Photochromic Blast from the Past

While digging through some old ACS party favors, I came across this wallet card, which I obtained at an ACS "Kids in Chemistry" event ca. 2001.  A check-in with Mr. Google brings back this 2004 Celebrating Chemistry outreach pamphlet, where the PULS (Photochromic Ultraviolet Light Sensor) card helps budding chemists understand the virtue of UV-blocking sunscreens.

What's missing for us big kids? An explanation of the inner workings of the cards' photochromic dye!

Does anyone with long-toothed ACS volunteer experience happen to know the answer? When did these cards hit the scene? Who manufactured them? What dyes were used? Are they still in use for outreach today?