On a certain museum display

Janitor

Okay, so this is really two questions, one having to do with electrical engineering, and the other having to do with materials processing (a name I just made up).

I was at a museum, and I saw on display a rotating platform that held a slab of clear glass. Inside the glass there was a milky 3D image of a steam locomotive. The locomotive pattern had the look of etched glass, but heretofore I have only seen etching on the surface of glass, and this pattern was most definitely in the interior of the block of glass. There were no seams in the glass. As the glass rotated, the locomotive lit up in four colors: red, yellow, green, and blue. A museum worker lifted the glass up and let me look at the clear platform that it sat on. Under the rotating platform there were four fixed LEDs, shining in the aforesaid four colors.

So the EE question is: when and how did they make blue LEDs? I can remember red and green being common when I was young, and then they came out with yellow (a.k.a. amber), which if I recall somehow combines the red and green technology to put out a mixture of light that looks yellow. But blue LEDs are something new to me.

Second question: how do you etch (if that is even the right term for it) the interior of a chunk of glass?

SGT

Blue LEDs exist for more than a decade. See here.
About the glass etching I don't know. Is it possible that the locomotive you saw was a holography?

Averagesupernova

Make a laser beam come into focus inside the glass block.

Janitor

SGT's link says that gallium nitride is the material needed to make the blue LED. Thanks for that link.

Averagesupernova, are you speculating, or have you heard that this is how it is done? If is is some sort of heating effect, what is happening to the heated glass that makes it form opaque spots at the point of focus? Opacity due to surface etching is presumably due to roughening of the glass surface by a strong acid, such that the glass is no longer a nice flat plane. I am not seeing how internal roughening would result from application of heat. Do you have a link on this topic?

Janitor

Not if my understanding of holography is correct. The image remained visible to the eye as a white-colored 3D pattern even when viewing the glass under ordinary room lighting.

SGT

I think that with red, green and blue laser diodes you could obtain a white holographic image. Of course there must be a focusing system to achieve that.

Janitor

I note that these were just four ordinary LEDs with their hemispherical plastic "lenses" pointing straight up, such that at any given time the locomotive might be catching only one or two colors. If you were to stop the rotation of the platform, the front of the locomotive might be green while the rear of the locomotive might be yellow, with the red and blue lights missing the pattern and passing through the block of clear glass without scattering. The base/platform part of the display looked like something you could make at home for no more than $10 in parts. It was not at all what you would call high-tech. The pattern inside the glass, on the other hand, looked like something that a home hobbyist would have no chance of being able to make.

Xodar

Averagesupernova was correct about the use of laser. They are used to fracture the glass at the focal point... check http://www.bathsheba.com/crystal/process/
This is being done a lot now for souvenirs.