How do LEGO ghosts glow?

One of my favorite parts of having my ghost infested castle (see previous post), is that for several minutes after turning the lights off, there is a green glow coming from the top of my dresser.  Being a nerd, I was already somewhat familiar with the idea of phosphorescence, but I thought it would be educational to learn a little bit more about it.



The material in the ghosts that give them their ability to glow is (likely) some form of strontium aluminate (SrAl, SrAl₂O₄, or Sr₂Al₂O₅) mixed with a rare earth element.  This compound forms a crystal lattice within the plastic that absorbs light that hits it.  All light contains energy (E=hv) and so when this crystal lattice absorbs the light (the blue arrow) it is now has a higher energy state than it previously did (S₂).

Many materials are capable of absorbing light in this way but most of them re-radiate the light in a very short amount of time (on the order of nanoseconds).  This is known as fluorescence (the longer green arrow).  Because it re-radiates in such a short amount of time, as soon as you turn the light source off, the glow stops.

Glow in the dark products contain compounds called phosphors (strontium aluminate being the one often used in toys).  What makes phosphors unique is that they can hold onto the energy for a while before releasing it again.  This is accomplished through an unusual quantum state change (the triplet states).  Because this state change is unusual, it happens at a slower rate.  This is what gives phosphorescent materials their ability to glow for several minutes once the lights are turned off (the shorter green arrow).

In order to make your ghosts really glow, you want to bombard them with high energy photons.  Remembering that E=hv (where v is frequency), that tells us that the higher the frequency, the more energy will be imparted.  One surefire way to bombard your ghosts with lots of radiation is to take them out in the sun.  That should saturate all the phosphors in the plastic and provide a nice long afterglow.  The problem is, when the sun is up there are not a lot of dark rooms in which to play with your glowing ghosts.

The next best solution is to bring the light source indoors.  An overhead light works, but the light is usually diffuse and incandescent bulbs tend to be shifted toward the lower end of the frequency spectrum (yellow).

Using an incandescent flashlight is better since the light is directed but it still tends to be on the yellow side and hence lower frequency/lower energy.  LED flashlights offer a better alternative as they tend to be while light which means they contain most of the visible light spectrum. This includes the blues and purples which have the highest energy of the visible light.

However, if you really want to get your ghosts glowing, you need to harness the most powerful rays of the sun.  If you've every looked at a bottle of sunscreen you've seen the terms UVA and UVB.  These refer to the most powerful rays from the sun that make it to the surface of the earth, Ultra-Violet.  These are the rays that burn your skin but also impart the most energy to glow in the dark objects.  So how do you bring the UV indoors?  The best way to do this is through the use of a black light.  Black lights radiate at the edge of the purple spectrum and into the Ultra-Violet.

Below is  a comparison of  pictures using an LED flashlight (left) and a black light flashlight (right).  In both cases I bombarded the ghosts for several seconds then took the picture right as I turned off the light.  You can see how much better glow the black light gets. 


Lastly, here is a picture of the whole castle after shining the black light over all the ghosts.  There are only about 25 visible from the front.  The other 25 are behind the castle wall.

Hope you learned something!