Phosphor / Notes

Field notes

Three things we read while working, written up plainly, each with the plate it came from. They are here because the afterglow is not a metaphor we invented for a website: it is a measured property of a material, and the people who measured it wrote down what they saw.

Field note 01

The cross that left a shadow

In 1869 William Hittorf noticed that something leaving the cathode of a vacuum tube travelled in straight lines, and he knew it because it cast a hard shadow. Put a Maltese cross in the beam's path and its shape is left on the glowing wall behind it: dark where the cross blocked the beam, bright where it did not. Ten years later the engraving beside this text printed the demonstration in Popular Science Monthly.

The reason the picture stays for a moment after the beam has moved on is that the wall is a phosphor: it absorbs the energy and gives it back slowly. That is the whole mechanism of this studio's name, and it is 150 years old.

1879 engraving of a Crookes tube with a Maltese cross casting a shadow on the glowing wall.

Field note 02

Why the blip stays

A radar screen shows you where an aircraft was, not only where it is. The beam sweeps a circle once a second or so, and in the time between two passes the screen would be empty if the phosphor let go immediately. So the phosphor is chosen to hold: the blip dims between sweeps but does not disappear, and the operator can see the track rather than a single dot.

That is why our ribbon lights squares and lets them decay instead of switching them on and off. A list that shows you only what is happening now tells you nothing about the last ten seconds.

A phosphorescent object glowing in the dark after the light has been switched off.

Field note 03

The forbidden transition

Phosphorescence comes in two speeds. The fast one, triplet phosphorescence, lasts from microseconds to about a second, and it is slow only because quantum mechanics forbids the transition: the energy gets locked in the spin of the electrons and can only come back out by a route that is not supposed to be allowed. The slow one, persistent phosphorescence, lasts seconds to hours, because an electron is caught in a defect in the crystal lattice, like a pitfall, and waits for a random spike of heat to let it go.

Slow work is not automatically good work. But when something takes longer than expected it is worth asking whether it is stuck or whether it is holding, and those two look identical from the outside.

Light from a struck quartz crystal, photographed in the dark.

Facts from Wikipedia's articles on phosphorescence, phosphors and the cathode-ray tube, all CC BY-SA. Images from Wikimedia Commons, each with its licence in the credits file.