Lumen Printing: How Exposure Time Changes the Image – from 5 minutes to 2 months (!) exposure!

Lumen printing is a simple cameraless photographic process where photographic paper is exposed to the sun. But the exposure time can greatly affect the result. Malin Fabbri shows how a short 5-minute print compares to 30 minutes, 2 days and – just to push the limits – an extremely long 2-month exposure time. See the difference!

Writer and photography / Malin Fabbri

Lumen printing is a cameraless photographic process that uses sunlight to create images directly on black & white photographic paper. Objects such as flowers, leaves, and feathers are placed on the paper and exposed to UV light, producing colours and tones without using a developer.

If you have never tried lumen printing, give it a go – it is an easy and rewarding process. You create images without a camera, most often using botanical materials such as flowers and leaves. One of the questions I get asked frequently is how long to expose a print and how exposure time affects the final result. To find out, I decided to run an extreme test to show just how much difference exposure time can make.

To explore how exposure time changes a lumen print, I created a series of tests using the same flower, the same paper, and the same basic setup. The only thing that changed was the exposure time, from very short to extremely long. The subject was Wood anemones, collected in April from a field full of spring flowers. The last print remained outdoors until June, creating an exposure time of 2 months.

Paper used for the lumen prints

All you need is photo paper (the kind used in traditional darkroom printing) and a frame. Nothing else. To make an image, lumen prints rely on the direct action of light on the silver halides in the emulsion. 

A number of papers can be used to print lumens, in fact, I have not heard of a paper that does not work at all. I used FOMATONE MG Classic 132, matt surface. It is a Czech company that makes the paper. This variable-contrast black and white enlarging paper uses a warm-tone silver chlorobromide emulsion on a baryta base, which produces neutral to warm image tones. It is available in matt, semi-matt, and glossy finishes. The matt surface used in these tests produces slightly lower maximum density than the glossy version. If you are curious about this one, here is the manufacturer information, a retailer in Norway, Czechoslovakia and Freestyle in USA, that sells Foma paper and the technical information as pdf on the paper.

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The Lumen Exposure Tests

The flowers and the frame were the same for all four prints. The UV light was the same for the first 3 prints, but for the month-long print, of course the UV had time to change during that period.

The 5-minute exposure lumen print

After just 5 minutes, the print developed a lovely light blue colour with surprisingly good contrast.

The 30-minute exposure lumen print

At 30 minutes, the colours shifted noticeably towards warm purple tones. This exposure created one of the most balanced results.

The 2-day exposure lumen

After 2 days, the colours moved towards brown. As the paper continued to darken, the separation between the flowers and the background became less distinct.

2 months (!) exposure

The longest exposure produced the biggest surprise. Instead of continuing the trend towards lower contrast, the print appeared to reverse direction, and the extremely long exposure print was of higher contrast than the 2-day print. This unexpected behaviour is one of the things that makes lumen printing so fascinating.

Lumen prints are created through a print-out process, where UV light converts silver halides into visible metallic silver. During extremely long exposures, additional reactions seem to have influenced the image.

There are some possible explanations and rather unqualified guesses:

• The uncovered paper continues to darken, while the flowers create a changing microenvironment that affects the emulsion beneath them. 
• As the wood anemones dried and slowly decomposed during the 2-month exposure, they could have changed the chemistry enough to slow or alter the reaction beneath them. 
• Lumen printing pushes photographic paper far beyond its intended use, and the image reversal or print-out is caused by a regression in silver-gelatin materials when exposed for extremely long periods.

If anyone has a better guess, do let me know in comments!

The question about which exposure time to use remains. The answer? There is no single “correct” exposure time for a lumen print; it all depends on preference and creative choice. Personally, I prefer the shorter exposures where the warm tones are preserved – and I don’t have to wait 2 months! What do you prefer? Comment below; I’m curious to find out!

If you prefer watching video, see the YouTube video

Malin Fabbri is the editor and founder of AlternativePhotography.com, author of several books on alternative photographic processes, and experiments with a variety of techniques such as anthotypes, cyanotypes, photopolymer gravure, chlorophyll prints, lumens, cyanograms, photosynthesis, pinholes and solargraphs.