OK. All I know is that the jumbo bulb is safe. I have used it for lith printing about 3 feet above my trays. Some of those prints take 30 minutes to process before I put it in the stop bath. No fog. Zero. None.
It's the only safelight I have ever used. I imagine since it's an item that is replaced once it burns out, the 'filter' that's built into the bulb is replaced at the same time. I can see how the filters used with regular incandescent bulbs or fluorescent tubes could fade with time and become unsafe.
- Thomas
See, this is where people are missing the point of LEDs. They do not have and do not require a filter; because of the nature of how they work, they produce a very very narrow spectrum. An incandescent bulb is the opposite, in that it produces broad-spectrum (black-body) radiation, which includes significant components at higher frequencies that must be filtered, and no filtering is perfect. Filters also typically degrade when they're absorbing photons, hence their replacement schedule. Without a filter, the LED will keep working (and not fogging paper) practically forever - their half-brightness lifetime is often on the order of 50,000 hours (they will be half as bright after 6 years of 24x7 use).
An LED works by exciting atoms in a semiconductor (raising an electron to a higher energy level) and then when the electron falls back down to its default level, a photon is emitted. The wavelength of a photon is defined by the energy that went into it, therefore specifying what the LED is made of (specifically, the differences between energy levels in the material) will specify the wavelength(s) produced.
It takes a certain minimum amount of energy in one photon to activate a silver halide molecule and form a latent image - any photon of insufficient energy (too red) will just get absorbed as heat or be reflected. It's practically the same process as what happens in an LED but in reverse.
Your garden-variety red LED has a particular band-gap between its electron energy levels. This energy is smaller than the activation energy of a silver halide molecule in photographic paper[1], therefore none of the photons from a red LED can fog the paper. You could leave the LED on in your paper safe for a week and the paper wouldn't fog.
If you filter an incandescent light, you might lose 50% of the red photons and get rid of 99.9% of the blue/green/UV ones. But that just means that fogging is 1000x slower than it would be with an unexposed bulb, which is still not particularly slow.
In terms of a solution for a non-electronics-nerd, a $20 off-the-shelf LED safelight is an easy and reliable option. It will probably never wear out in your lifetime. I also like the bike-light idea - they're cheap and ubiquitous, though buying a new set of AAs every 30 hours will add up - nothing like paper costs, but certainly more than $20 for the mains-powered one over the years.
[1] panchromatic film adds sensitising agents (don't ask me the chemistry, I don't know) that make them capable of being activated by photons of lower energy / longer wavelength, i.e. more red. Likewise IR-sensitive films have other sensitising agents that allow them to react to photons of even lower energy still.
. They really don't practically produce wavelengths that would need to be filtered out. They are not perfect at all but every time I've tested leds with BW paper, I get much lower fog than with any commercial darkroom light I've tried. This may be because of filter fade present in the old units.
