I'm reposting this here from where I originally posted it in the beginner's forum, I figure it might help here in the GD too.
I've seen a number of posts lately asking about different types of lume on watches. I hear a lot about very little, some good information, some bad - I'd like to try to clear that up a bit for everyone.
There are three "umbrella categories" that lume can fall into:
1. Traditional paint
2. Tritium (often called tubes, H3 or gas lights, sometimes called capsules)
Traditional paint based lume is by far the most common type. It's applied like a paint, because that's what it is. The paint contains a photoluminescent compound that absorbs light and then gives it off over a period of time. The quality of the paint, application, and the thickness of application (among other things) all have an effect on the resulting product, as you'll likely see below.
The more layers of luminous compound that are applied (thicker application), the more luminescent material there is to absorb and emit light. The greater the density of the luminescent material there is in the paint, the greater it's ability to absorb and emit light. See where this is going? Some companies just throw some on the dials of their watches so that they can say they glow in the dark (Armitron, Invicta, Fossil), while others really take pride in maximizing the low-light legibility of their watches (Lüm-Tec, Seiko, and many high-end brands). Lüm-Tec especially goes the extra mile when it comes to the application of their own luminous compound, which is helping them build a serious reputation, especially around here. Many dive watches also have very good lume, a near requirement for working at depths where the sun doesn't penetrate. Luminescent paints require charging by visible and/or ultraviolet light to gain the necessary energy to glow.
Tritium is a gaseous isotope of Hydrogen that is very slightly radioactive and has a fairly long half-life (approximately 12 years), making it a fairly safe substance to work with. It is an excellent substance to use in illumination, as when phosphors are placed in contact with the gas they will glow. Tritium is sealed inside phosphor-coated tubes, which will then glow continuously for a very very long time. This method requires no external charging source at all - ever. Yes, the tubes will only be half as bright as they started after 12 years, and a quarter as bright after 24 years and so on, but they are fairly cheap to replace once a decade - not much more than having a dial re-lumed with paint. Tubes don't give off the same initial "WOW" glow that paints do, but they make up for it by going the distance as you'll see below.
Incandescent bulbs, LEDs, Electroluminescent film, LCDs, OLEDs. These devices require an electric power source in order to work. We're all familiar with them, in fact you're using at least one of them to read this message, probably more than one. By far the brightest option for illumination out there (hey, I used an LED flashlight to charge my lume for the pictures below, and I've never seen a tritium or Superluminova-based computer display, nor a home lighting system based on them), but at the expense of requiring a battery for operation. Timex has an electric system they call "Indiglow" and Casio uses one they call "Illuminator". They glow bright, but only after the system is activated. Casio builds an automatic switch into many of the watches in their G-Shock line that comes on when you move your wrist to check the time. As long as you've got battery power, you've got a display that can be read at night.
Now on to the pictures:
I have seven watches lined up for this demonstration, listed as numbered.
1. Armitron Star Trek Collector's watch (no-name lume paint)
2. Invicta 8926C (no-name lume paint)
3. Omega Seamaster Professional (SuperLuminova)
4. Lüm-Tec Combat B6 (Lüm-Tec G3, Green)
5. Luminox Titanium Nighthawk Chrono (Tritium)
6. Lum-Tec M12 (Lüm-Tec G3, Blue)
7. Sitting on top of all of them is my Casio G-Shock G9000-8V Mudman (Electroluminescent display)
I charged them all for approximately the same amount of time immediately before the first picture was taken (less than 5 seconds), and I pressed the button on the G-Shock before each photo was taken. No further light was used to charge the watches after the first photo was taken. There is a small amount of ambient light in the room, however it isn't intense enough to have any charging effect on the watches - it's plenty dark in here, trust me. 5 second exposure is WELL beyond what your eyes can see.
Camera (Nikon D90) was set to manual, f7.1 with 5 second exposure for all pictures. Photos were taken at roughly 15 minute intervals.
Right after charging
+15 minutes - at this point all of the watches except the g-shock and the luminox are too dark to read without giving my eyes time to adjust (and I'm only 23 with great vision!)
+60 minutes - The Lüm-Tec M12 isn't visible at all to the naked eye, despite what the pictures show
+75 minutes - last photo because I want to go to bed. Lüm-Tec B6 is still fairly visible, Omega SMP still visible. Luminox is every bit as bright as when the test started, and the G-Shock makes me squint. Luminox appears brighter to the naked eye than the Lüm-Tec B6.
Well there you have it, a reasonably comprehensive comparison of all the different methods (that I'm aware of) of low-light dial legibility on the market today.
I don't want to make a judgement on which is "best", because they're all so different.
Electric is the brightest, that's for sure, and can be had for CHEAP - $10 Wal-Mart Timex's have Indiglow. Tritium has some serious staying power - it'll glow all night (and all day for that matter!) without any noticable decrease in illumination, but it doesn't have that knock-your-socks-off initial glow of paint. Luminous paints can have very bright initial glow, but can (and often do) fade to near invisibility over the course of an hour or two, and the luminosity of the compounds varies wildly from formula to formula. Paints also take some craftsmanship and skill to apply, and it's pretty tough to do correctly from what I've heard.
I hope that this helps people who are concerned with what's what when it comes to lume!