Searching for good information on the internet can be a frustrating experience at times. Either the search turns up only poor quality or the search provides only basic material.  And, if you are looking for scholarly or peer reviewed articles, they are frequently behind pay walls. It is so encouraging when you read an abstract and it looks like you have found a goldmine and it turns out to be the shaft! In the case of the pay-for articles, they are often in the range of $35 or more, and this includes material that is several years old. Of course you can always use the local library and use their inter-library access to get a photocopy. But, that can take two weeks and by then you may have moved on to greener pastures.
When I started this website I realized that I would be doing a great deal of research and as a result, I am constantly looking for good free sources of information that both I and the readers could trust. I have posted some of these sources before, but I found a really great open source location for peer reviewed journal articles with no strings. How about a free portal to almost 10,000 journal articles and nearly 1 million article level entries? And, the range of subject includes the following areas: (From the site).
Agriculture and Food Sciences
Arts and Architecture
Biology and Life Sciences
Business and Economics
Chemistry
Earth and Environmental Sciences
General Works
Health Sciences
History and Archaeology
Languages and Literatures
Law and Political Science
Mathematics and Statistics
Naval Science
Philosophy and Religion
Physics and Astronomy
Science General
Social Sciences
Technology and Engineering
The website is the Directory of Open Access Journals, DOAJ and is a treasure trove of good information. It is the kind of source that deserves support and although it is free, I plan to support it as I am able.  Please take a look here, and spend some time in this virtual library. Happy reading!

Photochromism is a chemical process that has as its primary function a transformation between species by the absorption of radiation with two forms having different absorption spectra. But for most of us, photochromic materials simply have a reversible change of color with different kinds of light. A perfect example is in eyeglasses that darken when we go into the sun. Indoors, where there is very little ultraviolet light they are clear and colorless; but, outdoors, in the presence of UV they react to the light and become progressively darker. There are many novelty items that are photochromic including nail polish colors, and T shirts to suggest as examples. Chemically, there are many classes of photochromics. Some are based on inorganic chemistry as in the eyeglass example that typically utilizes silver chloride. There are more that are organics such as the spirans and leuco dyes. There are also photochromic systems that are biological in nature as in retinal in the eye used in our vision process. Generally speaking, they are fascinating and provide interesting experiments.
I have had the opportunity to synthesize photochromics in a laboratory setting, and I have attempted to fabricate some with readily available materials at home but without success so far. But I will continue to try! But, for these experiments I bought Solar Drops ™ from a company called Solar Color Dust. Since I do not endorse products I have not included a link, but there are many products available from several companies. I purchased 6 bottles (six different colors) of 6ml each with approximately 100 drops per bottle. These are designed to be mixed with epoxy at the rate of 10 drops for each 1 ounce (28 grams) of resin. They were expensive at $20 plus shipping. I have included several photos with the results and some associated comments:

The first two photos show the epoxy with each color in both the colorless and colored form. These were mixed as instructed and poured into plastic petri dishes. Exposed to UVA (365nm).

The next two photos were deposited directly  from the bottle to show the maximum density possible. Also exposed to UVA. (about 3 minutes).

The last two photos show an attempt to use the material as an imaging system and a microscope image explained in the final comment.

I believe that the photochromic used in this product is based on an encapsulated leuco dye. The microscope image shows the dye distributed in the epoxy. But, this is my best guess and not a certainty. Although I found the process interesting, I have no real desire to go further as the rate of change from colored to colorless is very fast and the density is not very good. The image was difficult to do and the result was less than I had hoped for. One way of extending the reversion is to chill the image in the refrigerator and slow down the rate of change. But, for the inquiring mind, it is worth the effort. Color me gone...