LED bulbs for alt process darkroom - what's safe to use?

[Jeff Bannow]

I'm not really keen on buying incandescent bulbs for my newly set up alt process darkroom, but I understand that some LED light bulbs emit UV light as well as visible light. Anyone have a LED bulb recommendation that they have tested as safe?

The plan is for 4 60w equivalent bulbs mounted on the ceiling. These room lights will be on during the entire process. My process at this time will be mostly palladium.

 

[David Knierim]

The short answer is you'll be fine - especially with the cheaper low-CRI (Color Rendering Index) LED bulbs.

Most white LED bulbs are made from a blue LED and yellow phosphor. The blue LED will be either 445 or 455nm center-wavelength. Warm-white (2700 to 4000K color temperature) bulbs are more likely to use 455nm, which is better. Iron-based photographic processes are sensitive to about 430nm and a tiny bit above, so not strictly just UV. Blue LEDs, especially the 445nm ones, emit a tiny bit of light below 430nm. Incandescent bulbs also emit a tiny bit of light below 430nm, and even some below 400nm (true UV).

I've read proposals to make very-high-CRI LED bulbs using UV LEDs and broad-spectrum phosphors. I haven't come across any products using this technique, however. It would be less efficient than the standard blue LED designs. So far, it looks like high CRI LED bulbs add a red-orange LED besides the blue LED and phosphors. That kind would still be fine for a darkroom.

BTW, 395nm UV LEDs are becoming more efficient and cheaper. They make great exposure light sources.

David Knierim