I use this led strip:
https://store.waveformlighting.com/p...12527605252198
I cut and soldered a 5m strip (of the 365nm) so the exposure area would cover my 11x14 printing frame. The strips come with adhesive so I just stuck them to a plywood board I had, in retrospect an aluminum sheet would have been better as they didn’t stick all that well. The LEDs get warm but not hot enough that they need a heat sink.
Exposure distance is approximately 2in as that was the closest distance where the beam spread appeared even. As to exposure time, a palladium print will get to maximum density in around 2min through an unexposed area of film and around 5min through a blank area of pictorico. I don’t typically print cyanotypes but if I remember correctly the exposure time would be in the 10-15 minute range.
I have focused more on using film lately, but my longest exposure has been around 60min. That was to get detail in an area of snow with a density of log 2.8-3.0. The LEDs are expensive so it was a tough decision, but it has been worth it when printing negatives with very dense highlights.
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