Lontano,

You indeed raise an interesting question which I have never heard raised before. My first, and easy answer would be the movie Das Boot depicts this, and we know that Lehmann Willenbrock was a technical advisor on the production. Given the level of detail throughout the film, and accuracy of the movie in general with other sources such as this site, I am inclined to believe Willenbrock had a hand in the lighting inside the boat during shooting the movie. This would be the only ""source" I can cite regarding this. While not authoritative, I am inclined to believe it is likely historically accurate.