The photochromic protein bacteriorhodopsin (BR) is contained in two-dimensional crystallized form in the purple membrane (PM) fragments from the cell membrane of Halobacterium halobium [1, 2]. BR belongs to the class of retinal proteins and is related to the human visual pigment. Excitation with light induces a cyclic sequence of photoconversions which are summarized in Figure 1. Even in dried PM-films the photocycle is observed. After absorption of a photon the protein reaches approximately 50 μs later the longest living M-form, which is about 160 nm blue-shifted from the initial state B. From the M-intermediate BR can relax either thermally or photochemically to the initial B-state.