To choose a cyanotype UV exposure unit, check that it emits UVA at around 365 nanometres with BLB tubes, that its working area is larger than your biggest format and that it presses the negative flat, a timer being a bonus. The sun is enough to discover cyanotype, but it does not take orders.
A sky that clouds over mid-exposure, a season with no light, the wish to print an identical series once evening falls, and you soon find yourself looking for a stable source of ultraviolet. That is the role of the UV exposure unit, a box fitted with UV tubes that replaces the sun with constant light. Whether to buy a UV lamp or an exposure unit depends on how often you print, from the first print to the hundredth.
Here you will find the buying criteria for the box, the components of a home-made build and how to calibrate it. The practical guide to cyanotype places each step of the process in context. The exposure process itself, reading the colour change and the exposure time are covered in our guide to exposing a cyanotype; making do with a backup lamp is covered in the page on cyanotype without sun.
What a UV exposure unit is for in cyanotype
A UV exposure unit is a closed box that lights the sensitised paper with an even sheet of ultraviolet light. The paper and its negative, pressed together under glass, receive a dose of light that is always identical from one print to the next.
The gain is not speed, as a home exposure unit remains slower than full summer sun, but reproducibility. Once you have found the right time for a given paper and negative, you get exactly the same result the next day or in the depths of winter, which no sky can promise. If you print in series or teach, this consistency is worth the investment.
The other benefit is uniformity. Sunlight is directional and can cast a shadow under a thick object; a well-designed exposure unit spreads the UV evenly over the whole surface. This is valuable for large formats and delicate photograms.
An exposure unit gives every print the same dose of ultraviolet light, spread over the whole surface.
Which UVA tubes to fit in the box
Everything depends on the wavelength of the light. Cyanotype reacts to ultraviolet close to visible light, at around 350 to 400 nanometres, which is known as UVA (in French). It is this band that reduces iron III to iron II and triggers the formation of Prussian blue.
Suitable tubes are often marked BLB, for blacklight blue, and have their peak emission at around 365 to 370 nanometres. They are the same as those used in black lights, sold as fluorescent tubes or as LED strips. Germicidal tubes known as UVC, on the other hand, which emit at around 254 nanometres, are to be avoided: they do nothing for the process and are a danger to the eyes and skin.
The number of tubes and how close they are to the paper determine the power received. A typical build lines up four to eight tubes of fifteen to twenty watts, spaced two to three centimetres apart and placed a few centimetres above the exposure surface. The more tubes there are and the closer together they sit, the shorter the exposure, but the exact setting is found by testing, never by calculation.
Cyanotype reacts to UVA close to visible light: tubes lined up close together above the paper.
Buying a ready-made UV exposure unit
Exposure units designed for historical processes and screen printing are available commercially, and they are very well suited to cyanotype. They are closed boxes with UV tubes, often fitted with contact glass and a timer that switches the light off at the chosen time.
Three points deserve attention before you buy:
- The working area must be larger than the biggest format you plan to print, with no shadow at the edges.
- A timer is handy for repeating an exposure to the second.
- A good contact system, heavy glass or a pressure lid, remains essential, because a negative that is not pressed flat gives a soft image whatever the quality of the tubes.
A commercial exposure unit costs more than a home-made build, but it arrives set up and calibrated, with the electrical safety already handled. It is the peace-of-mind choice if you print a lot. For occasional use, a home-made build often remains the better balance between price and use. For a few summer prints, on the other hand, the sun and a cyanotype kit are enough, with no equipment at all.
A commercial exposure unit: a box with UV tubes and heavy contact glass, with a working area larger than the format.
Building your own UV exposure unit
A DIY exposure unit comes down to a few components, and many studios have been working for years with a home-made build. The principle is simple:
- an opaque box;
- a row of UVA tubes on the ceiling;
- an exposure surface at the bottom;
- a sheet of glass to press the negative onto the paper.
The heart of the build is the UVA tubes and their ballasts, the fluorescent light fittings you find in hardware stores. Fix them side by side under the lid of a wooden box, painted white inside to reflect the light. For an A4 to A3 format, four to six UVA tubes of fifteen to twenty watts, a few centimetres from the paper, already give good results.
The contact surface consists of a perfectly flat board covered with thin foam, on which you lay the paper and then the negative, with a sheet of glass on top to press everything flat. The pressure of the glass provides the tight contact that makes the image sharp; a hinged contact printing frame does the same job without any glass to lift.
Electrical safety is not negotiable. Wire the tubes according to their instructions, protect the circuit, and close the box so that no UV leaks out towards your eyes. If electrics are unfamiliar to you, it is best to have the build checked by someone competent.
A wooden box painted white inside, UV tubes in the lid, and the print pressed under glass.
Calibrating your exposure unit once and for all
An exposure unit, whether bought or built, only shows its full value once it has been calibrated. The aim is to find, for your paper and your type of negative, the exposure time that gives the deepest blue without blocking up the shadows.
The method comes down to a test strip. Gradually mask an offcut of sensitised paper, expose it in steps of two or three minutes, rinse it and read which step gives the most satisfying blue, neither pale nor fogged. That time becomes your reference, to be noted on the box.
This matters because UVA tubes lose intensity with hours of use. An exposure that gets longer for no reason often signals tired tubes that need replacing. A new paper or a denser negative also calls for recalibration, but the logic stays the same: the colour change on the surface guides your reading, just as with an exposure in the sun.
Setting up the mask for a test strip, with the UV source switched off between handling steps.
Choosing between an exposure unit and the sun
The sun remains the finest source of UV, free and powerful, perfect for discovering the process and printing through the seasons, provided you accept that the exposure time in the sun changes with the season and the sky. The exposure unit does not replace it: it offers what the sun cannot, light that is available at any hour and strictly constant.
The right guide is your own practice. As soon as consistency matters, because you print in series, teach or want to work in winter, the exposure unit becomes the tool that makes every exposure reliable.
Éric Marais — Photographer, author of two books on pinhole photography (published by Pearson) and creator of the Sténoflex. He has run introductory workshops in alternative photography, from pinhole to cyanotype, from the very beginning.