The M-lifetime of bacteriorhodopsin films for optical recording is a key parameter in obtaining a high light sensitivity, a high contrast ratio, and a high contrast decay time. An increase of the M-lifetime causes a proportional reduction of the light intensity required for optical applications. In bacteriorhodopsin variants such as BR-D96N, the M-lifetime can be tuned over several orders of magnitude by simply changing the pH value with respect to the proton availability in the matrix of the films. At low humidities, the proton transport steps linked to the photocycle limit the overall kinetics. A proton-diffusion-limited, two-state model (PDL2 model) for bacteriorhodopsin is introduced which allows us to model mathematically the optical excitation and thermal relaxation processes for both high and low humidities in bacteriorhodopsin films. Films containing wildtype bacteriorhodopsin and the variant D96N are compared in dependence on the pH value and the relative humidity at 20 degrees C. Of the investigated materials, only BR films containing BR D96N can be used for recording at low light levels of 100 mu W/cm(2). In a holographic interferometry experiment-a typical application where a high light sensitivity is a key issue-it is demonstrated to what high extent the water content in the films affects their suitability for recording at low light levels. Kinetically optimized bacteriorhodopsin films yield a more-than-30-fold improvement of sensitivity in holographic interferometry compared to dry bacteriorhodopsin films.