Ewesly / Making a test strip
This section of the Web Site is the Benchmark Project for the TIE 594 Web Process in Education class that I have taken at National-Louis University, and as such is written with an educator's overview. Big shout out for NLU to host this site (at least until I get my degree!) and to Dr. Craig Cunningham who has been so very patient with me!
There is a decided lack of how exactly to run these simple holographic recording materials calibration tests in textbooks and on the web, so I decided it’s high time to demonstrate this all-important aspect of the holographic recording process.
- This lesson’s subject matter is the methodology of peaking getting the best (brightest or most easily viewed) hologram on commercially available films and plates.
- Students will learn the importance of exposure by making a test strip and the importance of development time, plus the role and interesting results obtainable by two different types of processing chemistries.
This curriculum is centered on controlling the variables of the holographic recording film’s processing procedure, and how to troubleshoot processing errors. This entails varying the following parameters and analyzing the results.
Exposure as a function of intensity and time * exposure test series * role of the developer * developing time variations * different types of developers * the role of the bleach * different types of bleach.
At the end of this unit, the student holographer will be able to:
- Make a test strip of a sensible order of exposures
- Choose the developer and bleach combination that will give the final desired color
- Follow the timing of the processing steps
- Troubleshoot exposure and processing errors
Once you are committed to becoming a holographer, you admit that you are a laser nerd and want to soak up all the possible information about the process, so there would be a very positive mental attitude carried into this lesson. The information might be totally brand new to some, and would be even better appreciated by those who have had some success and failures, as they may have not approached the problem in a systematic way.
Since the whole process is extremely technical and there was some capital outlay for the equipment and film, there would perforce be a very positive mental attitude, especially if the holographer has a certain goal in mind for the finished piece.
The point of this exercise is to show how to tune in the exposure and processing aspects of recording a hologram. The directions included with the film (when they are included) are oftentimes erroneous, behind the times, irrelevant, etc.By playing with the parameters, the students should learn the methodology and apply it to the holographic films currently available or those that may appear in the future.
The holographer needs to know the dynamic range of the material if it is foreseen to do multiple exposures for color blending or double exposed interferometry.
Once a system is tuned in with exposure parameters constant any object can be placed in position and the same exposure and development can be used to give optimal results. The brightness of the final holograms would be affected by the relative reflectivity of the objects, stronger reflectors giving brighter holograms, and weaker objects could be compensated for by bumping up the exposure time.
The Single Beam Reflection Hologram recording scheme was chosen for this tutorial as it \is very popular in introductory holography classes with its immediate gratification of a hologram that doesn’t need a laser to be viewed. It is also quick to set up as there is no need for beamsplitting, only beamspreading. With a split beam set up there is more versatility as far as objects, beam balance ratio, etc., goes, however there are more parameters to keep under strict control and more optics to keep still during exposure. With a solid object that has kinematic positioning, like the Standard Holographic Object waffle iron, object movement is not an issue.
This SBR set up (often known as the Denisyuk scheme, after the Russian Holographic Deity, Yuri Denisyuk), puts high resolution demands on the recording material, with its fringe spacing on the scale of a half a wavelength of the recording laser color, so it is a good test to see how good the material really is. Not all holographic films and plates are capable of recording this type of hologram well or even at all!
For the sake of this lesson, it is assumed that the student has the necessary prerequisites.
For this lesson, it is assumed that the student probably has heard the basic raps on the theory of holography and how it’s different from photography has followed directions either from this site or some other source and has the necessary equipment (laser, beamspreader, isolation table, object, holographic film and chemistry) to set up the necessary optical configuration to record a hologram, (preferably the type known as Single Beam Reflection) (the Googling is left up to the reader, start with Single Beam Reflection Hologram or Denisyuk hologram) and is ready to load the holographic film into the set up.
By the time the student holographer is ready to make an exposure, they will have built the optical configuration. It is probably unlikely that they have worked in a photographic darkroom, which is the void that this unit will try to fill. Unless the students have no interest in the process and have been coerced into the lab, they will have a positive mental attitude toward the whole thing, and the only danger is that they might be so over-enthusiastic that they might not want to go through the discipline of a systematic shakedown of the exposure and development process. But even if their exuberance overpowers common sense, they will come back to this lesson when they want to make the best possible result.
The Standard Object
For researching holographic recording materials I have a Standard Single Beam Reflection Test Object, a silver spray painted waffle iron mold. This object presents a not very deep texture which is homogenous throughout the exposure test quadrants. Plus it's fun to look at the pseudoscopic side because it looks like the waffle. The holographic plate sits on three ball bearings glued onto the waffle iron. A bar prevents it from sliding down and blocks light from entering the edge of the plate, which could result in colored bands of internal reflections along the top edge of the plate.
The iron mold is supported in a kind of goal post arrangement so that it can be tilted to give a decent reference angle, which was about 30 degrees from the normal in this case to prevent the shadows of the waffle texture from getting too long. A support from the bottom prevents it from falling/rotating during exposure. Notice that in this configuration the holographic recording is upside down from the holographic replay.
The figure below shows the rig with a plate processed to replay laser red replaced back into its exposed position and illuminated with the laser, generating real-time interferometric fringes, demonstrating the stability of the kinematic device. The reflections on the ball bearings aid the repositioning. Give it a try, it's not that difficult!
Maybe every holographer could make their own waffle iron test object similar to this one so that at future symposia of display holography everyone could whip out their holo-waffles to really see whose films and processing really are the best. The waffle is a universal breakfast food, so that everyone in the Global Coterie of Holography could find one to use in a thrift shop.
Not only is good coherence important for generating strong interference fringes, but the two interfering beams should have their polarizations vectors aligned. Most of the objects doomed to be holographed are diffuse reflectors, but who says that diffusely reflecting objects can’t preserve polarization?
A polarized Helium-Neon laser illuminates the Standard Single Beam Reflection Test Object which is photographed through a polarizing filter aligned to pass vertical polarization vectors. The polarizing filter is rotated 90 degrees, and the diffusely reflecting white painted Target Card photographs well, since diffuse reflectors scramble the polarization vectors, but the Test Object dims out, since its reflection is polarized orthogonally!
The secret to the polarization preserving object is the paint; Krylon #1401 Bright Silver and others of this ilk use tiny aluminum flakes as the “pigment”, and being metallic, they are specular reflectors which preserve polarization. The flakes are so tiny that their orientation preserves polarization while conforming to the diffusely reflecting surface underneath. Be aware that some brands of paint dry more specular, looking like polished chrome, which doesn’t look that good for this purpose.
Those who are more advanced might enjoy this: He-Ne red and Frequency-doubled YAG green beams were made collinear using a polarization preserving broadband beamsplitting/combining cube. This results in the two beams being polarized at right angles to each other, and in this case the red beam is (p) polarized which turns out to be horizontal, and the green’s (s) polarization is vertical.
The left image shows a predominantly green beam with some yellow and orange undertones since both red and green beams are present. A Polaroid filter is in front of the camera set to pass horizontal polarization in the middle image, where the red beam shows through while the green is blocked. Rotating the Polaroid 90 degrees results in the vice versa at the end, green pass and red blocked. The diffusely reflecting paper in the background is visible as more or less yellow in both polarization orientations since the paper’s diffuse reflection scrambles the polarization vectors.
Is defined as a function of Intensity and Time: E = I * t. Every light-sensitive material requires a certain quantity of exposure to do its job properly.
The classical photographic analogy is to compare exposure of film (or an electronic sensor, more than likely in this era) to filling a glass with water. A trickle of water coming out of the faucet will take a long time to fill the glass while a wide-open torrent will take a short time. On a camera, the faucet is replaced by the iris, f/stop, aperture, diaphragm, whatever you want to call it, which limits the flow of light through the lens, while the shutter controls the length of time of photon delivery. To go one step further, the sensitivity of the film, the ISO speed rating, can be modeled by the volume of the cup; high sensitivity, requiring less light to do its job, would be represented by a smaller cup, while a less sensitive film with a lower ISO would be represented by a larger cup to be filled.
For a holographic model of exposure, the flow of water would be represented by the power of the laser; little He-Ne’s and pointers would be like garden hoses, and fire-breathing Argons and YAG’s would be like fire truck hoses.
The sensitivity of the holographic recording material would be represented by the height of the receptacle, while the spread of the laser beam would determine the diameter of the receptacle. A beaker a decimeter tall and a decimeter in diameter could represent the photon thirst of a holographic plate that’s of the convenient 6 by 6 cm (2 ½”) size; along the same lines, a container that is 6 decimeter in diameter that is filled to a depth of one decimeter is the volume of photonic liquid that is needed for a 30 by 40 cm plate. Even a small flow would take a reasonable amount of time to fill the former; the latter, a pretty long time.
Scroll down if this spiel becomes overwhelming. When calculating exposure in holography, the amount of light available per unit area comes into play, the unit standardized upon being “per square centimeter”. Lasers’ output powers, being sources of light, are typically measured in Watts or fractions thereof. A reading of light power delivered to the holographic recording plate is generally described in microwatts per square centimeter, muW/cm2 for short, and if you’re dangerously lucky you could be measuring milliwatts per square centimeter, mW/cm2, for the fire hoses of the high power lasers. (It seems like html doesn't support the metric symbol for micro, the Greek letter mu, so I am inserting mu for the abbreviation. When it's mJ it's milliJoules. Plus the superscript 2 is coming out as a plain old 2 in the abbreviated centimeter squared. Sorry!)
What’s in a Watt? A Watt is a measure of power; a force that is delivered constantly as long as the source is turned on. Intensity in other words. But exposure integrates intensity during a length of time, which physicists define as energy, so another physics unit, the Joule, is used to define exposure doses.
A Joule is a measure of energy; how much power is delivered during a period of time. A Joule is defined as a Watt – second; a 1 Watt light bulb turned on for one second emits a Joule of energy. Notice that a Joule is not a Watt per second which means divided by seconds, but multiplied by seconds.
The units for exposure doses for holographic recording materials are microJoules per square centimeter, muJ/cm2, ranging from tens of them for the old high speed materials like Agfa 10E75 and Kodak SO-253 to hundreds for Agfa 8E75HD and Slavich PFG-01 to thousands for Slavich PFG-03M and Sphere-S GEO-3, plus photo-resists and dichromated gelatins, or more properly milliJoules per square centimeter, mJ/cm2, since there are a 1,000 micros in a milli.
Calculating exposure times can be based on knowing how many muJ/cm2 are required from the manufacturer’s instructions, measuring the power of the light per unit area at the film plane in mW/cm2, and dividing the energy reading into the power requirement to get the exposure time. Example: A holographic film allegedly requires 200 mJ/cm2 to do its job. 50 mW/cm2 is measured at the film plane. 200 divided by 50 = 4, so a 4 second exposure would be required.
Stop scrolling here for the bottom line! But not everyone has a laser power meter, nor does one trust their power meter if they do have one, or trust the manufacturer’s energy requirements. An exposure test series, similar to what photographers would do in making a print in the darkroom needs to be made to fine tune the exposure to get the optimum result. The intensity part of the exposure is determined by how much the given laser beam’s power is spread over the area to be holographed; the holographer finds the appropriate time at first by trial and error, but then can calibrate their own exposure dosages.
In photography, the light from the source arrives at a scene, whose reflectance the intensity of the light reflected from the scene is further controlled by the aperture of the lens. In holography, the laser’s light bathes the object and that light arrives at the holographic plate without a lens in the way. The intensity of the light bathing the object is determined by how wide the laser beam is spread, which determines the exposure time.
The raw laser beam is centered on the target card, which could be a dud holographic plate painted white with cross hair fiducial marks on it, or a similarly sized piece of cardboard.
Here the beam is diverged only slightly, not even totally filling up the image space, which would result in a hologram whose brightness would vary from the center outward, or even a color variation radially with some processing schemes.
The beam is spread big enough to more than cover the Standardized Test Object, just in case the testing finds a very kick ass bright exposure and development times combination and the spirit moves one to holograph the whole thing!
For consistent results the intensity should be measured, ideally with a calibrated light meter, and recorded in the Log Book. For those not so fortunate, consistent results could also be achieved by using a selenium, CdS, or silicon cell attached to a VOM to measure intensity. If this same radiant flux is acheived again, exposure time should be the same, leaving the processing schedule at the status quo. If half the light is detected, then the exposure time needs to be doubled; if twice the light is detected, then the time needs to be halved, and so forth.
An accurate and repeatable shuttering system is very helpful with these tests to initiate and terminate the exposures. The old fashioned way dating back to the ancient daguerreotypists (who simply removed and replaced a lens cap to fulfill their exposure obligations) would be to lift a card out of the path of the laser beam, with an eye on the clock. For the typical holographic exposures of multiples of seconds, a clock with a second hand or a stop watch is sufficient. Picking the beam blocker up off vibration isolation device before yanking it out of the beam’s path avoids vibrations being translated to the working surface, or better yet, having it located off the table, but nevertheless in the beam path.
The pulling out of the beam blocker may become tiring with attendant inaccuracies, so automated mechanisms would be more productively employed.
An electronic device called a solenoid, the devices that give the “clunk” of electrical car locks, can pull a cardboard beam locker out of the way. They can be powered by the appropriate voltage of wall wart. The switch on a power strip can turn it on or off instead of pulling it out of the wall, again manually timed. The most exquisitely quiet shutter I had ever seen was devised by the late great Rudie Berkhout who used a d’Arsonval movement from a classic VOM, so that when voltage was applied the meter pointer moved a flag out of the beam path.
What would be the deluxe set up with one of these would be to have it electronically timed, using a darkroom timer, either of the enlarging type with durations of one-tenth of a second and upwards, or the classic backward running darkroom clock, a Gra-Lab Universal Timer. With so many chemical photographers getting out of the game, these timekeepers might be easily picked up at garage sales, etc. A timer hacked out of a http://nutsvolts.texterity.com/nutsvolts/200906?pg=44&search_term=darkroom%20timer%20microwave%20oven#pg44 has even been implemented.
For fraction of a second timing (if you have that powerful of a laser!) an old-school SLR film camera can be placed in the beampath, with the back open and the lens taken off. Older mechanical cameras offer only the option of binary fractions, starting at 1 second, then ½, ¼, 1/8, 1/15 (instead of 1/16 since everyone likes number that end in 5’s or 0’s), 1/30, 1/60, 1/125, etc. CAUTION! Higher power laser beams might burn holes in the black coatings of the focal plane shutters.
Between the lens shutters could also be used, but with the lenses removed. The glass could be used as a beam spreader, but if they were left on the shutter housing there is the risk of the beam moving as the shutter is tripped. Cable releases will minimize the vibration transference. If salvaged off of older cameras, the times may not be of a binary type of timing.
Of course, there are professional grade shutters, with aluminized blades for holding off high power lasers, like this one, which can time both exposure and setting time. I got this one for a song and a dance. (Eat your hearts out! Then again, how often do I use a millisecond?)
And if you really want to be jealous, dig this sweet set up: the UniBlitz shutter initiates the exposure, the Newport Power Meter's sensor sends a signal to the mighty Kikusui Storage Oscilloscope, and the exposure intensity and time are displayed on the 'scope's screen!
Here are the 4 positions of the test card at work. Remember to let the apparatus settle between each exposure.
An exponentially increasing series of exposure times is preferred as evidenced in the animation above. This is because the eye sees logarithmically; it looks for ratios between values, not absolute intensity. Notice the rhythm of the grey scale below; each step is twice or half as bright as the one next to it.
Photographers call this type of sequence the series of stops, based on 2 to the nth power, n being an integer. (Don’t forget, integers include zero and negative numbers, with 2 to the zero power = 1, and 2 to the negative first power = 1 over 2 to the first power or ½, 2 to the negative second power = 1 over 2 to the second power or 1/4, etc.)
For greater precision, use half stops. But the series doesn’t become 1, 1.5, 2, 3, but as bunch of numbers familiar to photographers as the f/numbers: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64… Every other number is doubled or halved depending on which direction you are going, and its root is the square root of two raised to an integral power! Here is an exposure series run in this manner.
Rotary motion of the blocking card is the best configuration to ensure that all test sections have similar intensities and not bias the results with uneven beam spreads. If a blocker were moved laterally, the edge exposures would not see the same amounts of light as the center strips illustrated below because of the radial intensity gradient caused by the Gaussian distribution of the laser beam’s power.
There are so many recipes out there it is beyond the scope of this part of the exercise to enumerate them all. But some general recommendations follow.
The JD-3 and JD-4 kits sold by Integraf Photographers’ Formulary would be good starting points for this exercise, as when properly done the final results will provide a reflection hologram that replays in the same color as the laser that recorded it. This helps in the Assessment stage.
Development: Keeping development temperature constant will help ensure consistent results from session to session. The extremely fine-grained Russian emulsions, Slavich PFG-03 and Sphere-S GEO-03, benefit from 15 degrees Centigrade (65 degrees Fahrenheit) developer temperatures! Constant agitation is also important! And timing is key! Pick a time and stick to it! Vary the exposure time to control how dark the hologram develops and ultimately how bright it gets. What is the correct development time? Make several plates of exposure series and develop them for different times, and see which looks the best!
Rinsing: By getting the developer out of the emulsion before the next bath, this rinse is key to longevity of the bleach and cleanliness of the final hologram. At least a minute with running water, preferably 2 - 3 minutes.
Bleaching: This isn’t like laundry bleach, but a processing step which ultimately makes the hologram brighter. Agitation is not as critical as in development, but moving the plate around to freshen up the chemistry in the coating as it gets used up is a good idea.
Rinsing Again: To get the bleach out of the emulsion. Sometimes the bleach leaves a color cast in the emulsion, once that disappears then the chemistry is completely removed, let it rinse another minute or so, then use the
Wetting Agent: which is a surfactant that lets the liquids sheet out of the light sensitive coating and prevents streaking, not unlike the final rinse in a dishwasher.
I personally do not use nor recommend squeegees or hair dryers to speed up drying times, having witnessed way too many disasters.
How does the holographer determine if the optimal exposure is found? What are we looking for? For this exercise it is assumed the neophyte holographer is using the prepackaged chemistry kits from " Integraf or Photographers’ Formulary called JD-3 or JD-4, as when processed properly the color of a SBR is the same as the laser color, which can aid in the evaluation of the exposure. These kits employ development followed by a rehalogenating bleach, which preserve the dimensions of the recorded interference fringes. (For more details, see CWC2.) Using JD-1 or JD-2 where the processing results in a color shifted image gets more complicated, because a green image appears brighter to the eye than does a red, even though radiometrically (absolute energy measurement) the red might weigh in brighter! Plus there are other interesting tricks than can be done if the color is not shifted! In the figure below the reflection hologram is illuminated by a laser during reconstruction, and the hologram "steals" the light and transforms it into an image wavefront, depleting the straight through beam and casting a shadow. The darkest shadow shows the greatest efficiency or brightness.
Here the hologram is held in front of the object, with an alignment card in place of a plate on the object. The image is apparent in the hologram since it is held at the appropriate reconstruction angle. Where there was no exposure, the blank areas in the test strip, looking dark in the holo, is where the maximum amount of light passes through the devloped plate, making the bright cross on the target card. In this orientation, the minimal exposure is in the lower left, and its shadow is the weakest, showing the minimal amount of diffraction efficiency, although still a healthy amount. The exposures increase CW from there, and you can see that less than half the light passes through the maximum exposure step, meaning that more than half the light is turned into image light! This was a BB-520 plate from HRT of Germany, (now Colour Holographics of England), developed in BBAA and bleached in a FeEDTA solution. Exposure doses were 400, 800, 1600 and 3200 microJoules per square centimeter. CAUTION! Some holographic materials cast deeper shadows not because of high efficiency but because of "noise", a cloudy haze precipitated by scatter from relatively large grain sizes.
A quicker variation of the above scheme is to replace the plate on the object slightly displaced. Here you can see the image is brighter than the object in a couple of quadrants!
With photographic prints, an exposure will yield a print that is unacceptably light at the minimal exposure, then gets darker with more exposure, with the shadows filling in and detail filling in in the highlights, until there is too much exposure and the shadows block up and the highlights become light grey instead of having just the touch of density to give shading.
In a holographic test strip, what will be observed is that the image will get brighter and brighter, but after a certain point the shadows will start losing contrast and not look very dark black but take on a powdery look, caused by crystals that were over-exposed and should have not been developed.
Simply looking at the hologram with a black backing, velvet or velour is best as their fibrous nature is a good light trap, and seeing the where the brightest image occurs. In the example above, a Kodak 18% reflective Grey Card is on the right for comparison purposes. Notice that there are 8 steps to this exposure test.
If attempting this for the first time, the first best guesstimate may be grossly over but more than likely under-exposed. In the sample below the upper left exposure, the longest one in this case, is above threshold and all the rest are under threshold. Notice that this one was shot with a less than full plate spread, so you can see the exposure fall off due to the non-uniform Gaussian beam profile. The next shot should start with this longest exposure and go higher and higher.
This one on the other hand was very much over exposed and/or over-developed. For the next trial, start with the lowest dose and work downward. There is such a thing as too much!
The student will realize that they have arrived at the best possible exposure time when the image pops out, and the viewer doesn't have to squint and squirm to see what's behind the glass. A properly exposed plate will reproduce an image that looks solid, and without a haze that makes it look like it's underwater. Comparison to commercially available holographic images (but not the embossed ones on foil, nor the dichromated gelatin types, but other silver halide based materials) might prove surprising!