Rubylith Transmission Curve and Safe Lights

I have been looking for a transmission curve for Ulano's Rubylith masking film for some time. Ulano has not made one available. I use the stuff to provide additional filtering for a Superbrightleds.com red LED 7.5 watt bulb, popular for use as a safe light (E27-R8-G). Empirically, the Rubylith removes the low-level green and yellow components in the bulb's spectrum, which can otherwise fog paper with close and/or long exposure. I have verified this with a cheap spectroscope. Most common red LEDs have similar green and yellow components.

I found a transmission curve for Rubylith in an old declassified Canadian defense document describing a laser targeting "hit" recorder. I'm guessing this is accurate coming from military testing.

The graph of the output of the red LED bulb shows the low-level yellow and green output (the overlaid purple trace is for Arista EDU single-grade paper). The Rubylith curve shows it acts as a low-pass filter, effectively blocking anything higher in frequency than 600nm.



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Cool find, @DonR! Thanks for posting this.

Btw, I can say from first-hand experience that Rubylith is a really, really good UV blocker, but not perfect. It does pass through a tiny little bit of UV. I imagine the same is true for other wavelengths just the same. The practical implication of this is that in extreme situations, there can be unanticipated fogging effects. For instance exceedingly long alt. process exposures with a single layer of Rubylith used to mask borders, or extremely bright safelights (LED) with significant green/yellow secondary emissions.

For the kind of application you're using it for (and me, too), it's great material. Fortunately it can still be found here and there.

koraks said:

For the kind of application you're using it for (and me, too), it's great material. Fortunately it can still be found here and there.

Click to expand...

Here is the link to the full document:

Rubylith curve plot document

I still have 4 large sheets of the stuff. It's available from a few places, mostly for astronomy night sight preservation screen filters over iPhones and iPads, it seems.

Here is my recent modification of an old Arkay OC safe light using the aforementioned bulb (with base adapter) and Rubylith single-layer filter, sandwiched between two pieces of 5x7 picture frame plexiglass.

Cool stuff! Yeah, it's nice to have some of those sheets around the place. I use them a lot for masking alt process prints. The sturdy transparent base is quite UV transparent. I just cut out a window where the image area will be and peel back the red layer.