AdLab: Sunblock Ad Changes Color When Exposed to Sun

The excellent MIT Advertising Lab blog has this example of Photochromic ink used in a DM piece:

Another example of the message being built into the medium -- a print ad for Sundown sunblock, one half of which changes color when exposed to the sun (see full creative here). If you know how they might have achieved the effect, could you please leave a comment?

Why, don't mind if I do:

Photochromic ink is a technology that I saw paraded around once a year or so when I was in Print Production. The related Thermochromic (heat-sensitive) inks are a little better known, being used in things from kids' snow boots to coffee mugs and beer cans, but photochromic has never really gotten going.
It's not cheap, especially for a DM piece that I imagine had a budget of under $1 a piece (just a WAG, but you know...).

Nice to see someone got a project out the door with this, anyway.

Adverlab article

Here's a manufacturer's page.