Holography materials

Fine grain emulsions (red & green)

The VRP-M and PFG-01 emulsions may be used equally well with pulsed lasers and with CW radiation. In the pulsed laser radiation case the emulsion should be post-sensitized with the technique of latensification. The latensification technique is described below.

Characteristic curves of fine-grain red (PFG-01) and green (VRP-M) emulsions, showing spectral sensitivity versus wavelength, are shown in Fig.1. The VRP-M optical sensitivity (to CW radiation) is seen to peak at approximately 75 microJ/cm2 and that of the PFG-01 (to CW radiation) at approximately 80 microJ/cm2.

Fig.2 shows the optical density after exposure by CW radiation and after development versus energy. Grain size characteristics for the VRP-M and PFG-01 emulsions are shown in Fig.3.The diffraction efficiency versus exposure for reflection holograms recorded on PFG-01 (using a CW laser) and on VRP-M (using a pulsed laser) is presented in Fig.4. The maximum diffraction efficiency is seen to be >45% for both emulsions. Material lifetime is more than two years.

Table 1 shows a summary of recommended processing schemes for use with VRP-M when exposed by pulsed Neodymium lasers (526.5nm, 532nm) and for use with VRP-M and PFG-01 when exposed by CW Argon or by HeNe lasers. In the case of exposure by CW radiation latensification is usually not necessary.

White-light holograms made on VRP-M have a natural green reconstruction colour that can be easily changed by the technique of colour -shifting using D-Sorbitol (described below). White-light holograms made on PFG-01 have a natural yellow/orange reconstruction colour when using a HeHe laser and the recommended chemistry.

All chemicals necessary for the preparation of the recommended solutions can be obtained
from the following companies:
(i) Photographer's Formulary Inc. www.photoformulary.com,
(ii) Sigma-Aldrich www.sigma-aldrich.com

For pulsed work one may also use the standard D-19 Kodak developer if 6-8g of Methyl Phenidone is added into the final solution.

The temperature of all solutions described herein is 20^oC

Figure 1-4

Recommended Processing for VRP-M and PFG-01.

Latensification

PFG-01 and VRP-M emulsions have peak sensitivities to exposures in the millisecond regime. In order to obtain optimal sensitivity to exposures different from this regime the technique of latensification must be used.

Latensification is usually done directly after the holographic exposure. Before applying the process a latensification time appropriate for your system must be worked out. This procedure is as follows: Place a 25W white lamp at a distance of 1m from a test holoplate or film such that its light uniformly illuminates the emulsion. You will need to try several exposure times.

First of all you will need to develop the unexposed emulsion under normal safelight conditions. The plate will darken a little. This is called the "fog" level. After development wash this control plate, dry and keep it handy. Now a series of exposures with small test plates must be made. Start at about 2 secs and go up to around 10 secs. After each exposure develop your plate and match the darkening of this plate to your control plate. If it is the same, more exposure is needed so go back and repeat the process. Stop when a result that is just marginally darker than the fog level is obtained. This is then the correct latensification exposure for your geometry.

Now that the proper latensification time has been discovered, after every proper holoplate exposure you must take your plate and illuminate it exactly as described above for the time that you have worked out. Then all processing is as normal.

Latensification stabilizes and enhances the latent image formed by the holographic exposure. If required, chemical processing may be done with significant delay after latensification (~8 hours).

Colour control

For the VRP-M emulsion we recommend two colour shifting techniques. One produces a fixed colour-shift towards the red of approximately 50 nm. The other produces an adjustable colour shift.

The fixed colour shifting is accomplished by soaking the final hologram for 1 minute in a bath of Potassium Iodide solution. Adjustable colour shifting is accomplished by soaking the hologram for 1 minute in an aqueous solution of D-Sorbitol (sugar substitute - C6H14O6) with added wetting agent. The colour of the final hologram depends on the solution concentration (Fig.5). After soaking, the film must be taken out and put onto a flat surface. Water drops must be removed using a rubber wiper such as a windscreen wiper. Here one must be delicate - if too much force is employed you may obtain a somewhat different colour than that predicted by Fig. 5. If, after drying of the hologram, the replay colour achieved is not satisfactory, the film or plate may be washed in warm water and then soaked anew in another D-Sorbitol solution of different concentration.

Developers, bleach and colour chance bath

Chemical Formulas