] has been followed carefully, the distance
from the beam splitter to the object to the cardboard screen will be
approximately equal to the distance from the beam splitter to the small
mirror, large mirror and cardboard. In no case should an inequality
exceed half of the length of the laser. If scattered light from the
laser tube is perceptible on the screen, enclose the laser in an opaque
housing.
"The two beams that now fall on the screen must be adjusted for
relative intensity. The beam from the mirror should be two to three
times brighter than the rays reflected to the screen by the object. The
brightness is difficult to estimate, but it is not too critical. If the
reference beam seems too bright, try shifting the position of the lens
so that it picks up the rays that are reflected by the second surface
of the beam splitter. If the beam still seems too bright, move the lens
closer to the splitter or insert a neutral-density filter in the beam
at the point where it is reflected from the beam splitter. If a filter
is so used, place it exactly at right angles to the axis of the beam;
otherwise light will be reflected back and forth internally between the
glass surfaces and will introduce unwanted interference effects.
"The angle made at the photographic plate between rays from the object
and those of the reference beam should not exceed 30 degrees. The
spacing of the lines in the hologram varies inversely with the size of
this angle and becomes so narrow at angles approaching 90 degrees that
problems arise. Now replace the cardboard screen with the photographic
plate. The emulsion side should face the object. (The emulsion side of
a plate can be determined by the fact that it will stick to your lip.)
The best emulsion for holograms is the Eastman Kodak Company's 649F,
which comes in the form of four-inch by five-inch glass plates, packed
36 to a box. The plates are fairly expensive. They can be obtained from
the Edmund company in smaller quantities. Other emulsions of lower
resolving power can be made to work by using a narrow angle between the
reference beam and the object beam. This arrangement generates somewhat
broader fringes, which are better for such emulsions, but the
adjustment is difficult and I do not recommend it to the beginner.
"Just before making the exposure, direct the laser beam into the
interferometer and examine the fringes for movement. If they appear
solid, switch the rays to the hologram beam splitter and make the
exposure. A dim safelight can be used if it is kept at least 15 feet
from the plate. The exposure time is a matter of trial and error. If
the object is colored and not more than three inches in diameter, a
laser output of five milliwatts should make an exposure of optimum
density in about three minutes. If the plates are stored in a
refrigerator, allow at least 30 minutes for them to reach room
temperature before use. The Eastman Kodak Company recommends that 649F
plates be developed for five minutes at 67 degrees Fahrenheit in
Eastman H. R. P. developer. Thereafter the plates are fixed, washed and
dried. "The dried hologram can be inspected immediately for an image.
Place the plate in the diverged beam of the laser at the angle of the
reference beam. You should then see-the object. Rotate the plate from
side to side to find the angle that yields maximum brightness.
Alternatively you can inspect the hologram by placing a filter of
almost any color in the slide holder of a 35-millimeter projector and
fitting a pinhole mask over the front surface of the projection lens.
You can even use a flashlight of the penlight type if it is fitted with
a self-focused bulb. When inspected by flashlight, the hologram will be
fuzzy and the image will appear in the colors of the rainbow.
"If no image can be found, the probability is high that something moved
during the exposure. Examine the plate under a microscope at a
magnification of about 600 diameters. The pattern should consist of
fine, crisscrossed lines. If these lines are not seen, some part of the
apparatus certainly moved during the exposure If the emulsion is much
darker or lighter than a conventional photographic transparency,
appropriately increase or decrease the exposure during the next try. If
the object has poor contrast, increase or decrease the intensity of the
reference beam.
"You do not have to make a hologram to have fun with these fascinating
playthings. Relatively inexpensive holograms on film can now be bought,
with a viewing filter, from the Edmund company.
A number of engrossing experiments can be done with them. Try
photographing the reconstructed light. You will discover that either
the focus must be altered for recording sharp images of foreground and
background objects or the lens must be stopped down to increase the
depth of the field. Try making photographs of I various areas of the
hologram. Each portion, however small, will reproduce the entire
scene-yet the information contained by each area differs from that of
other areas. The principal difference involves the effects of parallax:
the relative displacement of objects as seen from various points of
view. In addition, each area makes a contribution to the resolution of
the entire plate. Features of the scene appear in sharpest focus when
the reconstructed rays from the full plate are intercepted by the eyes.
It is this property of the hologram that accounts for the fact that
blemished plates can yield good results. Clear photographs of objects
can be made from holograms that are dust-pocked or
scratched-imperfections that would ruin a conventional photographic
negative.
"The hologram is a special form of the diffraction grating, which is a
flat optical surface ruled with thousands of parallel, uniformly spaced
lines and used for diffracting white light into its constituent colors.
Diffraction gratings transmit part of the light as a straight beam but
bend and disperse other portions into bundles of rainbow colors that
lie on each side of the central beam. The bundles are known as
diffraction 'orders.'
"The hologram also diffracts the light into such orders. If your eye is
close to the hologram, you will find a certain angle at which an
apparent mirror image of the scene appears. The depth of the field in
the inverted image may appear greatly exaggerated, depending on the
angle between the reference beam and the object beam at which the
hologram was made.
"To gain a full appreciation of the astonishing amount of information
that can be compressed into the two-dimensional pattern of the
hologram, examine a plate under a microscope. At 40 diameters of
magnification you will find curving lines making fine patchwork
designs. At 200 diameters these fine details turn out to consist of
still finer features. At 1,000 diameters the structures will be
resolved into an orderly pattern of relatively straight, interwoven
lines that resemble the seat of a caned chair. Good holograms contain
more than 25,000 such lines per inch. It is in their number, shape and
density that the optical information is encoded. These paragraphs have
mentioned only a few of the many new optical experiments that have been
made possible by the advent of the hologram.
Bibliography
FUNDAMENTALS OF OPTICS. Francis A. Jenkins and Harvey E. White.
McGraw-Hill Book Company, Inc., 1950.
A NEW MICROSCOPIC PRINCIPLE. D. Gabor in Nature, Vol. 161 No. 4098,
pages 777-778; May 15, 1948.
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The Society for Amateur Scientists (SAS) is a nonprofit research and
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)
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GAIA
garbage...@hotmail.com
> *******************************************************************************************************************
> It is as well the first Hologarphy Article .
> http://server6.theimagehosting.com/image.php?img=1967_02_.GIF
> http://server6.theimagehosting.com/image.php?img=1967_010.GIF
> http://server6.theimagehosting.com/image.php?img=1967_011.GIF
> http://server6.theimagehosting.com/image.php?img=1967_022.GIF
> http://server6.theimagehosting.com/image.php?img=1967_023.GIF
> http://server6.theimagehosting.com/image.php?img=1967_024.GIF
> http://server6.theimagehosting.com/image.php?img=1967_025.GIF
> http://server6.theimagehosting.com/image.php?img=1967_026.GIF
> http://server6.theimagehosting.com/image.php?img=1967_027.GIF
> http://server6.theimagehosting.com/image.php?img=1967_028.GIF
> http://server6.theimagehosting.com/image.php?img=1967_029.GIF
>
> http://server6.theimagehosting.com/image.php?img=Holography.jpg
By reaction this topic must be real .
So this is probably related to this as well .