Thoughts on the chemistry behind the lumen process

I’ve previously talked about my initial work with the Lumen process and later its combination with the chemigram process. In recent weeks I’ve been experimenting with a some old Kodak Autopositive paper and been amazed by the colour produced. When initially exposed it goes an intense crimson red colour, while this colour is lost during fixing, it is replaced by a fairly intense orangey-yellow colour which is almost as attractive.

Kodak Autopositive paper after initial lumen exposure

Kodak Autopositive paper after fixing lumen exposure

I had heard before that different papers can give wildly different colours when used with the Lumen process, but the papers I’d tried before this were all reasonably similar, so the results from the Kodak paper was a delightful surprise. It did, however, start me wondering what on earth is going on to form such colours from the POV of the chemistry. In order to figure out what might be happening during the Lumen process, it is important to first understand the chemical processes by which normal silver gelatin images are formed…

Traditional Silver Gelatin photographic process

Common photographic emulsion consists of crystals of one of the silver halides (silver bromide, silver chloride or silver iodide), suspended in a gelatin layer. Gelatin is chosen because it is permeable allowing the chemical agents (developer, fixer, toners) to easily access the light sensitive crystals. It also forms a good colloid ensuring an even spread of crystals across the substrate (paper).

Emulsion manufacture

The emulsion is formed by taking silver nitrate and mixing in a gelatin solution that contains halides (potassium bromide, sodium chloride, or potassium iodide). The silver nitrate reacts readily with the halides, the silver atoms combining with the halogen atoms, while the salt combines with the nitrate. Taking potassium bromide as the example halide, the reaction would be

AgNO3 + KBr => AgBr + KNO3

The silver halide molecules form light sensitive crystals, the size & shape of which determine the “grain” of the emulsion. Other active molecules in the gelatin also react with the silver and result in flaws in the crystal lattice. It is these flaws which in fact lead to the photosensitivity of the silver halide.

A number of other chemicals are added which will influence the chemical reaction, the formation of the crystals and thus the characteristics of the emulsion when later exposed to light. A pH buffer will influence the speed of the crystal forming reaction, a crystal habit former will influence the shape of the crystals, a ripener will encourage the formation of crystals, restrainer prevents the reaction taking place too quickly, surfactants lower the surface tension between the liquids to encourage mixing, a defoamer will hinder the formation of foam, stabilizers will prevent other undesirable chemical reactions taking place, biocides will kill any biological impurities in the mixture. While the vast majority of photographic paper emulsions are based on Silver Bromide, the choice of other chemicals used during the manufacturing process may vary wildly across manufacturers or product lines.

The emulsions are usually washed to remove reaction byproducts, specifically the salts and nitrates. Further additives will also be included to control the sensitivity of the emulsion. Usually paper is designed to only be sensitive to blue and green wavelengths of light, allowing use of a red safelight in the darkroom. There are some variations in the choice of sensitive wavelengths, hence the need to use special safelight colours with some papers. There are also panchromatic emulsions which are sensitive to all wavelengths and thus must be used in complete darkness with no safelight. Kodak Panalure is an example of the latter.


As mentioned earlier, the silver halide crystals in the emulsion contain small defects leading to gaps in the lattice, as well as mobile silver ions. When a photon of light is incident on the silver halide crystals, it can liberate an electron from the halide. The electron migrates to an sensitivity site where it combines with a mobile silver ion to form a metallic silver atom. The halide atoms are released from the crystal and will collect in the emulsion. As further photons are incident on the crystal, the number of metallic silver atoms grows. These silver atoms form the so called “latent” image in the emulsion, that will be developed into the final image


The goal of development is to intensify the latent image by many orders of magnitude – as much as 109. IOW the few 100 silver atoms in the latent image need to be turned into many 100’s of millions of silver atoms, becoming easily visible. This is achieved through a chemical reaction that converts more of the silver halide crystals into metallic silver atoms. The metallic silver atoms are opaque & non-reflective to light, producing the blacks in the image while the paper of course provides the whites. The key characteristic of the chemical(s) used as the developer is that it should have a much stronger effect on crystals which contain the latent metallic silver atoms after exposure. A developer which affected all crystals indiscriminately would simply result in fog across the entire emulsion.

The stop bath merely neutralizes the action of the developer chemicals, so won’t be discussed further.


Once the desired image has been developed, there will still be plenty of silver halide crystals remaining in the emulsion. These are of course still light sensitive, so if left would cause the image to degrade / fog over time. Thus the goal of fixing is to remove all remaining crystals, leaving only the metallic silver image. Ordinarily a sodium thiosulphate solution is used as the fixer, as athough potassium cyanide is a viable alternative, the latter is significantly more toxic and dangerous to deal with. Again considering silver bromide, the reaction the takes place is

AgBr + 2Na2S2O3 => Na3[Ag(S2O3)2] + NaBr

Both of the molecules produced by the reaction with the fixer are soluable and can thus be removed by washing. Washing also serves to remove any unreacted sodium thiosulphate which, given time, is liable to remove the metallic silver of the print too.


There are a number of ways in which an image can be toned, with differing chemical techniques. Silver conversion toners work by replacing the metallic silver atoms with silver compounds, which exhibit a particular tone. Many of the compounds are in fact more stable than metallic silver atoms, so this can improve the archival quality of the image. Colour coupler toners work by coating the metallic silver atoms with a colour dye. Dye toners are similar, but they coat the emulsion as a whole rather than just the silver atoms. As the name suggests, metal replacement toners use a reaction that replaces the silver atoms with atoms or compounds of a completely different metal such as gold, platinum, copper.


Lumen image of flowers & leaves on vintage Ilford FB paper

Lumen image making with Silver Gelatin

With an understanding of the traditional silver gelatin process, it is time to think about what might be going on with Lumen image making. First are some observations of the process in action

  • The entire paper is exposed to broad spectrum light immediately
  • Prolonged exposure to ultra-violet light is needed to form an image
  • After fixing the paper is no longer sensitive to UV light
  • Paper which is wet with water will form a more intense image
  • Different papers often result in different colour formation
  • The image colour will change during fixing
  • Old vintage papers often exhibit more extreme colours than modern emulsions

As the paper is exposed to broad spectrum light, essentially all the silver halide crystals will contain a handful of silver atoms (the latent image), so if developer is applied the image will go entirely black. The lumen image could be formed by a reaction with the silver halide or with the silver atoms. We know, however, that after fixer is applied, the paper is no longer sensitive to UV light. Fixer does not affect the silver atoms, only the silver halides. It follows that the lumen image formation must involve the silver halides, and not the metallic silver atoms. As compared to visible light, UV light has higher energy levels and is well known to have an effect on organic molecules and assist in some chemical reactions. Thus it is plausible that UV light triggers a chemical reaction that would otherwise not occur under normal darkroom enlarger exposure.

Silver gelatin emulsions are based on silver-chloride, silver-bromide or silver-chlorobromide crystals. Based on the range of colours shown in lumen prints on flickr or instagrams, that small handful of different types of crystal appear insufficient to produce the wide range of colours seen on different papers. This rather suggests there is a reaction taking place which involves some other molecules present in the emulsion, besides the halide crystals. From the description of the manufacture of emulsions earlier, it is known that there are a wide variety of chemicals involved, beyond the silver nitrate and halides. While attempts are made to wash them out of the emulsion after production, it is likely that chemicals remain, particularly with older papers when the manufacturing process was less well refined. Thus it is plausible that the lumen process involves a chemical reaction with leftover agents from the manufacturing process.

There is a question of why water might act to intensify the image colour, speeding up image formation. Remember that gelatin emulsion is intentionally permeable to allow developer/fixer chemicals to access the silver halide crystals during traditional processing. Water itself is not sensitive to UV light, so unlikely to be chemically involved in the chemical reaction. Most likely is that water is acting as a transport to ensure a fresh supply of the agents of the chemical reaction so it that it can continue rather than fizzle out.

Based on the above information, a guess can be made as to what takes place during lumen image formation.

  • Ultra violet light is incident on the silver halide crystals, liberating halides and silver atoms.
  • Ultra violet light is incident on unknown compounds in within the emulsion, breaking them down to release further (unknown) atoms/ions/molecules.
  • These atoms/ions/molecules react with the silver atoms to form a variety of silver compounds.
  • Water improves mobility of the atoms/ions/molecules, ensuring the reaction between the silver atoms and other compounds in the emulsion continues at a reasonably high rate as compared to when the emulsion is dry.
  • Different silver compounds naturally exhibit different colours, and so the variety of chemicals left over from the manufacturing process of the emulsion will affect what compounds (and thus colours) can form.
  • Each compound will also have different chemical stability. When the image is fixed, some of the silver compounds are broken down again, resulting in the fading in intensity of the lumen image and distinctive colour shifts. The silver compounds which were more stable do not get affected by the fixer and so remain to provide the final image.

It is frustrating that there is a still a giant unknown of just what molecules in the emulsion are reacting with the silver atoms. Understanding this would likely require access to company records about their manufacturing processes, and/or access to xray spectroscopy equipment.

If this broad hypothesis is correct though, it is probably not the age of the paper affects the different colours seen in lumen printing, but rather the process that was used at the time of production that matters. ie the 40 year old kodak autopositive paper that produces intense crimson colours, probably would have had this same effect the if used for the lumen printing the day after it was made vs today. IOW, taking some modern emulsions which exhibit uninteresting lumen results and storing them for 30 years would not have an impact on what kind of colours a lumen print in the future will produce. This potentially makes the old vintage papers all the more valuable to collect today – once they’re gone, they’re gone and the possibilities they have for lumen printing will be lost forever. Off to eBay again….

If anyone knows better about the chemistry of Lumen image making I’d love to hear about it in the comments, as my own knowledge is limited to ancient school chemistry classes and whatever I could find in the internet…

Chemigram combined with Lumen image of flowers & leaves on vintage Ilford FB paper

Chemigram combined with Lumen image of flowers & leaves on vintage Ilford FB paper

Combining the chemigram and lumen printing processes

Over the past few weeks I have written about experiments with the Chemigram process and most recently the Lumen printing process. When learning how to create Lumens, the intent was always to figure out how to then combine the technique with the chemigrams process to form hybrid works. The idea is that chemigrams excel at producing abstract images, while Lumens can produce some very detailed reproductions of objects being imaged. Combining the two techniques would hopefully allow the appealing aspects of both techniques to be presented in a single print.

As a refresher, with the chemigram process the image is formed by moving the paper back and forth between developer and fixer baths, often with some kind of resist applied to control the effects of the chemicals. When the desired image is achieved, a final archival fix and wash is performed. With the lumen process the image is formed by placing some object(s) on photographic paper and then leaving it in the sun for 20 minutes to many hours. When the desired image is achieved, a final archival fix and wash is again performed.

A Lumen print exhibiting varying colour tones. The image is dark purple where the paper was fully exposed, pale pink where fully obscured from light, and a yellow/brown where moisture from the leaves affected the paper

A Lumen print exhibiting varying colour tones. The image is dark purple where the paper was fully exposed, pale pink where fully obscured from light, and a yellow/brown where moisture from the leaves affected the paper

The approximate goal is to have certain regions of the paper processed with one technique, and the remainder of the paper processed with the other technique. The key challenge is figuring how to apply the processes so that their effects are localized. To expose the lumen image, the paper must not have been exposed to either fixer or developer. The chemigram process does not particularly care whether there is an image formed already, as the developer bath will obliterate any unfixed image. Thus the image formed by the lumen exposure must be protected from any developer used in the chemigram process.

Wood varnish applied to the paper as resist, and left to dry for 24 hours. Lines scored in the varnish with a blade

Chemigram created using wood varnish applied to the paper as resist, and left to dry for 24 hours. Lines scored in the varnish with a blade

A potential sequence of steps for combining the techniques would thus be

  1. Apply resist to the areas of the paper that will be using the chemigram process
  2. Expose the lumen image on the areas of paper not covered by resist
  3. Perform an archival fix on the paper, preserving the lumen image
  4. Switch between developer and fix, working on the chemigram resist
  5. Perform a second archival fix to preserve the chemigram image

Steps 1 and 2 can actually be reversed – the only requirement is that the resist be applied before doing the archival fix on the lumen image in step 3. If the chemigram resist is fairly strong, little of the fixer used in step 3 will penetrate, allowing plenty of opportunity to build up the chemigram image.

If one wants to use a soft chemigram resist, it may not be desirable to fix the lumen print before forming the chemigram image. One alternative idea would be to protect the exposed lumen image from the developer by applying a relatively hard resist over it. This gives a slightly different production sequence

  1. Expose the lumen image on the paper
  2. Cover the lumen image in a hard resist
  3. Apply a soft resist to areas of the paper that will be using the chemigram process
  4. Switch between developer and fix, working on the soft chemigram resist
  5. Switch to water/stop bath and remove the hard resist from the lumen image
  6. Perform an archival fix to preserve both the chemigram and lumen image

The hard bit is probably figuring out what is most effective as the resist over the lumen image, such that it avoids any effects from developer, while being easy to remove in a water bath.

In my first experiment at creating a hybrid I tried the first sequence using a dried flour and water paste as the hard resist for the chemigram. The flour paste was applied to a few regions of a sheet of outdated photo paper and left to dry out overnight. After that flowers were placed onto the areas not masked and exposed in the sun for 30 minutes to allow a lumen image to form

Exposing a lumen image on paper partially coated in a mask for future chemigram work.

Exposing a lumen image on paper partially coated in a mask for future chemigram work.

Once the lumen image had formed, the paper was placed straight into the fixer bath for an archival quality fix, to preserve the lumen. As ever, it was slightly disappointing to see the intensity of the raw lumen image fade in the fixer, but what remained was still attractive. With the lumen phase out of the way, it was time to work the resist to form the chemigram image. As mentioned before, the flour paste forms a relatively hard resist once dry, so it took many switches back & forth between developer and fixer to work off the resist. The paper was flexed and rubbed to encourage the resist to break up and form the abstract patterns that characterize chemigrams. The final results were very satisfying for a first experiment

Resulting print after combining the chemigram and lumen processes.

Resulting print after combining the chemigram and lumen processes.

Resulting print after combining the chemigram and lumen processes.

Resulting print after combining the chemigram and lumen processes.

Despite only having made one attempt at combining the processes, the results clearly illustrate there is good potential in the overall idea. There are many avenues of exploration available to take it forward, and I’m thinking it’ll form the basis of an interesting project over the coming months.

An introduction to the Lumen printing process

I don’t have space for a darkroom at home, and while there are still a number darkrooms across London that provide training and rental, this quickly gets expensive and lacks convenience due to travel time involved. As such I have an interest in traditional photographic techniques which can be performed under normal interior or exterior light conditions. Film development requires nothing more than a changing bag to transfer the film to the processing reel. The cyanotype process only needs limited darkness after coating the paper while it dries, thereafter the actual exposure takes place outside in direct sunlight or via a UV exposure unit. While the chemigram process could be done in a darkroom, it is most commonly done in daylight as image formation doesn’t rely on light, but rather interaction of the chemicals to write onto the paper.

Continuing with this theme, the time came to try out the Lumen printing process. Many people are familiar with the photogram technique. Objects are placed directly onto light sensitive paper, which is then exposed under a enlarger, typically forming silhouettes of the objects where they blocked the light source. The paper is then processed in developer and fixer following the normal black & white printing process. This of course requires use of a darkroom.

Those who have handled traditional black & white photographic paper may have noticed that when it is exposed to daylight it slowly changes colour. Normally this would be a disaster making the paper useless for further photographic use. The Lumen printing process though, takes advantage of this behaviour in combination with a photogram like technique. Objects are placed directly onto light sensitive paper, and it is exposed to a light source, but this time it is either sunlight or a UV exposure unit. Over the course of the exposure, which can last anywhere from 20 minutes to several hours, the photographic paper will gradually change colour forming a silhouette of the objects. Once the paper is exposed, it would NOT go anywhere near photographic developer as that would just turn the entire paper uniformly black. Instead it is placed straight into a fixer bath, followed by an archival wash

The image initially visible after exposure can be quite intense, but this will sadly fade somewhat during the fix, and the colours will also typically shift in tone. The result is an image which is fairly low contrast with quite subtle tones. What is surprising is that despite the use of black and white paper, the image will actually exhibit significant colouration. This is markedly different from what is seen when creating photograms where the image is still strictly black and white and high contrast.

A simple lumen plrint made with planet cuttings. The image is fairly low contrast and mostly shows an outline of the plants.

A simple lumen plrint made with planet cuttings. The image is fairly low contrast and mostly shows an outline of the plants.

There are a variety of factors which will influence the colours obtained in a Lumen print. A non-exhaustive list would include

  • Type of paper – each paper has a unique chemical formulation in its light sensitive coating(s) and as a result will behave differently when exposed to UV light
  • Age/condition of paper – the chemical formulation of the light sensitive coating(s) may change and degrade over time, depending on the way in which it was stored, which will in turn alter how it reacts with UV light
  • Time of exposure – the paper may undergo several colour changes as it receives increasing amounts of UV light. For example, it might start white, then go yellow, then blue. So varying the length of exposure will alter what colour the final image has.
  • Opacity of the objects – objects which are partially transparent to UV light will allow a range of exposures across the paper. Since colour is dependant on the amount of UV light, partially transparent objects can result in a range of colours being visible in the image.
  • Water on the paper – any water that is in contact with paper during exposure can influence the colours formed. The water can be applied manually, or it can leach out of the object (eg fresh cut flowers/plants) on the paper in the heat of the light.
  • Type of light source – while direct sunlight and UV exposure units both produce UV light, the sun provides a much broader spectrum, so will have a different effect than narrow band UV lamps. The sun is also a much more diffuse source than a lamp just millimeters away from the paper.
  • Toning of paper – the initial image will change and fade significantly during fixing. By toning the paper before fixing, it is possible to retain more intensity in the image and/or alter the colouring.
A Lumen print exhibiting varying colour tones. The image is dark purple where the paper was fully exposed, pale pink where fully obscured from light, and a yellow/brown where moisture from the leaves affected the paper

A Lumen print exhibiting varying colour tones. The image is dark purple where the paper was fully exposed, pale pink where fully obscured from light, and a yellow/brown where moisture from the leaves affected the paper

While some objects are heavy enough to make good contact with the paper and resist movement on their own, with others it might be necessary to weigh things down with a sheet of glass. This is particularly needed when using flowers and other plant material to form the image. There are a variety of options for this task. Those with existing darkroom equipment may well have one or more contact printing frames. These are quite expensive to obtain even on eBay, so more economical options are to obtain some suitable sheets of glass from a local glazing supplier, or use a cheap photo frame. With the prevalence of pound / dollar stores, finding cheap photo frames is remarkably easy. Why spend £50 on one contact printing frame, when you can buy 50 photo frames for £1 each. Being able to expose many prints at the same time will dramatically improve productivity, so it makes sense to go cheap and obtain many frames. It also means you won’t care if they get damaged or broken during use

An 8x10" sized glass photo frame from a pound shop, to use for contact print

An 8×10″ sized glass photo frame from a pound shop, to use for contact printing

One important thing to note is that the photo frame should have quick release clips for the backing board, not tacks that require pliers to remove and a hammer to re-insert !

An 8x10" sized glass photo frame from a pound shop. The quick release clips allow easy changing of paper.

An 8×10″ sized glass photo frame from a pound shop. The quick release clips allow easy changing of paper.

To use them, open up the back place the objects onto the glass in the arrangement desired. Remember to flip the arrangement left-to-right at this stage, since you’re looking at the image from the back, not front. Then place a sheet of B&W photo paper on the object, so the emulsion side is in contact with the objects. Finally replace the photo frame back and secure the clips in place. There should now be good contact between the objects and paper to ensure they are held still and make good contact for crisp edges. The pressure also helps to squeeze fluids out of plants which can help with colouration of the print.

With the frame prepared, flip it over, place it in direct sunlight and leave it for a while. Over time the colour of the paper will change where it is exposed to the UV light. At a minimum 15-20 minutes will probably be needed, but don’t be afraid to leave the image exposing for hours. Given the length of time involved, it is worth preparing many frames and exposing them all at once. When the desired arbitrary amount of time has elapsed, disassemble the frame to remove the photo paper and place it into the fixer. As mentioned earlier, the intensity of the image will fade somewhat and the colours may also change significantly. Once an archival fix and wash is completed though, the image should be stable thereafter.

The cyanotype process is often said to be one of the simplest to photographic techniques to teach people, but it is clear that the Lumen process gives it a good run for its money. If preparing your own materials, cyanotype requires a little bit of care in mixing the right quantities of chemicals for coating the paper. The Lumen process requires no preparation of the paper – it is merely necessary to mix up fixer at the documented dilution factor. There is a slight burden on Lumen printing when it comes to disposing of the fixer, as it will become increasingly contaminated with silver as more sheets are fixed. Overall though it is a quite straightforward and satisfying process to work with. It is hard go go wrong, beyond exposing for too short a time, so even with no prior experience you’ll get some decent results.