Those who have printed hand coated processes using the sun, know that UV intensity alters the scale of the print, reciprocal of density range. Simply put, pointing the printing frame towards north sky while in shade, returns a print tonal scale longer than if the print frame was faced directly into the sun while printing. The former print would be quite a bit longer in scale than the former. The high intensity of UV light (aimed at direct sunlight) affects the highlights far more than low UV light. The affect is much like expansion and compaction principles, respectively. With high UV intensity the printing might be said to be acting from top down, not bottom up.
I went into the printer testing with this in mind. There is differences of opinion about just how much 'power', as in UV intensity, sort of measured in Lumens, comes from the wattage of UV fluorescent lights; tubes/bulbs. The only material I have come across on this was found on a website listing characteristics of various UV sources. Their rating was 20 lumens per watt. My solar printer is putting out 160W of UV light, times 20 which would indicate a total UV output at 3200 lumens. If that is anywhere near actual output, it would be a lot.
Before Solar Printer, I mostly printed using north sky light, with a fairly consistent meter reading of 15-20 lumens during the print time. Directly into the sun is well beyond the capacity of my GE meter so I haven't a clue as to the UV level for that reading. But compared to the north light count, the printer is a sort of muscle machine. I will admit I was a bit worried it would be so intense as it was designed as to set up the 'top down' printing, which I am not a fan of, because I also print gums, which need much less intensity to print, being the negatives needed for gum printing are much thinner. Aiming a gum print into the sun would make for very, very short print times, and that makes the process all but useless for any control. More on that process later.
The print below is s single print test I did for a salted silver (salt paper) print. I just wanted to know, roughly, the UV power output affect on this particle process, using this particular negative. Same negative that later was tested on a Kallitype print. When I print that negative using the sun, the densities for Zone 7 & Zone 8 are blank white, not matter how long it is printed, even when I pointed it directly into the sun during printing. All that did was seriously suppress the middle tones into a dark mass. Nothing printed in those two upper zones.
As you can see in the image, Zone 7 is not only printed in, but so is Zone 8. In fact, all the tonal ranges are suppressed. This is a bright overall image, originally. This was a 5 minute print in the printer. Corrected print time would be a minute or so less, and that will be determined with more adjustment and testing. To accommodate the longest possible tonal range in the print, the salt/silver (binder/sensitizer) ratio was 3% salt (binder) in the paper (Canson white) using a saturated solution (13%) of silver. There is no higher ratio, as the silver sensitizer is at a saturated level and cannot be increased. You probably already knew that but it seemed a good idea to throw that in.
According to my eye, this image is slightly darker overall than the original print, however I try not to do too much manipulation of the image in Lightroom just to make it more visually appealing. That was also the last of the silver of that percentage, thus uncoated area on the right. It was a density range test.... I don't give a fig about showing only perfectly executed images to seem the perfect printer. I show what comes out of the testing and printing as I come across each obstacle or issue. But that's just me.
The first thing I noticed was the highlighted areas, large trees top center, which had been blank white before, now printed in. Also Zone 1 & 2 were well represented. What suffered mostly were middle tones, which are printed down too dark. What that tells me is simply that the negative is scaled too long, the upper tonal ranges too dense and the middle tone densities are too thin. That is a simple scaling change in Lightroom. One of the possibilities I will be testing is coloring the negative image green, as PJ did with his test negative. Otherwise I will stick with my scaling routine, instead of Dan Burkholder's method. I will be spending more time on that issue. Refer to a post a while back in which I covered the basic differences between the two methods; including CI curve charts.
Salted Silver image;
Paper; Canson white ~ salted at 3% ~ sensitized 13% silver sensitizer
Print time ~ 5 minutes
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