I bought a new RGB LED bulb at the big box store that I wanted to use in my darkroom. It's nice as it's controlled by a remote and offers a variety of lights: from very warm white (about 2700K) to natural light (about 4000k), the remote acts as a dimmer as well. But I of course wondered if the red light was really safe for the paper. This is how I did it.

As we know, black&white paper is orthocromatic - sensitive to all visible light except red. This means that the safelights must emit light in the red wavelength spectrum, which is from about 635 to 700 nanometers[1].

If we look at the spectral sensitivity of, for instance, the Ilford FB Classic paper, we find that that paper is safe with lights that emit wavelength from about 580 nanometers - so a bit broader than pure red and falling into dark orange as well:

Now, I didn't want to wait to set up a paper test, so I recalled that a few months ago I bought a few diffraction grating as I wanted to build a simple spectrometer. But, that project never went beyond the initial design, so I was back to square one. Until I stumbled upon a nice paper[2] describing a super simple experiment that made use of the very same diffraction gratings I had. Bingo! Read that paper to understand the math and physics of the experiment.

Here is me testing a safelight. As you can see in the (bad) picture, the setup can very clearly identify the different spectrum peaks that the lights emits. The paper also describes the way to then calculate the peaks wavelength with a simple formula.

So, here are my findings on the safelights I have:

Classic tungsten inactinic light683nm
Osram Parathom P LED 1W654nm
iDual One 9.5W647nm

Both the Parathom and the iDual also emit additional spectrum with the iDual being the worse at 633 nanometers. But still, all three are pretty "red".

I have used the Parathom for years now with no fogging at all even for exposures of at least 30 minutes and the lamp at 1 meter from the paper.

I am very happy about the iDual as it is an RGB light and it would be fantastic to use in my darkroom so I don't have to set up multiple lights but I can use one and control it with a remote. Plus, it is a LOT brighter than all the others!

Let me know if you'd like me to do some additional testing with other lights. I have an RGB led strip to test as well and I'll report back once I do.

Happy printing!

Additional comments

I have received a few comments about this article and I thought I'd clarifiy a few things. The purpose of this "experiment" was to verify my assumptions of LED lights being reasonably safe. Someone correctly pointed out that LED lights might have tail radiation that sits in the unsafe spectrum for paper. No light is 100% safe as no paper is made the same so the fault might occur in the paper as well. But we all need to consider that we are working in a darkroom with paper which is 6 iso or lower so it would take a pretty substantial amount of tail radiation in the yellow or green to fog it, or an unreasonable amount of time under the safelights. I wasn't meant to replace a truly scientific experiment, but combining the data sheets of red leds (all of which indicate minimum wavelengths emitted still in the reds meaning the yellow radiation is a lot lower in energy), my previous paper tests and layman's observations with diffraction gratings I am just deducting I am reasonably safe.


[1] https://en.wikipedia.org/wiki/Color

[2] https://arxiv.org/ftp/arxiv/papers/1204/1204.6072.pdf