This particular award has sometimes been viewed with controversy. It was actually closer to the general terms of Nobel's will (http://nobelprize.org/nobel/alfred-nobel/biographical/will/index.html) than most awards in physics. Dalén made developments in acetylene chemistry to get a very bright light, developed safe storage methods, and then engineered a special valve that was controlled by the sun, so that the resulting buoys would only operate at night, prolonging their life to about a year. To a rugged coastal area like Scandinavia, these mass-produced long-lived minimal maintenance buoys were a significant boon to safety and livelihood.
Dalén was blinded in a gas explosion accident earlier the same year as he was awarded the prize. This has led to speculation that he was selected partly out of sympathy.