31 step wedge & Nextav 3A photo copyright Douglas Stockdale –
Last December I had written an article about using a Zone system gray card to determine my solar cyanotype exposure times as a means to obtain the Dmax (Maximum Darkness, e.g. blackest tonalities) for my solar cyanotype printing process. I have actually moved on from that indirect process in an attempt to work directly with the solar source of my UV light.
First was to move from using my gray card to working with an UV Index posted on the web, an example of which is below, sourced from uvindextoday.com. I input my area code to obtain the UV Index values. I also started using a 21 step wedge from Stouffer to test my exposures for a series of times through the day. As you note below, the UV solar intensity changes through the day, which is another reason that makes solar cyanotype printing rather vexing. Nothing like trying to hit a moving target, eh?
Initially, this combination seemed to work to establish my solar cyanotype exposure times…until it did not. With the 21 step wedge, there were so big jumps in duration times between steps. Which resulted in some over or under exposures from my post-printing evaluations. Thus, I next acquired a 31 step wedge from percisiondigitalnegatives.com which provides a finer gradation for determining the exposure durations, which is what I am using now in the photo above. Nice.
And working with the web-based UV Index was great…until it wasn’t. First, the ‘local’ UV Index is calculated from a number of variables, none of which appear to be local. The second issue is that this is updated only one time per hour through the day; so you have a UV Index for at the top of the hour and as you can see below, the sun’s UV values are constantly changing. My work around was to time my solar printing times to start at the top of the hour…which was good when it wasn’t. While doing my step wedge testing until recently, I forgot to take note if the reading was when the sun’s UV Index was increasing or decreasing; thus the reason for a few ‘overexposures’ and ‘underexposures’. Lessons learned, now I add this to my notes.
Second issue which led me to purchase the relatively inexpensive Nextav solar UV meter was the web based UV Index did not account for cloudy days. When the sun is behind heavy clouds the UV Index is one value and then when the sun comes out and shines brightly, another value and of course all of those conditions in-between. Thus, the idea of purchasing a hand-held UV Index meter, going with the Nextav to find out if this might be feasible. (Not recommended for evaluating UV lights in an UV exposure box). I had read the fine print that this UV meter is a broad UV spectrum for UV-A and UV-B light, while for cyanotype printing, we basically need a narrow spectrum UV-A light. Who knows what the UV light spectrum is for the web-based UV index.
So now I am completing a series of test with the 31 step wedge, noting both the web-based UV index and what my little hand meter reads. I am probably going to eliminate the web-basted UV index at some point, but for now, tracking both and currently using my web-based UV Index exposure calculations. Meanwhile, I am improving on my printing consistency for my solar cyanotypes. Cool!
If it’s not one thing, its another…especially with solar cyanotype printing.
Cheers & make every day an Earth Day