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  • Writer's pictureChiara

Cyanotype science

Now that assessments are out of the way, I'm returning my attention to the technicalities of getting my cyanotype process working well at home. The story so far has me struggling with the quality of my transparencies and even more so with the power output of my UV light source. This morning I have re-read and summarised this useful article by Sandy King. Many people refer to it as being the most useful source of information for alternative printmakers working in their own/home studio. However, it was written in 2001 and from a USA perspective. Whereas the qualities of cyanotype have not changed since 2001, the availability and cost of different light sources have changed. And the article assumes USA voltage and safety parameters where a handy person can reasonably expect to wire together a bank of fluorescent lights and ballast. With the higher voltage used in the Australian system, that's not something I am able to tackle.

In any case, here is a summary of what I know about exposure for cyanotype from King's article:

  • AV A spectrum (320-400nm) is optimum for cyanotype exposure

  • UV B spectrum (280-320nm) is also useful, but is a cancer and cataract risk to humans, however regular plate glass shields out 95% of UV B rays

  • higher wavelength emissions (400nm and up) give better contrast in the print, as they penetrate deeper into the emulsion/paper matrix

  • Sun exposure suffers from all the vagaries of the weather and geography, but is a good source of the required UV light as well as higher wavelengths.

    • direct sun is faster

    • open shade gives better contrast

  • Fluorescent tubes wired together in a bank provide a useful light source for cyanotype (this is where the article and I part company due to Australia's 240V grid and my unwillingness to risk an electrical accident)

    • electronic ballast is cooler, more constant and more efficient

    • a bank of tubes requires a fan for cooling

    • distance between the tubes in the bank affects the evenness of lighting/exposure. Tubes at 6mm distance from each other exposure should be even at 5cm distance from the bank. If tubes are 18mm apart, the distance must be increased to 10cm to get even lighting. There is a theoretical fall off in light intensity with this increased distance.

  • Metal Halide lights are also discussed, however these appear to be too bulky for my studio set up.

I have also been researching ready made UV light sources which might be available for purchase. A proper exposure unit is beyond my budget and hard to justify for the volume of cyanotype work I anticipate doing.

  • Facial tanning units don't seem to be readily available in Australia (or with Australian wiring), or maybe I'm looking in the wrong places. They seem to provide the right sort of emissions and should be compact and easy to set up.

  • Reptile lights are sold as banks of fluorescent or Edison screw fixtures with UV A, UV B and heat lights available for purchase. The ones I have seen so far are no more than 20cm in width and I am unsure of how even the coverage would be. They are sold without the required fluorescent tubes or lamps, so the price quickly adds up. I need to do more research.

  • Grow lights for indoor plants or hydroponics are also touted as a good option. The ones I have seen so far have emissions in the red and blue visible spectrum rather than UV.

  • Strings of BLB LED lights are available (as party or ambience lighting), however I need to find out more about their power, since the 20W BLB CFL which I have been using is sold as a party light and has proved inadequate. A string of LEDs has the potential advantage of being readily attached to cover a surface which could be brought close to the paper for exposure, whereas my BLB bulb is limited to a single socket and I have been exposing from a distance of 30-40 cm to get coverage of my paper.

I've been talking about the "power" of a light source without really knowing what the correct units are: this article has some basic information. I'm used to thinking of the power of lights in Watts and it's about 40 years since my last high school physics class, so I've got a bit of catching up to do.

  • "A measurement of 1 lux is equal to the illumination of 1 metre square surface that is 1 metre away from a single candle".

  • "The lumen is a standardised unit of measurement of the total "amount" of light packets (quanta) that is produced by the light source"

  • "One lux (1 lux) is defined as being equivalent to one lumen spread over an area of one square metre."

  • so lux is a measure of light density where 1 lux = 1 lumen per metre squared (1lm/m^2)

  • "The power required to operate an installed light fitting is measured as a rated Wattage (Watts being Joules of energy per second)"--this is the unit of measurement I'm used to using.

  • "The Luminous Efficacy is a measure of how efficiently a light source produces visible light." Inefficiencies include power lost in creating heat and other losses. Luminous Efficacy is measured in lumens per watt (LPW).

  • Whereas I grew up buying 100W incandescent bulbs for the brightly lit areas of the home and maybe 20W bulbs for a bedside lamp, a 20W Compact Fluorescent bulb puts out as many lumens as a 100W incandescent bulb. This article has some of the relevant information.

Now that I have a better idea of what is being measured, I need to check how much of that information is available in the online shopping sites I'm looking at.

I've ordered this. It's a 72W LED blacklight designed for stage lighting. My understanding is that LED is at least as efficient as CFL, so that would make give me three times the output I have from the 20W CFL. It has a built-in stand and no need for cooling. It has a remote control, which might help me with timing, and at least will mean I don't have to stand in front of the light to switch it on and off. The UV spectrum should be safe enough anyway, since it's designed for a stage/party setting. It's coming from a local supplier with free postage and a 30 day return policy, so I can't go too far wrong.

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