First sun exposure using PPM-1 UV exposure meter

Recently, I bought a light meter from LightMeasure.com that displays time, intensity, and total dose of UV light from 360 - 380 nm.  The device (often called an "integrator") is made by someone who has experience with Solarplates.  Since the UV light needed to develop Solarplates is the same as needed to develop the KM73 plates that I use, I thought it would be beneficial.  Here are the results of my first basic experiment over the past week.

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February 21, 2015

Since my normal workflow uses RPX 16x20 uv unit (the one with tube lights and not CFLs) and a Takach10” x 12” stochastic screen for 5 minute screen exposure and 5 minute image/transparency exposure, I placed the PPM-1 over the RXP unit to measure the dose of a 5 minute exposure.

Dose 1:  5466                        cold unit, first firing

Dose 2:  5978                        lowered PPM-1 unit closer to lamps

Dose 3:  5683                        careful to place PPM-1 unit at exact same distance from UV tubes as the outer face of the contact frame class is from the UV tubes.

Average dose:  5709

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February 25, 2015

Testing outside with PPM-1 to see how long it takes to get a dose of 5700

Direct sunlight

12:23 pm       169 seconds  dose = 5706  last intensity reading = 333

12:29 pm       157 seconds              dose = 5735  last intensity reading = 365

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March 2, 2015

Direct sunlight           3:10 pm

Expose KM73 polymer plate to the “Gamelights…” transparency using same contact frame and screen as normal workflow, to make a new plate for comparision

Target dose = 5700 for each exposure (screen & image)

Actual:

Screen  dose: 5761  time: 279 seconds     last intensity reading: 92

Image dose:  5813     time:  354 seconds    last intensity reading: 157

Notes:

Using a plastic bag to cover contact frame from exposure is awkward.  Perhaps light leaks?

The current clamp system made holding the contact frame difficult and impossible to put down so I had to hold it (had to keep adjusting attitude of frame).

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March 4, 2015

Printed new plate (outside exposure with PPM-1) and old plate (exposed with RXP 16x20 uv tube unit) using same ink and paper.

(Note:  image resolution degraded/grainy to protect copyright)

Old Plate

New Plate

Conclusion: 

PPM-1 is a good device that makes exposing any plate size possible.  Needs a new curve, probably a less aggressive curve, probably making the transparency more closely resemble that of a film inter-negative.  This first experiment was overexposed a little bit because I was not able to accurately control the exposure with just a plastic bag.  A good light-proof cover needs to be designed to control the exposure.  It might be worth making an entirely new contact frame that has edges that can rest squarely on the ground without the clamps getting in the way, so the exposure process can happen at the same attitude (angle facing the sun) throughout both the screen and image exposures. 

Speculation:

Since the sun is a single point source of UV, undercutting does not happen to the same degree as with fluorescent tubes.  This might explain the darker tone above 30% density (undercutting/scattering of UV with tubes over cures these sensitive tones, thus making them lighter).  Also, the darker areas (above 85%) are lighter, probably because the UV intensity is higher so they are a bit over cured.