I forget what they were called but there are crystals that use a small radioactive source encased in phosphor doped glass. The radiation excites the phosphors and they emit a dim but visible light for a very long time.

Used to be the light source of choice for elite special forces types. The advantages where that the solid chunk of glass was pretty much indestructible and the half-life of the radioactive source was measured in decades. Useful output was said to be beyond the lifespan of an average human.

The problem was that these units were expensive and their output was minimal. With very good and young eyes you could read a large-print book without too much trouble. The light output was all-round and couldn't be turned off. To eliminate the light you stuffed it back into a pouch. With practice you could make your way down a path holding the light source in the palm and facing the palm away from you. There was just enough light to keep from stumbling over a log, getting clotheslined by a vine or stepping off a cliff. You could make out shapes at six or eight feet if your eyes were good.

Back in the 60s these units were a big deal. Having one was considered in some circles to be the mark of an elite operator. Those were when batteries were less efficient and bulbs used a lot of power. These units were supposed to be issued to people who had to go very long times between resupply. People who lived out of a pack for weeks, sometimes months.

They were never much more than marginal for doing anything. But with one of these you always had light and for some people a very dim light was better for not attracting attention.

Technology and times have changed. Batteries have improved a lot. LEDs produce light much more efficiently than the old incandescent bulbs. A couple of high capacity lithium batteries connected to a low-draw LED will run for months and produce far more light. Also people are far more leery about toting around radiation sources. Even ones that are encased in thick glass that acts as a shield.