Gelatin is the common base for photographic film and plates, including holographic materials. Polyester film or glass plates are coated with a thin layer of gelatin infused with silver halide and other chemicals that give the film its light-sensitive properties. The gelatin is the vehicle to carry the silver halide in place during film storage, exposure, and development. Otherwise, it is inert.
However, when ammonium or potassium dichromate is mixed with the gelatin, the gelatin becomes weakly light-sensitive. This opens a door to another arena for the holographer, entirely different from the more traditional silver-halide process. The grossly simplified explanation is that light in the presence of dichromate molecules cause hardening of the gelatin. The hardened gelatin diffracts light differently from unhardened gelatin, so the gelatin can record a diffraction pattern necessary for a hologram. An exposed dichromated gelatin plate is "developed" by first removing the dichromate in a water rinse, then removing the water through a sequence of alcohol baths.
Dichromated gelatin, or DCG, holograms are phenomenally bright. Unfortunately, gelatin is hygroscopic, meaning it has a tendancy to absorb water, and as the gelatin reabsorbs the water removed by the alcohol baths, the hologram will fade away. To preserve the hologram, it must be sealed from moisture, usually with some type of epoxy.
Dichromated gelatin is roughly 1,000 times less sensitive to light than the typical silver halide film, and even then it is most sensitive at the short end of the light spectrum (blues and violets), moderately sensitive to greens, and virtually blind to reds without some complicated chemical treatment to improve its red sensitivity. Most DCG holograms are produced using green or blue lasers at the higher end of the power scale, 100 mW or more.
A serviceable 100 mW green laser suitable for holography may be out of reach of the hobbyist wanting to just give DCG a try. Nonetheless, modest sized holograms can be produced with modest power lasers. The image on the right was created by a newcomer to DCG techniques. A handful of 1/4"-20 socket-head screws were the subject; the glass plates, 2 x 3" large microscope slides, and the laser used was a Coherent C215M-10 operating at a lowly 10 mW.
Overview of the Process
Conceptually, making dichromated gelatin plates then processing them after exposure is fundamentally simple. The emulsion is made from ammonium or potassium dichromate, gelatin, and water. Often the formula used is expressed as a series of three numbers, where the numbers represent the ratio of the three components. The recipe, 5-30-250, means 5 grams of dichromate to 30 grams of gelatin to 250 milliliters of water (also grams). The gelatin is first added to cold water and allowed to swell, the mixture is heated while stirred continually until the gelatin is completely dissolved. The dichromate is then added with continued stirring until completely dissolved.
The mixture of water, gelatin, and dichromate, still warm, is applied to glass plates. The simplest method is called veil coating in which the mixture is poured onto an angled plate and allowed to flow over it. Other methods include first pouring a line of mixture at one end of a plate then using a Meyer bar or doctor's blade to drag the mixture across the plate. Spin coating is possible, but at much lower rotational speeds then is used for, say, integrated circuit production, and tends to be very wasteful of mixture. Low-speed spinning can be used after any of the other coating methods to get a more even distribution.
Coated plates are then allowed to dry. The are usually ready for exposure 4 to 12 hours later. Since dichromated gelatin can be 1,000 times less sensitive to light at 532 nm (for example) than silver halide emulsions, exposures may take a while.
After exposure, the plate is left to sit in the dark for a few minutes before processing begins. Fixing is first, and this can be done either chemically with a standard photographic fixer or optically with a few seconds exposure under intense white light. Washing is next to remove the dichromate from the gelatin. Finally comes the drying step, and this involves a sequence of progressively more concentrated isopropyl alcohol baths, the last bath being 99% alcohol. Hot air drying is next to remove the alcohol.
Once completely dry, the DCG hologram can be quite spectacular. The diffraction efficiency can be over 90% with no noticeable grain, and the hologram itself is very bright. Unfortunately, it will not last if hologram is not sealed. Gelatin is hydroscopic, and therefore absorbs water. As water returns to the gelatin matrix, the hologram disappears. Epoxy is normal sealant.