A UV Light for Cyanotypes

The traditional cyanotype process, now regarded as alternative processing, requires a UV source for curing (exposing) the prints. This post outlines the requirements for such a UV light source and some options to consider, along with the associated pros and cons, and then to look at one specific light source in more detail.

Everbeam UV Flood light

What is UV light

Ultraviolet light, known simply as UV, is electromagnetic radiation of wavelengths 10–400 nanometres, shorter than that of visible light, but longer than X-rays.

UV is typically broken up into 3 bands:

• UVA - 400 to 320 nm
• UVB - 320 to 290 nm
• UVC - 290 to 200 nm

UVA is the range for cyanotypes with 365nm usually considered a good mid-band wavelength that is effective for exposures (think of the numbers of days in the year and then the wavelength is easy to remember).

Can I just use sunlight

If you don't have a UV light source then sunlight is an option. The atmosphere blocks UVC and some UVB so the UV light from the sun is of varying wavelengths mostly in the UVA band. This works better in very sunny places so is not the best solution for me being located in Scotland. The problem is that this can be a very inconsistent light source with potentially massive corresponding variations in exposure time. If you have a weather station then you can typically monitor your UV Index as a guide and this will vary by time of year and time of day. However just a cloud blowing over can change the values rapidly so it's not an ideal way to work if you don't have clear skies.

If instead of working outside you opt to use sunlight from inside, perhaps somewhere like a conservatory, note that it is very common for modern window glass to block UV so this may prevent cyanotype exposures.

Artificial UV lights

Opting to use an artificial UV light source allows you to work inside and have consistent timings without all the variations that arise from time (of day / of year) and weather. There are many different types of UV light that are available, these include; torches, grow bulbs, black lights, party lights, face tanners and LED strips. Some will work much better than others.

Philips Face Tanning Light

I have previously used a face tanning unit which used 4 Philips Cleo 15w Hg tubes emitting UVA. This did work but the tubes are delicate, the unit is bulky and awkward, the whole thing can be hard to position and it is vulnerable to liquid spills. For these reasons I'm switching to using a more compact and robust LED floodlight.

A UV flood light

Everbeam offer several variations of UV flood lights, the one I selected was the "Everbeam LED UV Black Light Flood Light - 365nm 50W". I will be using this to expose A4 size prints (if you want to work on larger prints, say A3, you may want to increase the distance and step up to a higher power with a 100w unit).

https://shopeverbeam.com/collections/uv-black-light/products/everbeam-led-uv-black-light-flood-light-365nm-50w

Note I was able to get this with a UK power plug even though the online images show a different plug.

Establishing an exposure distance

The light is made up of a 45 LED array which is approximately 9cm x 5cm. The angle of coverage is specified as 120 degrees. This equates to a minimum operating distance of about 7cm (your maths may be more exact than mine) to get a full coverage for an A4 sized print.

Angle of coverage

Note that we can expect UV light to follow the usual inverse square law of radiation, i.e. doubling the distance gives a quarter of the light or four times the exposure time. This tells us two things, having a long distance is likely to require very long exposures. Having a short distance means that any slight variation in setup (moving the light, thickness of paper etc) will have a proportionally high effect and make consistency harder.

Test set-up with books for height

So choosing a working exposure distance is a combination of exposure time, area of coverage and how consistently repeatable the set-up is. Something in the 10 to 15cm range could be a good starting point for testing this light. I made a temporary rig using books to give the height, all easily adjustable while I investigate what works and this gave me an initial test height of 11cm.

Note that the red light on top in the photo is a regular darkroom safe light (as used for orthachromatic films), from previous experience I know this light to be cyanotype safe so was the safest option to help see what I was doing in an otherwise dark room.

Establishing an exposure time

A standard exposure time (the time it takes for the darkest tone to be achieved) is established through a test by exposing for a range of time values and then seeing where the colour blocks with no further change. This can be evaluated by eye, a scan can be taken or a print calibration tool can be used.

My test was using Daley Rowney 220 gsm paper, coated with the classic cyanotype formula and covered with a clear sheet of Q-Connect laser film. I exposed this as a series of 1cm strips, each getting one more minute than the previous one, giving a total of 28 test strips (NB my paper was not fully coated at the very edges). If you have a step wedge (e.g. Stouffer) you can use this instead.

Scan of the test strip

This was then scanned with a flat bed scanner for analysis. Using Capture One Pro I then added color-readouts to see where the brightness values levelled out (the lower of each set of 4 numbers).

Test strip with Color-Readouts

So from this test my standard exposure time is around 9 minutes. At some point I can repeat this test in smaller increments, say strips 30 seconds apart, for greater resolution but I have some more variables to introduce to my set-up before I do that.

If your exposure time varies

Running your own exposure test is a good idea as there are so many factors that make a difference, including; power rating of the light, size of the light, distance between light and paper, other light sources (windows or lights giving off some additional UV), UV blocking from glass or perspex over the print, UV blocking from the film (digital negative medium), the paper surface (hot press papers are usually preferred) and coverage of the cyanotype solution (brush or puddle pusher), cyanotype chemistry (e.g. classic, Sam Wang / Mike Ware formulas), humidity etc.

Conclusions

The UV flood light from Everbeam worked well for my test, gave a quick and repeatable exposure time and was easy to use. This light should be a good option for my cyanotype work going forward.