handmade Holographic Materials                    - materials -

Silver halide grain size

As for conventional photography, holographic emulsions are a dispersion of Silver bromide in a inert support. The main difference between the two is the grain size, while in standard photography silver grain size is often greater than 500nm, in holography the silver grains must be less than 20-30nm. Good and efficient Holographic Materials usually have grains dimensions less than 15nm.  
 

chemicals purity importance

Emulsions are prepared starting from highly purified gelatin and high grade chemicals. Stock solutions are freshly prepared; aged, foggy and/or uncertain solutions are always discarted.
In the past I used to buy cheaper low grade chemicals and I often reused old tanned stock solutions, now I know that  spending a bit more money is better than waste hours and hours of preciuous time, and obviously throw away  bulks of chemicals.
During silver bromide precipitation low grade chemicals might cause side effects as sensitivity alteration, fog formation, shortened shelf life.

Water is another important factor to control. Water used during emulsification should be highly purified as pharmaceutical bi-distilled water is, while water used for emulsion rinses can be reverse-osmosis water .

Chemicals purity for reagents used in developers and bleaches can be lower than those used during fabrication of emulsion.
 

emulsions characterization

Experimental Handmade Holographic Emulsions are coated on microscope slides characterized evauating the following parameters:

       - Sensitivity (Optical Density=0.5)
       - Fog level after forced ageing
       - Diffraction Efficiency

Sensitivity is accurately measured exposing the microscope slide to a calibrated laser light source at different exposure times, test slides are then developed and their Optical Density and Fog Level are measured.

Ageing is firstly  simulated by heating the test slides at controlled temperature and humidity using an empiric formula, if satisfactory   expected Shelf Life improvements are got, than a batch of plates is coated and stored for real ageing tests.

Diffraction Efficiency is finally evaluated making a Single Beam Hologram developed in fresh developer at known environmental conditions.

notes on optical sensitivity

Despite of conventional photography, in Holography it is very hard to say how sensitive a material is. In the past Agfa declared for the now discontinued Holotest 8E75-HD-NAH  a sensitivity of 75uJ/cm^2 to get an Optical Density of 0.5.

Who makes holograms knows that exposure calculations and instrumental measurements are useful only to have a rough idea of the actual exposure time. Many factors should have to be considered for calculating the right exposure time, the fast way is to make a rought measurement and a few tests at differents exposure times.

By the way Single Beam Reflection Holograms (Denisyuk) need more energy density than a Dual Beam Transmission Hologram

Another important aspect is that overexposing a Denisyuk Hologram causes the formation of colloidal silver increasing the efficiency of the final holographic image.