©1995-04 Dimensional Arts Inc.


Every laser has what is termed a coherence length. It is related to the length of the laser tube and the purity of the phase of light emitted and the wavelength itself. The more pure the light the greater the coherence length. That is, not just any kind of laser can be used in making a hologram. On the specification sheets of most quality lasers manufactured today you will see the term TEM (infinity). This means that the laser is operating in the lowest transverse mode, which is the most uniform across the beam and is preferred for holography. A laser inteded for making holograms must ideally be lasing in just one longitudinal mode. Both of these qualification, i.e. spatial coherence and longitudinal coherence, define the purity of the light.

In twin beam holography it is extremely important to measure the paths of the reference beam and object beams(s) for even if you are using the prescribed laser for holography its coherence length is not infinite. The coherence length also places an upper limit on the size, especially depth, of the object which can be holographed be setting definite bounds to the path difference of the reference and object beams.

These concepts are subtle and can be quite difficult to understand so let me explain a little further. First of all, a laser ideally is emitting all of its light in on and absolutely only on wavelength with all of those wavelengths completely in phase from the the point of exit to infinity. This would be wonderful, but unless you have a fifty mW or more laser with a special attachment called an etalon, your coherence length is probably around six to eight inches. This is the approximate length of an average 5 milliwatt, HeNe laser. This means that once you separate your original beam and secondary beam, the path difference which they travel cannot exceed six inches. If the distance of the reference beam from the beam splitter to the hologram is 48 inches, then the distance of the object beam from the beam splitter to the object to the hologram must also be 48 inches. Then your available path difference can be utilized totally by your object.

In a way this can be related to depth of field in photography, but in holography outside of that depth of six inches th object drops off into nothingness. In a way, you "focus" on your object by making sure the lengths of your objects beam(s) and reference beam are measure correctly.


The coherence length is vitally affected by the type of laser used. Depending on the kind of laser you are working with you may have to adjust the length (the difference can be sizable). When in doubt check with the manufacturer.

The light being emitted by lasers has what you might call a coherence curve. It is a bell shaped curve which shows, in distance from the exit point of the laser, where the wavelengths are most in phase. This is usually a constant integer and depends on the wavelength or substance which is lasing, the size of the laser as well as how purely it is emitted. This number, as how purely it is emitted. This number, let's say eight inches reamins constant. At the peak of the curve, or every eight inches, the light is most in phase. you would make the path lengths of your beams multiples of twice the cavity length of your laser. In addition, the coherence function repeats itself. It is at maximum again at a distance of twice the mirror separation in the laser. It repeats itself. It is at maximum again at a distance of twice the mirror separation in the laser. It repeats itself every 2L distance.

Before buying a laser for us in holography it is always wise to inquire about all the pertinent characteristics of its functions. We will spend more time in later section on coherence length, on reference and object beam intensity ratios and all the practical information one needs in order to perform holography. At this point, however, we feel you have been offered enough of the basic theory of holography and now we will begin applying al of this to the construction of a lab and construction of holograms. Naturally as your practical experience grows you will be able to absorb more theory, but now let's start with the isolation table, or in more coloquial terms, from the ground up.



Construction of the isolation table is relatively simple and will be much simpler if you have the 3/4" plywood and the 4x10" lumber cut to size by the lumber yard.

The roofing tar is used to fill in any air spaces that may occur between joints, thus making a completely solid unit. The tar can be purchased at a hardware store by the gallon.

Obtain the inner tubefrom a tire store and the sheet metal from a sheet metal from a sheet metal fabrication company (look in the Yellow Pages).

5-doz. 2" #8 flat head wood screws
5-doz. 1/2" #8 flat head wood screws
1-gal. roofing tar
1-# 1020 inner tube
1-4' x 4' 8-gage sheet metal
3-3/4" x 4' x 4' interior plywood
4-4" x 10" x 4' lumber
2-cans flat black spray paint

This table, suspended on the inner tube, is an air flotation system that will settle out vibrations rapidly and can be used successfully in isolating an object and optical components from vibration up to half the wavelength of 6328Å light.


It is important that your isolation table be placed on a solid floor, such as cement slab in a garage. Wooded floors are not recommended. If your table is not quite stable enough after testing it with the interferometer set-up described in the next section, try putting a 3" level of newspaper under the whole table as a primary cushion. Or try putting it on a table with legs in buckets of sand.


To test your isolation table for stability, an interferometer can be set-up. By positioning the beam splitter, the two mirrors and the lens you will produce visible fringes that show a fraction of a wavelength variation in the spacing of the mirrors.

PROCEDURE: (refer to illustration)

  1. 1. Mirror #1 should be positioned to redirect the beam back down the laser tube.
  2. 2. Insert the beam splitter and touch up mirror #1's position.
  3. 3. Position mirror #2 and tilt both it and the beam splitter so that all the spots on the cardboard screen are on the same horizontal line (there will be two sets with two bright beams in each set).
  4. 4. Superimpose the two sets as close as you can by rotating mirror #2.
  5. 5. Insert the 60X microscope objective lens (see note in Transmission Hologram section for lens position technique page 51).

The black and red lines that appear on the screen represent constructive and destructive interference between the two plane waves from the two mirrors. These are the types of interference that must be recorded to produce a hologram.

Walk on the floor, stamp on it, yell, sneeze, clap your hands over the table; these vibrations will probably cause the fringes to blur. If the fringes blur during exposure of the film, you will not get a hologram. If they blur part way through the exposure, you will get a hologram. However, the image will have very little constrast because only part of the exposed information is interference, the rest is just light exposure with no information.

You may notice that the center fringe pattern appears to "breath." This is all right and is caused by changes in air pressure over the table.

You have now assured yourself of the most important consideration in holography: STABILITY.


For those of you who do not have a suitable space to use as a darkroom, we have this example of a 4x8' darkroom.

If construction is not possible an alternate could be to sew a few strips of black plastic that can be bought by the yard from a large fabric store and hang them from the ceiling. This type of darkroom could only be used in the evening or in a very dimly lit area because of the light leakage problem.


Here are the minimum optics necessary for making Transmission and Reflection holograms and the interferometer test set up. The list of suppliers contains the names of manufacturers and distributors of optics in various sizes and types that you may want or need to make other set ups.

3-2 x 2" front Surface Mirrors (Edmund #40, 040 51 x 76 mms.)
1-2 x 2" Flat Glass for Beam Splitter (Edmund #2269 52 x 113mms.)
2-60x Microscope Objective Lenses (Edmund #30, 049)

Tow pieces of ground glass, spaced 10 cm. apart, can be used as an object beam spreader, therefore eliminating one objective lense. This eliminates lens cleaning and beam alignment problems, and is less expensive. If you use a lens you should take the lenses apart and use only the final, very small lens to spread the beam.

Because of the high cost of optical mounts made especially for holography, we have over the past ten years fabricated all our own optical mounts. These mounts have all the stability and versatility of mounts that are produced today costing 10 times as much.

All our mounts are made from heavy aluminum stock available at surplus stores (check Yellow Pages under Metals or Aluminum); these places will usually cut the aluminum to size. The large holes can be drilled by a machine shop leaving the small drilling, tapping, and en filing to you. To keep reflections down, it is best to spray paint everything flat back. Epoxy everything together with the new 10 second epoxy that is now available from hardware stores. Aircraft surplus has the nylon screws.

The quantity indicated in the illustration is for the minimum number needed. You will probably want to make or order different kinds for various other experiments. Some mounts can be made from wood, however, remember that stability is a prime ingredient of successful holography.


Part of your holographic set-up requires a very sensitive light meter. The light level of a low power, spread laser beam is not enough to activate a conventional light meter, hence, we have developed a simple, accurate "Light Meter", a Triplet V-O-M Meter with a Cadmium Sulfide photo cell obtained from most electronic suply stores.

To use, first attach the photocell wires to the meter leads and plug the leads into the "COM" and "V-O-M" receptors. There are no polarity requirements, so either wire can go to either lead and either lead to either receptor.

TO OPERATE: (Refer to illustration)
1. Move red switch to X1K OHMS.
2. Put photocell very close to a bright light and rotate the knob on the side until the needle is right on the "O" on the very top scale.
3. To measure beam intensity it is very important to remember that the meter's scale will read in sucha way that the

the light falling on the photocell the LOWER the number on the scale. Example: A reading of "50" is twice as bright as a reading of "100".



In most holograms the object is displayed against black space, therefore, the hologram looks best with white or very light colored objects. Using a securely mounted piece of wood painted white makes a good white background to display dark objects. Don't forget to place it within the Coherence Volume or it will not appear in the hologram.

Object size is dependent upon depth and isolation. The Coherence Volume of 6" has a height of several feet. We have made holograms of objects 18" tall with no problems, however, the objects were never more than 6" in depth. Lighting such a large object is also difficult because of limited table space.

A usual practice with large objects is to increase the distance between the object in the film so that the whole object can be easily seen; but remember, no matter what size the film or the object, the object will always reconstruct its original size. You may have to look through the hologram at some very extreme angles, but as long as the object is properly lit and in the Coherence Volume, it will all be in the holographic image.

Try different textures and materials. Glass, metal, wood, ivory, objects from styrofoam or clay, ping pong balls, toothpick sculpture, anything. We do suggest that you use objects which reflect a good amount of light.
Be careful of materials which are highly reflective because the meter reading may be deceptive when it reads those highlights that are direct light reflections back to the film plane. You might want to spray on dulling spay that can be obtained from a photographic supply store.

Try putting a magnifying glass or a mirror in with the object. Some surprising optical illusions can be created. The most important thing to remember when choosing an object is stability.

We use the word "Basic" because there are as many different configurations as there are holographers. Below is a beginning set up and in a later section we will show you others that are used for different reasons.

As you can see in the illustration the components are paced with specific measurements. This is because of a coherence length of 6328A light. It is a good rule of thumb to have the final distance from the first beam splitter through the set up to the film be a multiple of twice the length of your laser cavity.
The Coherence Volume is roughly 6" in depth, meaning that any object placed within this space will make a hologram - outside this area the object is too far out of phase with the reference beam.

TO SET UP (refer to illustration)

  1. Place the laser, Direction Mirror, Object Direction Mirror, Beam Splitter, Reference Direction Mirror, and the Film Holder in their approximate position. The Film Holder should have the thumb screws on the opposite side from the object area.
  2. Measure these components for proper placement using a piece of tape for the Center of Object Area. Both the reference beam path and the object beam path must be of equal length from the Beam Splitter through the set up to the film. In the illustration 48" is used.
  3. Turn on the Laser and adjust components for beam landing. Put a piece of white cardboard in the Film Holder for reference beam landing - it should hit the center of the cardboard.
  4. You will notice two beams reflecting off the Beam Splitter. These are first and second reflection. Use a piece of black cardboard to block the second reflection.
  5. Position 60x Lens in the reference beam.
    It can be difficult direction a 1.5mm beam through a 2mm diameter lens. First lower the lens and adjust the mount so that the beam glides across the top of the lens holder in the direction of the lens. Then carefully raise the lens until you see the spread beam exit. Adjust the lens so that the beam fills the cardboard in the Film Holder. You may notice concentric rings on the cardboard. These are either imperfection in the lens or dust on the lens. Try cleaning with lens tissue. If rings still exist, carefully rotate the lens in its holder to put the rings off the cardboard.

  6. Place the object so that it is in the center of the Coherence Volume. Solid white objects about as big as your fist work best for beginning holograms. We will discuss other types of objects later.
  7. Position the other 60x Lens or pair of ground glass diffusers in the object beam with the same technique as in the note under step 5. The spread beam may not completely light the object. This may mean that you will have to place the lens before the Object Direction Mirror instead of after it as in the illustration. Look through the Film Holder, rotate the object until you like the composition, and secure the object to the table with a drop of 10 second epoxy or non-hardening clay. It will take a few minutes for the epoxy and clay to solidify.
  8. Measuring the beam intensity ratios is most important to insure good contrast in the final image. Take the Light Meter and make the necessary preparations as described in the Light Meter section. The optimum beam ratio is between 2:1 to 5:1 - The reference beam should be 2x to 5x brighter than the light reflecting from the object toward the film plate.
  9. Put a piece of black cardboard in front of the Laser to use a shutter.
    1. Turn the lights off.
    2. Block the object beam with a piece of black cardboard somewhere between the Beam Splitter and the Object Direction Mirror.
    3. Put the photocell against the cardboard in the Film Holder tilted toward the reference beam coming from the lens.
    4. Read the meter's top scale (you can use a penlight).
    5. Block the reference beam with cardboard somewhere between the Beam Splitter and the Reference Direction Mirror.
    6. Put the photocell flat against the cardboard in the Film Holder facing the object.
    7. Read the meter's scale.
      Remember that the meter reads backwards. The Brighter the beam the Lower the number.
    8. If ratios are over 5:1 or under 2:1, move the object lens closer or farther from the object and/or move the reference lens closer or farther from the Film Holder. Check to be sure that the object and film are still completely lit.

  10. In the dark with only the safe light on, insert the film into the Film Holder with the emulsion facing the object, gently tighten the nylon screws, and step back for a minute to let the table stop moving and allow the air to settle. You can tell which side the emulsion is on by lightly wetting your lips and touching the film between them for a few seconds. As you part your lips one side will stick -- that is the emulsion side.
  11. Gently pick up the black cardboard shutter and suspend it directly in front of the laser beam for a mental count of 10.
  12. Raise the cardboard all the way allowing the beam to illuminate the film and object.
    Exposure time is by experiment based on Laser output, reflective characteristics and size of object, brightness of beams, age of film and chemicals, and temperature of D-19. With a 5mW Laser, a reading 2x to 5x brighter than this for the reference beam, fresh film and chemicals, D-19 at 68 degrees and a light colored object we have used a 20 second exposure. We do not recommend turning your laser on and off with each exposure. Use your cardboard shutter to begin and end your exposure. This will help to insure that your laser is operating at its highest output.
    Your can mentally count the seconds: One thousand... Two thousand... Three thousand...

  13. Lower the cardboard shutter, remove the film and place in the D-19 developer emulsion side up and agitate checking every 30 seconds to see if the exposed part of the film is a light to medium gray. This check can be made by holding the film over the safelight and observing the contrast between the exposed part and the clear edge that was hidden from the light by the Film Holder. If the the film does not turn gray within 5 minutes development time, your exposure was much too short (or you may have left the cardboard block in the reference beam), and conversely if your film goes too dark within the first minute of development, your exposure was too long. Optimum time in the developer is between 2 to 3 minutes.
  14. When the film is dark enough, place it in the stop bath of water and agitate for 30 seconds.
  15. Remove from stop bath and agitate the film in the fixer for two minutes. If you plan to bleach your hologram you would not fix for two different techniques.
  16. You can quickly check to see if there is any image at all by holding the film over a small light bulb and tilting the film around while looking through it. If everything was correct, you should see a rainbow of colors. If you do not see the rainbow, it is a sure bet that there is no image and skip to step #18.
  17. Wash film for 10 minutes under cold water faucet, agitate in Photo-Flo solution for 30 seconds, and let dry making sure the emulsion is not in contact with any surface.
  18. No image or poor image can be caused either by vibrations during exposure, insecure object, or poor contrast of beam ratios.

Put the film back in the Film Holder with the emulsion toward the object. Remove the object. Replace the Beam Splitter with the Object Direction Mirror. This will put all the Laser light into the reference beam. Adjust the components so that the reference beam through the lens illuminates the film. Look through the film and see the image of the object in its original location on the table. You may have to slightly rotate the Film Holder in relation to the reference beam because of emulsion shrinkage.
You can achieve very good reconstruction using a point light source, such as a slide projector or a high intensity lamp with 5770Å narrow band pass filter appendix taped behind a piece of cardboard with 1/4" hole punched in it. This assembly taped over the light source will result in a semi-coherent playback source.

You might want to try the following experiment. After you have made a good hologram do Not remove the object from its position on the isolation table. Place the hologram back in the film holder with reference beam incident on the hologram. The reconstructed image of the object is seen superimposed on the object. If the reference beam is blocked the original object is seen through the hologram. Now proceed to block the reference and object beams alternately. You will find that in a completely darkened room it is difficult to distinguish between the real object and the reconstructed holographic image. Now adjust the hologram in the holder until the two are perfectly superimposed. Try pressing gently on the object with your fingertip. Notice the fringes at the point of contact. You have just performed stored beam holographic interferometry. A subject beyond the scope of this booklet but a very important process used widely today in the testing of mechanical components.

TO SET UP: (refer to illustration)
1. Roughly position Laser, Direction Mirror, and Film Holder (with the nylon screws on the opposite side of the object area).
2. Measure overall distance from Laser to Film as 32" making sure of the 30 degrees angle of reflection off the mirror. This will make reconstruction easier, for the reconstruction beam will not be in the way of the person viewing the hologram. You can try smaller angles but 30 degrees to 45 degrees is the extreme because of the tolerance of the emulsion.
3. Put a piece of white cardboard in the Film Holder and position the lens to fill the cardboard using the technique described in the Transmission set up.
4. Place your object ( a solid white object no larger than 3" deep, 3" tall and 4" wide) as close to the Film Holder as possible and within the film frame.
5. Take out the white cardboard and see that the object is within the film frame, rotate the object until you like the position and secure it with a drop of epoxy or non-hardening modeling clay.
6. Put a piece of black cardboard in front of the Laser as a shutter. In the dark with only the green safe light on, place the film in the holder with the emulsion facing the object. Step back for a minute to let the table stop moving.

NOTE: No beam ratios to figure -- there is only one beam. The light passing through the plate or film towards the object is the reference beam. The light scattered back towards the emulsion by the object is the object beam. The counter flowing waves interfere with one another to produce the stationary interference patter recorded by the hologram. Using the Reflection set up be sure to deep your object as close to the film plane as possible, but be careful no to scratch the emulsion when you are inserting or removing the film.

7. Lift the black cardboard, suspend it in the beam for a mental count of 10, lift the cardboard all the way out exposing the film, count mentally the exposure seconds, and replace the cardboard shutter to stop exposure. Exposure is short, between 5 and 10 seconds.
8. Develop for about 5 minutes in D-19 checking with safelight for a very dark, almost black exposed area on the film. Remember, we want light reflecting back to our eyes from the film, not transmitting through it.
10. DO NOT FIX!! Wash in running water for ten minutes, rinse in Photo-Flo for 30 seconds, wipe with a sponge and let dry. There is no stop gap test for spectrum as in transmission holograms. You will not know whether there is an image until the film is almost completely dry.

Reconstruct by shining a point light source (the sun, high intensity lamp, flash light, slide projector, etc.) at the film with the emulsion facing away from you and at the same angle as the original beam that lit the film. You will notice that the ;image is quite clear and has a green color to it. Turn the film around to see the real image projection. (if there is no image, it is usually because of object movement.)

9. Experiments

In all these illustration, no attempt at scale has been made, and a certain amount of component fabrication is necessary on some of the setups.

This type of hologram uses only one lens and a large mirror. Part of spread beam hits the object and goes to the film forming the object beam, and part of the beam hits a mirror which directs that portion to the film forming the reference beam. These holograms yield greater resolution. But, with this resolution you get less depth of field or Coherence Volume. Also, the reference beam/object beam angle is very small making this almost an in-line hologram which puts the reconstruction beam almost in your eyes


By splitting the object beam, you can light the object from many different angles achieving lighting effects more akin to conventional photographic lighting techniques. But remember that distance from the first beam splitter through each of the other beam splitters to the film must be equal.


Any hologram has the real image waves emerging from it. These waves are usually very distorted. Here is a set up that can produce a good real image. The telescope mirror collimates the beam so that instead of a cone of light for the reference beam, you have a tube of light. After setting up to view the virtual image, the film is rotated 180 degrees horizontally to view the real image.

NOTE: Actually by using the dish mirror (approx. 50" f.l.) you create a slightly converging reference beam. You can playback with filtered light source as usual. The dish mirror can also be used to make real image reflection type holograms.

This is a multiple lighting set-up to produce a longer Coherence Volume. The distance from the first beam splitter through each of the object beam splitters is equal, forming multiple areas which, in reconstruction, seem to be a large single Coherence Volume.


There are two ways to achieve 360 degree views of an object, and both use either the simpler Division of Amplitude technique in which your object and reference beam would both come down from the top; or split beam technique to independently light the object and produce a reference beam. The first illustration uses four 4 x 5" pieces of film and the second uses 5" wide roll film that has the same emulsion as the plates and can be obtained from AGFA and Kodak. The object area is smaller with the four plates but stabilizing of the roll film is difficult (try using a plexiglas tube, tape the film, emulsion facing in, on the outside and let it stabilize for 5 min. or more before exposure).

NOTE: In the Division of Amplitude technique for 360 degrees the light which reflects from the object to the film is the object beam. The light which misses the object and goes directly to the film is, by definition, the reference beam.

By placing a large lens between the object and the film with the center of the object and the film plane at the focal point of the lens, an image is formed that when reconstructed, lies half in and half out of the hologram plate.


This is pretty straight forward but remember to balance the beam ratios as in a transmission holograms and measure the distances. Exposure times will be dependent upon the brightness of the beams after the ratios have been established. Here objects movement is critical. This is a good setup to try the real image.

10. Bleaching

Holograms that are bleached are termed Phase Holograms and are often considerably brighter than conventional amplitude holograms. The following bleaching process has been developed by AGFA for their Scientia emulsion 8E75. This reversal process utilizes the desensitized silver halide residue for making the phase holograms. This method produces bright, low-noise holograms, which also do not easily fade. The image contrast is almost as high as that of conventional amplitude holograms. No special developers are required.

1. Develop for 5 min. with average darkening.
2. Agitate in stop bath for 2 min. (1 percent solution of acetic acid).
3. Wash for 5 minutes (check over light for spectrum).
4. Bleach for 2 minutes in: 5 g of potassium bichromate
5 ml of concentrated sulphuric acid in 1 litre of distilled water
5. Wash for 5 minutes
6. Clear for 2 minutes in: 50 g of sodium sulphite (anhyd.)
1 g of sodium hydroxide in 1 litre of distilled water
7. Was for 5 minutes
8. Desensitize in:
880 g ethyl alcohol
100 g distilled water
20 g glycerin
120 mg of potassium bromide
200 mg phenosafrinine
9. Rinse briefly in ethyl alcohol and let dry.

Your might want to try another new method of bleaching. Carefully prepare some bromine water by following this simple but dangerous formula:

Dilute approximately two ounces of liquid bromine in one quart of water. Be very, very careful for bromine can easily burn the skin and the vapors are extremely toxic. The bromine will require up to 72 hours to be dispersed sufficiently to produce a solution of approximately 6%. Extreme caution should be maintained while handling the liquid. After you have developed and fixed your hologram immerse it in a standard Photo-Flo solution (Photo-Flo is available at any camera store) for about one minute. Then, while the hologram is still wet, place it into the bromine water and observe the plate or film become transparent. (depending on the strength of the solution more or less time will be required.) Then return to Photo-Flo and dry as usual. We strongly suggest that you use bromine outdoors, because the vapor tends to linger in a closed area, causing a very unpleasant odor.


All photographic films come from the manufacturer with development inhibiting coating, not unlike the preservatives which are added to food to prevent or retard spoilage. This coating keeps the film from spoiling while it is stored. If you wish you can remove this coating and thus hypersensitize the plate of film for a few hours by the simple procedure described here. This procedure temporarily increases the effective speed or light sensitivity of the film about three or four times without a gain in "noise". In practical terms this will make it possible to increase the size of your film so as to make holograms up to 8" x 10" with a low power laser. The procedure is as follows: Before exposing your film immerse it for approximately 2 minutes in a solution of water containing 3 drops of 28% ammonia per liter of water containing a wetting agent such as Phot-Flo. The solution should be kept at room temperature. After the film is completely wet, remove it from the solution and let it dry without sponging. Remember, that the film must be handled in complete darkness. The film must be exposed before more than a few hours have elapsed. Proceed to expose your treated film, keeping in mind that the exposure time can be decreased for a small hologram or conversely that you can make a larger one in the same amount of time it usually takes to make a smaller one. If you wish to increase your speed even further -- approximately 10 times -- you can develop your film in Dektol which has been heated to 100 C. This latter procedure will add considerable "noise" but it is worth experimenting with for unusual situations.

On to Appendix.