Tag Archives: lumen

Building a UV LED light box for cyanotype and lumen printing

The cyanotype and lumen printing processes are two of the more frequently practised alternative photographic processes, because of their overall simplicity and the fact there is little-to-no need for equipment. Most people will do their exposures outside in the Sun initially, but if you want to work with these processes on a regular basis it can be frustrating to have ideas ready to try, but be delayed by lack of a sunny day. While it is still possible to expose on cloudy days, the length of time required to obtain a suitable image is greatly increased. Working with the sun it is also difficult to figure out predictable exposure times as the intensity varies according to the daily weather, time of day and time of year.

The solution is to switch to an artificial UV light source which can be used any time of day or year, whatever the weather, always giving the same exposure time. Historically though, UV exposure units have been relatively expensive to obtain, £100 or more. The ongoing developments in LED lighting technology though have now opened up new possibilities for constructing a custom UV light source for minimal cost. In particular it is possible to obtain 5m long strips holding 300 UV LEDs from online marketplaces such as eBay, for around £15 / $20 (search eBay for keywords “5M UV 5050 SMD 300LED“).

 5M UV (395-405nm) waterproof 5050 SMD 300LED strip, powered by a 12 V @ 5 amp supply

5M UV (395-405nm) waterproof 5050 SMD 300LED strip, powered by a 12 V @ 5 amp supply

The 5M long 5050 LED strips are 1cm in diameter and can be cut every 3rd LED. If they are cut into groups of 15 LEDs, this will result in 20 LED strips, each 25 cm long. Arranged side by side, this allows for creating a light source that will evenly expose a 20 cm x 25 cm area which is practically perfect for both A4 and 8x10in paper sizes. If one didn’t mind lower intensity it would be possible to left a 1cm gap between strips producing a source suitable for 16x20in / A3 paper, at the cost of longer exposure times.

When cutting the LED strip up, it is important to cut exactly in the middle of the metal contacts between every 3rd LED, as it will shortly be necessary to solder wires onto the metal contacts. If using the waterproof coated LED strip, the rubbery coating will have to be removed from the contact pads after cutting, which is quite tedious and an argument in favour of the non-waterproof variants. With the strip cut into pieces, it is now time to connect them back together by soldering short (5-10 cm length) wires between the metal pads. While it is possible to wire them all together in series to form one long strip, this means the link wires will be carrying the full 5 amp current load and if any link goes bad it risks taking out the entire set of LEDs beyond it. A better bet is to wire them up in parallel, or perhaps grouped in a mesh giving multiple paths for the current, so the link wires only need handle a tiny current and there is redundancy. The important thing when soldering the link wires is to preserve the polarity between strips – ie connect positive to positive, and ground to ground.

All 20 strips with connecting wires soldered on. Mistakenly all 20 strips are in series. This was later resoldered to put them in parallel

All 20 strips with connecting wires soldered on. Mistakenly all 20 strips are in series. This was later resoldered to split them in 5 groups of 4 strips, each group in parallel, reducing current in the link wires to 1amp.

The vendors of the LED strips will typically also sell suitable power supplies. These mains powered units need to output 12 volts and be capable of supplying 5 amps to enable the LEDs to run at full brightness. Lower amperage PSUs will work, but the reduced LED intensity will obviously increase exposure times, so it is best to simply get the right specification of PSU from the start.

Mains PSU for the LED strip able to supply 12 volt at 5 amp

Mains PSU for the LED strip able to supply 12 volt at 5 amp

The power supply will likely have either a 2.5mm or 2.1mm plug, so a correspond matching socket needs to be purchased. While it is possible to just turn the device on/off at the wall, or by pulling the plug out, a better bet is to put a rocker switch inline with the positive power line between the plug socket and the LED strip. Again make sure the rocker switch is rated to carry 5 amps.

Power output is given by multiplying voltage by current, so 12 volt at 5 amps will produce 60 watts of power output. This is going to generate a reasonably large amount of heat and if something is not done about this, it will gradually degrade the LEDs shortening their lifetime. The obvious answer to this is to obtain a couple of 12 volt computer fans to fit in the case of the final light box. A fan that is approximately 4cm x 4cm in diameter will be ideal. They’re quite cheap so you might even consider using a pair of fans. The wires from the fan can be connected in parallel with the LED strips, since they’re conveniently driven from the same voltage. Do NOT connect them in series with the LEDs, as the 5 amp current draw of the LED circuit will kill the fans. Also be careful to get the positive/ground polarity right when connecting the fan, as reversing polarity will NOT make the fan run in reverse and likely kill the fan too.

12 volt computer fan, 4cm in diameter

12 volt computer fan, 4cm in diameter

The case for the light box will be made out of wood and comprise two pieces, a base which will hold the paper to be exposed and a slightly larger lid which will hold the LED panel. Both will have sides and be sized so that the base nests snugly inside the lid (or vica-verca). The top and bottom panels can both be cut from a sheet of 3mm plywood, the lid panel being 39×30.5cm and the base panel 36.5×25.5cm. These sizes are fairly arbitrary – the smaller simply needs to be about 2 inches larger than the size of paper to be exposed on each side. So for 8×10 paper, the smaller would want to be about 12×14 inches. For the larger lid, sides were cut from a length of 70x18mm timber, and nailed to the plywood panel. For the smaller base, sides were cut from a length of 36x10mm timber. In the timber sides of the lid, two 4x4cm holes were cut to hold the fans. Two holes were also drilled in the lid, one for the power supply plug socket and the other for the on/off rocker switch. When inserting the fans in the case, one should be oriented so that it sucks air into the case while the other should blow air out of the case, creating good airflow across the LED panel.

Light box lid showing the on/off rocker switch through the panel and power supply socket in the side

Light box lid showing the on/off rocker switch through the panel and power supply socket in the side

Close up of the lid, showing the computer fan inserted in the side to pull air across the LEDs for cooling.

The LED strip usually comes with a self-adhesive backing tape which is supposed to be able to stick the LEDs to most surfaces. This proved insufficiently sticky for me, so I applied super-glue instead. While the LED strips could be attached directly to the lid of the light box, it was thought preferable to attach them to a sheet of perspex or aluminium to allow the LED sheet to be separated from the case if needed. If using metal just be careful to avoid any short circuits with the link wires of the LED. Once the LEDs are attached, the sheet can be fixed to the inside of the lid with a couple of screws.

The larger lid, showing the metal plate with LED strips attached. At either end are cardboard shields to block UV light leakage through the fans.

The light box base

The smaller lightbox base, sized to be able to hold an 8×10 inch picture frame from a pound shop. Note a couple of screws sticking out of each side of the base, to prevent it sliding completely inside the lid when nested

When first turning it on, there was some UV light leakage through the cooling fans. Thus a couple of shields were cut from heavy duty cardboard and duct taped over the fan openings. With this in place there is no significant UV leakage from the light box, due to the closely nested lid and base. The UV LEDs are emitting at the end of the UVA spectrum, quite close to the start of the visible light spectrum, so the light is not a serious danger like UVB light would be, but it is none the less worth taking care to avoid accidental exposure.

In use the light box has proved to be intense enough to expose acceptable cyanotype images in as little as 5 minutes, and lumen images in anywhere from 10 minutes upwards depending on the visible effect desired. This is considerably faster than many commercially obtainable UV light sources that photographers have used in the past, which could take 15 to 30 minutes or even more. All together the cost of the complete box was probably around £45 – if you already have some parts in the shed such as plywood / timber pieces and a suitable power supply, then the price could be around £20-25. Either way, it will easily beat the cost of commercially produced light boxes and likely perform better too. The hardest part in construction is probably the soldering of the 50+ link wires between the LED strips. The case needs only minimal wood working skills – use of a saw and hammer. In summary creation of the light box is a very worthwhile use of time and money and will proof useful for years after.

Obsolete and Discontinued: a collective photographic project

The project background

This post is talking about my involvement in the Obsolete & Discontinued project which has just had its first exhibition at the Releva-T analogue photography festival in Spain. The seeds of the project were sown back in March 2015, when London based photographer & expert printer, Mike Crawford, was given a huge quantity of photographic paper (and a few rolls of film) by a client, David Yates. The paper had come from darkroom supplies left by David’s late uncle, Bret Sampson. There was quite a variety of paper, including Agfa Record Rapid, Kodak Bromesko, Agfa Brovira, Kodak Royal, Kentmere Bromide, Ilford Ilfomar and more besides. Most of the paper was so old – at least 20 years, perhaps as much as 40 years for some – that common opinion would suggest it to be mostly worthless and fit only for the rubbish. Thankfully Milke did some test prints to investigate the condition of the paper, finding some in fine condition, while others had heavy fog. Even those which had heavy fog turned out to be quite amenable to lith developer which was not nearly as badly affected.

After seeing these results, Mike unveiled a proposal to a meeting of the London Alternative Photography Collective (LAPC) meeting. He intended to give out batches of paper (~10-20 sheets per person depending on number of participants) to a group of photographic artists with an open brief to produce any type of work they wished. The only rule was for the supplied paper to be used in some manner, whether for the final work or just in an intermediate step. The idea was immediately appealing and many attendees of that meeting signed up to participate in the project straight away, myself included. After getting around 60 people signed up Mike sent out details on what paper was available (and its response with test prints) to all participants asking them to provide a preference list.

Developing the techniques & idea

At the time, having a 3 month old baby, I wasn’t really going into the darkroom at all, so using any kind of traditional black & white process was out of the question, never mind learning lith printing techniques. I decided to focus on alternative photographic printing techniques that could be done without any use of a darkroom, specifically the lumen and chemigram processes which I had recently started experimenting with in the back garden. I had trays suitable for 8×10 paper and wanted FB paper rather than RC. Beyond that I was not fussed on the choice, whether it was fogged or not, since this is pretty much irrelevant to the chemigram process which takes place in full light. Essentially any photographic paper will work for chemigrams no matter how old or badly looked after it is. Eventually I was given approx 15 sheets of Agfa Brovira G3 as the official paper to use for the project work.

Rather than risk wasting this precious paper, I bid on a couple of ebay auctions acquiring some ancient kodak and ilford papers for exploration of the techniques I wanted to use in the project. With the ilford paper I learnt how to combine the lumen and chemigram processes to create very pleasing hybrid works. Showing one of the works to some friends at another LAPC meeting, a remark was made that one of the prints (shown below) gave the impression of a river running through a city:

Resulting print after combining the chemigram and lumen processes.

Resulting print after combining the chemigram and lumen processes.

I myself felt that the dark brown / black texture of the print felt like a desolate landscape, devastated by industrial development or some natural disaster – quite representative of the state (reckless) human development can leave land in. At the same time I was feeling that too many chemigram images I’ve seen focused on the totally abstract and so wanted to explore how to control the chemigram process to bring in more recognisable forms.

Melding these thoughts together, my intent for the Obsolete & Discontinued project was to try to produce semi-abstract, but still recognisable, images of coastal cities around the world using the chemigram and lumen processes.

Behind the scenes

The starting point was to identify some coastal cities whose border between land and sea would give rise to reasonably recognisable outline and a good balance. In other words I spent a while browsing around Google maps looking at coastlines for random cities I’ve visited in the past. This identified a number of candidates including London (Isle of Dogs), Southampton (Isle of Wight), Plymouth, New York, Boston, San Francisco, Rio de Janeiro and Tokyo. With these identified I printed out the maps on plain paper and using a scalpel cut around the coastline to remove the land

Printed map outline after cutting away the land mass

Printed map outline after cutting away the land mass

The paper outlines were then traced onto pieces of 2mm thick card, then cut out with a scalpel again, to create the master heavyweight stencils

Cardboard stencil after being traced and cut from the paper outline

Cardboard stencil after being traced and cut from the paper outline

The purpose of the stencil is to assist in applying the resist for the chemigram process. The chosen resist was a thick paste of flour and water, the same that was used in earlier chemigram experiments. The stencil was placed on a sheet of the photographic paper and the resist spread on in a thin layer. The ultimate result was that flour paste adhered to the paper in the regions that would represent the land mass, while the sea remained clear.

Applying a resist of flour & water to the photographic paper, using the cardboard stencil

Applying a resist of flour & water to the photographic paper, using the cardboard stencil

The coated paper is then left overnight to allow the flour paste to dry out and form a hard crust on the paper. With the areas representing the sea still exposed, it was time to start on the lumen processing. The idea was to try to get an abstract texture to represent the rippling surface of the sea. To this end, several layers of crinkled clingfilm were placed over the paper in a random fashion, which would (in theory) control the amount of UV light affecting the paper.

Paper covered in clingfilm and placed in a photo frame ready for exposure under the sun

Paper covered in clingfilm and placed in a photo frame ready for exposure under the sun

The paper was placed in a budget photo frame obtained from a local pound shop (aka dollar store) and then left out under the autumn sun for approx 4-5 hours. The UV light had quite a nice affect on the paper turning it a fairly intense purple/blue color, and the use of cling film had been partially successful in introducing variation in the colouration.

The paper after it had been exposed to the sun for several hours

The paper after it had been exposed to the sun for several hours

The lumen exposure was immediately preserved by putting the paper through a regular fixer bath. Sadly with the Agfa Brovira paper, almost all of the colouration from the lumen exposure disappeared. This is entirely expected when fixing lumen prints, but the degree of fading varies across different papers and the Brovira seemed particularly badly affected. The final step was to slowly dissolve away the flour resist, moving the paper between developer and fixer baths every 10 minutes or so. Where the developer came in contact with the surface of the paper first, it would go dark gray/black, while the fixer would preserve undeveloped areas in pale brown. In this way the texture was gradually built-up as the flour resist dissolved. The chemigram stage took at least an hour of hard work per print to complete.

In the end six prints were made on the Agfa Brovira paper supplied for the project, and the three most successful ones were selected for presentation to the project.

Final results of the process on Agfa Brovira paper

Final results of the process on Agfa Brovira paper

My time producing prints for the project spanned a couple of months, with me snatching 2-3 hours at a time to work on it when the weather was favourable to lumen printing, in between the usual time consuming duties of parenting. The most time intensive part was the chemigram process where the dried flour paste would very slowly dissolve, but the results obtained were certainly worth the effort. Participating in the project helped focus the mind, allowing for intensive effort to control & master a particular set of techniques, with a clear target in mind. It has been a great learning experience and I’m very pleased to have gotten involved in it.

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