1. R. C. Bertelson, in: Photochromism (G. H. Brown, ed.), pp. 45–431, Wiley-Interscience, New York (1971).
2. R. Guglielmetti, 4n+ 2 Systems: Spiropyrans, in: Photochromism, Molecules and Systems (H. Dürr and H. Bouas-Laurent, eds.), pp. 314–466, Elsevier, Amsterdam (1990).
3. N. Y. C. Chu, in: Photochromism, Molecules and Systems (H. Diirr and H. Bouas-Laurent, eds.), pp. 493–509, Elsevier, Amsterdam (1990).
4. R. C. Bertelson, Volume 1 of this monograph, Chapter 1.
5. S. Maeda, Volume 1 of this monograph, Chapter 2.