Understanding Color Spectrum for Silver Gelatin Emulsion Makers

December 22, 2015  •  Leave a Comment

Understanding the color spectrum and how it relates to silver gelatin emulsion formulas is an important part of photochemistry that one must ultimately master to gain total control over pre-visualization and the practical creation of your fine print.  That sounds much more difficult than it is.  The photo pioneers that came before us have done an excellent job at documenting their work, so we have the benefit of that knowledge as long as we are willing to read the original texts.  

My goal with this article is the concisely share my research on the topic in an attempt to get new silver gelatin emulsion makers thinking properly.  I've done my best to provide accurate information, but please review and let me know if something isn't correct.  The power of bringing several minds together is always better than any single individual. 

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The first silver gelatin negative was made from a color blind/ordinary/plain silver emulsion Silver Bromide (AgBr).  The color rendition for a blue and yellow ball would have created a print that was very light for the blues and dark for the yellows.  However, I am exploring these characteristics of the color blind emulsion for creating the finished print (we think of as a positive).  In the case of a landscape which typically has a blue sky, a properly exposed silver bromide negative emulsion may produce something that is very special and unique.  Instead of the sky being very light or blown out, it would be much darker, creating a unique photograph.  The Light Room Volume 1 is a very good resource to read more deeply about this subject matter.  

Josef Eder discovered that by adding erythosin to the plain silver (color blind) emulsion it opens up the light spectrum expect into the deep orange and red.  Now, blue eyes were no longer ghostly white for portraits.  At this time, portrait photography was every important.  

Eventually, emulsions were created by adding additional spectrial sensitizers to see into the orange and red spectrum.  These new emulsions are known as the modern day panchromatic negative emulsion.  When people refer to film today, they are referring to panchromatic negative emulsions.

A panchromatic emulsion produces a realistic reproduction of a scene as it appears to the human eye. Almost all modern photographic film is panchromatic.  As naturally prepared, silver halide emulsions are much more sensitive to blue and UV light than to green and red wavelengths.

The German chemist Hermann W. Vogel found out how to extend the sensitivity into the green, and later the orange, by adding sensitizing dyes to the emulsion. By the addition of erythrosin the emulsion could be made orthochromatic now.

However, his technique was not extended to achieve a fully panchromatic film until the early 1900s, shortly after his death. Panchromatic stock for still photographic plates became available commercially in 1906. The switch from orthochromatic film, however, was only gradual. Panchromatic plates cost two to three times as much and had to be developed in darkness, unlike orthochromatic—which, being insensitive to red, could be developed under a red light in the darkroom. This part is a bonus for me.  This can't be done with modern panchromatic films.  

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The Emulsion to Color Spectrum Linkage

It should be clear by now that understanding the type of silver gelatin emulsion you are making for your negatives and prints and how each type relates to the type of print you are going to make is very important to understand.  My soft place is for orthochromatic emulsions, with or without a yellow filter, depending on the subject and conditions.  By deeply understanding the theory, you are freed to explore and create with endless possibilities.  

If you want to explore an ortho-like negative, consider using a sheet of your RC graded darkroom paper and make a negative and contact print it.  Variable contrast papers can use a yellow filter because of its extension into the green spectrum.  I use Ilford RC grade 2 glossy for my negatives the most.

I have used Ortho-Litho films that are commercially available for years, but they are so flimsy and thin, it makes it very difficult to handle and work with.  By making my own emulsions, I get to choose the substrate to coat and this provides me with more choices and advantages.   

Modern films (panchromatic) records shades of gray realistically.  But that has not always been the case.  At the beginning of photography until 1884 that wasn’t true and it doesn't have to be true today either because we have the benefit of all the research done by the forefathers over the last two centuries.  

Color Blind/Ordinary/Plain Silver emulsion is sensitive only to UV Light and the blue-violet spectrum.  Examples of a color blind emulsion dubbed X2Ag by Denise Ross at The Light Farm can be further investigated and studied.  For example, a photo of a blue and yellow ball, the blues would be very light and the yellow would be dark. In The Light Farm volume 1, Denise went to a lot of hard work and provided excellent examples of images used with various emulsions to illustrate the above.  You can save yourself a substantial amount of time by reviewing her work and at the same time we get to support a fellow photographer.  I want her to share her knowledge and write more books.

To help emulsions to see beyond the color blind spectrum, a spectral sensitizer is used and this is how the pioneers went from color blind emulsions to orthochromatic and ultimately panchromatic.  

Orthochromatic ("correct colors") emulsion is early black and white film emulsion, sensitive to a wider range of colors than original color-blind (blue sensitive) emulsion. This name was used for the very first time in 1884, for dry photographic plates sensitized with erythrosin(e). 

This new orthochromatic emulsion has a very high sensitivity to blue, generally correct sensitivity to green and bright yellow, but has too low of a sensitivity to orange and is practically insensitive to red, as it does not register wavelengths longer than approximately 560-600 nm (medium yellow to orange). Furthermore, orthochromatic emulsions have a decreased sensitivity in the 500 nm area (pale blue), as compared to color-blind emulsions, and so it reproduces brightness of blue colors more accurately.

To learn more go explore this article at The Light Farm.  

Panchromatic (modern film) is sensitive to all colors of light in the spectrum. Early orthochromatic film had very little sensitivity to red light, leaving red subjects as black in the resulting images. Panchromatic film - originally made by adding dyes to red-insensitive film, a result of work by Dr. Adolf Miethe, is capable of recording red subjects, as its sensitivity range reaches wavelengths of 660-730 nm (orange/red to red).

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Closing Thoughts

I am currently making Ammonium Bromide Orthochromatic (AmBr-O) silver gelatin emulsions and coating several different types of substrates to find the one that jumps out at me and makes my style of photography sing. At this time, I have no idea how long this will take, but I do know that I am committed to the process and doing the work until all the puzzle pieces fit together for me.  

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