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Dichromated gelatin is a light-sensitive material made from gelatin (including ordinary food gelatin such as Knox) and a small amount of either ammonia dichromate or potassium dichromate.

Useful Information

A Beginner's Approach to DCG Holography
Dichromated Gelatin Chemistry


The basic formula for dichromated gelatin is water plus gelatin plus either ammonium or potassium dichromate. The amounts of each ingredient influence the characteristics of the result. Exposure energy requirements, color shift, emulsion thickness, etc., are all impacted by the formulation.

It is convenient, then, to have a standard for formula reference. Richard Rallison promoted using a system of three numbers to describe a formula—grams of dichromate, grams of gelatin, and grams (milliliters) of water. For example, 8-30-250 would be the notation for a recipe consisting of 8 grams ammonium or potassium dichromate, 30 grams of gelatin, and 250 grams of water.

To make comparisons among formulae, the gelatin number is always 30 in Rallison’s notation. The three numbers can be scaled equally up or down for producing different quantities of emulsion. (Personally, I usually scale the numbers to 7.1 grams of gelatin, 7.1 grams being the mass of gelatin in quarter-ounce packet of Knox brand gelatin.)

Thickness and Bandwidth

The ratio of gelatin to water affects the viscosity of the emulsion, and that in turn affects the typical thickness of emulsion on the glass plate. The thickness influences the bandwidth of the final hologram. Rallison reported the following results for emulsions applied by 80 RPM spin coating method:

Formula Thickness Bandwidth
xx-30-350 5 – 6 µm 50 – 150 nm
xx-30-250 8 –9 µm 10 – 50 nm
xx-30-200 10 – 12 µm 10 – 50 nm
xx-30-150 20 – 24 µm ~8 nm

Replay Color Shift

The ratio of dichromate to gelatin influences the color shift. The following table has typical values for exposures taken at 514 nm:

Formula Color Shift
3-30-xxx 630 nm
6-30-xxx 590 nm
10-30-xxx ~514 nm

Exposure Sensitivity

My personal guess at typical exposure requirements for the basic recipe 8-30-300 emulsions.

Wavelength Exposure
405 nm 5 mJ/cm2
442 nm 15 mJ/cm2
475 nm 40 mJ/cm2
488 nm 60 mJ/cm2
514 nm 125 mJ/cm2
532 nm 200 mJ/cm2


A great many factors may have a dramatic effect on sensitivity, notably humidity and temperature, so the above table is only a point of reference. Sensitivity also varies inversely with the dichromate concentration—halving the amount of dichromate would double the exposure requirement, for example.

Weird DCG Recipes

3.6–30–467, Chromium Acetate, Ethanol

Markova, Nazarova, and Sharlandjlev, “Control of the Spectral Position of DCG Reflection Holograms,” Institute of Optical Materials and Technology.

  • 64.3 g gelatin, Bloom strength of 210
  • 7.71 g ammonium dichromate
  • 0.64 g chromium acetate
  • 65 ml C2H5OH (ethanol)
  • Distilled water to make 1000 ml

Plates are coated with the solution at 50°C by doctor-blade method to 20 µm.

5–30–200, Ammonium Nitrate

Bahuguna, Beaulieu, and Arteaga, “Reflection display holograms on dichromated gelatin,” Applied Optics, volume 31, issue 29 (1992).

  • 2.5 g of ammonium dichromate
  • 1.5 g of ammonium nitrate
  • 100 ml of distilled water, heated to 70°C
  • 15 gm of USP grade Baker's gelatin (125 bloom strength) powder slowly added while stirring

Spin-coat at 100 rpm the still ~70°C emulsion for 90 seconds under hot-air gun. Dry vertically in a dark box. Plates are ready after about 6 hours. Sensitivity was reported as 100 mJ/cm2 at 488 nm. [In Rallison’s Thick DCG paper, he associated ammonium nitrate with hardening.]

4.5–30–500, Ammonia

Coblijn, Alexander B., "Theoretical background and practical processing techniques for art and technical work in dichromated gelatin holography", SPIE Institute Series Vol. IS 8 (1990).

  • 100 g water
  • 6 g gelatin
  • 0.9 g ammonium dichromate
  • 2 ml ammonia 35% (added last)

Household ammonia is typically 5-10%. [Presumably, the ammonia inhibits the dark reaction.]