Using LED lamps to expose alternative photographic processes prints

Exposure is one of the most important variables in alternative photographic printing. While sunlight remains the traditional choice, UV LED lights have become a practical and reliable alternative. They allow artists to print regardless of the weather or season while providing consistent, repeatable exposure conditions. We asked artists around the world to share the LED lights they use, along with their exposure times and results. This article summarises those responses and provides an overview of commonly used UV LED units and the exposure times they deliver. If you print cyanotypes in the sun, we also encourage you to take part in this survey.

Writer and photography / Malin Fabbri

Why LEDs?

I personally much prefer to use sunlight when possible, as I find it a more sustainable way of making prints, and I do love to be outdoors, but saying that, sometimes the weather is not on my side, and I do need to use a lamp. Many alternative processes are printing under ultraviolet (UV) and near-UV light rather than visible light. Many LED units – especially those made for curing or screenprinting – emit in the 365–405 nm range. This overlaps well with the sensitivity of cyanotype, kallitype and salt prints; even anthotypes can be exposed this way, though the exposure times can be very long, and not all pigments expose equally well under a lamp.

The sun can hide behind a cloud, or even refuse to come out, but LED lights stay the same regardless of the time of day or the season. Because the light is consistent, it is easier to compare tests, adjust your chemistry, and repeat good results.

I have several lamps, two of which are the Speedball 30 W LED UV Lamp (measuring 12×16 cm / approx. 5 x 6.5 inches in the outer dimensions of the casing), though the exposure times are long (40 minutes!). I sometimes use an  Everbeam LED 10 W UV, (Everbeam Black light flood light, 395 nm 10W IP66). It is very small (there are larger ones); the outer dimensions are only 8×10 cm / 3 x 4 inches. I don’t recommend that size, and I only use it for small postcard-size prints. I also use Black LED, an old Philips suntan unit and a homemade exposure box. You don’t need that many; one good one is enough. 

Good things about using LED lights

LEDs produce relatively little heat compared to other artificial light sources. This helps prevent paper warping, emulsion softening, or drying issues during exposure. LEDs are also energy efficient and safe to use indoors. According to manufacturers, black light is not harmful to the eyes or skin. Saying that, don’t look directly into the light. 

LED allows year-round printing regardless of the weather. Another benefit is adjustable height. LED panels can be mounted at a fixed height above the print frame. Once you determine a working distance and exposure time, you create a repeatable process – and a consistent light source.

Buying a LED Unit

When buying a LED light, wavelength is important. Look for LEDs that specify output around 365 nm (UV) or 395–405 nm (near-UV). Cyanotypes work well across this range; some silver-based processes may respond better to shorter UV wavelengths. If you Google “LED lamp for screenprinting” you will get a lot of results or see examples on Amazon here. 

It is also important to not to get too low wattage, marked by W. Low wattage means lots of time for exposure. One of my lamps is only 10 W and literally takes “forever”. Get at least 50W if you can, or be very patient when printing.

Setting up the LED light

When installing the lamp, I tend to use one with an “arm” and also measure the distance to the print so I can repeat the process – always take notes! Also, make sure the light coverage is even, the light area matches your print size (all of it is covered), and make a note of the print size too; it saves a lot of head-scratching later on. You can also build a box with foil on the inside.

If you are handy and want to try to build your own unit, see this article. Make sure you get the right lights; standard white household LEDs usually produce very little UV and do not work well.

Exposure times: Exposure times will vary depending on the process you use; anthotype, cyanotype, salt print, kallitypes, vandykes, etc. will have very different exposure times! And also the paper, chemistry and distance to the lamp will make a difference. Make a test strip to determine the correct exposure time. Also, test again if you change chemistry, paper, coating method, lamp, or distance. Cover part of your coated sheet with a strip or step wedge and expose in timed stages.

Printing: When you are ready to print, position the light perpendicular to the print and use a contact frame or a sheet of glass to ensure good contact between the negative (or positive for anthotypes) and the coated surface. Use a timer for accuracy. Because LED output is stable, small time changes lead to predictable tonal shifts.

Here is a chart found on the Speedball LED UV Lamp packaging. It also includes exposure times from 7 -12 minutes, but I believe they relate to screen printing and not alt. proc, so not so relevant, but an indication of which distance may be appropriate to use. Also, see the results in the survey below.

The LED and UV light exposure survey

I also asked the community what they use in a survey, and as always, many helpful people responded. I do love this “co-lab-learning” we often do! Here are the results:

Some observations from the data users have added are:

• Most modern UV LED lamps and exposure boxes produce a well-exposed cyanotype in about 3–10 minutes. This is by far the most common exposure range across the submissions.
• Exposure time cannot be predicted from the advertised wattage. Similar exposure times were achieved with lamps ranging from 20 W to 150 W, while some lamps with the same wattage required very different exposure times.

Now you can hopefully get good results, regardless of the weather. Good luck!

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.

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