by Ed Wesly
The replay color of a reflection hologram depends not only on the color of the laser recording the hologram, but also on the way the hologram is processed. For bleached holograms on silver halide plates, the processing will either recreate the laser color, (develop-rehalogenating bleach for no emulsion shrinkage), or the hologram will reconstruct at colors shorter than the laser's wavelength due to shrinkage of the coating through loss of material either through removal of silver halides in fixing or by loss of developed silver in a silver solvent bleach. Overall control of image color is afforded by the latter two methods, as the shrinkage of the coating is proportional to the amount of developed silver and the color can be controlled by juggling of exposure and development times. However the range of hues is limited, with only oranges and greens possible with a helium neon laser using these schemes. If the hologram is exposed to first one and then another of two different objects, both objects will be the same color, unless some clever masking scheme is used.
But the artist needs to make holograms with different colored images in it but usually they can only afford one color of laser (if that). In the original triethanolamine duo color paper in holosphere, Jeff Blythe explained how he recorded two different colored images in the same hologram using only a helium neon laser. The trick was to record the two separate interference systems in the emulsion while varying the thickness of the coating between the two exposures.
WHY IT WORKS: An ideal reflection hologram recording would have layers of varying refractive index to represent the bright and dark fringes present during. Their thickness would be 1/4 of the recording laser's wavelength, so that the exact same wavelength of recording out of all of those present in the white light fits in them snugly and is strongly reflected thanks to Bragg diffraction. If the material in the holographic recording layer were to shrink, a shorter wavelength than the original one would fit into the fringe structure and the color of reconstruction would be greener or even bluer.
Blyth's ingenuity lies in pre-swelling the emulsion before exposure to make it thicker. After exposure and processing, which removes the plumping agent, the emulsion shrinks back to its original out of the box thickness. The fringe system also shrinks, inversely proportional to the swelled state during exposure. A higher concentration of plumping agent in the pre-soak will expand the gelatin coating more, so the eventual collapse of the fringe spacing will be more dramatic and bluer.
TEA (TriEthanolAmine) is a water soluble oil, but it does not evaporate like water. A holographic plate soaked in a 10% solution of TEA in water will swell up three to ten times its original thickness while wet, but when the water dries out of the gelatinous sponge, the ten per cent of oily TEA is left behind, now swelling the emulsion to about 10% thicker than at first. The plate is now exposed to 633 nm He-Ne light for a single beam reflection hologram. The TEA is washed out. The hologram is processed and dried so that it is the out of the box thickness, which is 10% less than when it had been exposed. The holographic pattern is now 10% thinner too, and it reflects a color about 10% shorter in wavelength, about 570 nm, a yellowish green.
MAKING A TRIETHANLOAMINE PALETTE: Lay out bottles of different concentrations of TEA; usually a series of 2.5, 5.0, 7.5, 10.0, 12.5, 15.0, 17.5, and 20.0 per cent solutions will span the visible and maybe even into the ultraviolet in some cases with a sensible division of the spectrum. Soak a plate or two in each concentration for two to five minutes, the soaking time is not as critical as the time spent exposed to your "safe"light.
The truly critical step in triethanolamining is the elimination of streaking. The viscous TEA flows at a different rate than water if the plate is held vertically, and dries in messy streaks over the plate. The original practitioners, Lon Moore and John Kaufmann, used windshield wiper blades to squeegee most of the syrup out of the emulsion, eliminating streaks and quickening drying times. However this technique requires a knack which needs to be mastered, applying even pressure from edge to edge during the wipe and being able to do it consistently from plate to plate otherwise your calibration palette is worthless.
I have heard that Edwina Orr of Richmond Holographic Studios uses an air knife which blows a concentrated stream of air across the plate as it is conveyed below it on a sliding drawer type arrangement to standardize production.
Greg Cherry and Nancy Gorglione have described their spin coating apparatus, and thanks to a donation to the Holography Dep't @ SAIC by Weston Morris of his Swirl Art machine we have our own. This is the most fun way of TEAing, plus it is consistent, no streaks, and the plate is dry enough to shoot most assuredly after 5 minutes on the machine, but usually two minutes suffices for the lower concentrations in most cases.
After a plate is soaked and dried for each concentration, they should be exposed for one half the normal exposure they would get for processing in CWC2 and copper sulfate bleach (or your favorite rehalogenator). The pre-soak in water dissolves out the excess bromine put in during manufacture to extend the shelf life of the plates, so there is an increase in sensitivity of about one stop. Before processing, rinse the plates in water to remove the TEA so that it won't pollute the developer. The CWC2 developer followed by a rehalogenating bleach type of processing is chosen for its non-shrinking virtue. After gentle air drying, the different concentrations can be sorted by their replay colors.
QUESTION? What has happened to the reference angle?
FOR DUO COLOR HOLOGRAMS: Usually the practice is to pre-soak for the highest concentration you want to use. Wash the plate after the first color's exposure, soak in the second color's TEA bath, or plain Kodak Photo-Flo if the second color is to be laser red. You could start with both exposures being equal between the two at first, but may have to tweak one or the other up or down for color blending.
. Jeff Blyth, Pseudoscopic Moldmaking Handy Trick for Denisyuk Holographers, holosphere Vol 8, #3.
. Greg Cherry, Spin Coater for Triethanolamine Pretreatment, L.A.S.E.R. News, vol 5, #2, 1989