In the early 20th century, powerful acetylene gas flames were common lighthouse light sources, but burning them around the clock was extremely wasteful. Dalén combined an automatic gas-flow regulator with a light-sensitive 'sun valve' so the flame stayed off during daylight and reignited at dusk. Because the mechanism had few moving parts and superb resistance to salt corrosion, it worked for years in harsh coastal environments. Gas consumption dropped by roughly 90 percent, dramatically lowering maintenance costs for lighthouse services. At a time when electric grids rarely reached remote shores, Dalén’s system became indispensable for maritime safety. This societal impact earned him the 1912 Nobel Prize in Physics.

Dalén first addressed the safety issue of acetylene by inventing the AGA accumulator, which stores compressed acetylene in an asbestos-acetone matrix to prevent explosive decomposition. He then designed an automatic regulator equipped with a brass diaphragm and a bi-metal thermostat, achieving both stable flow and instantaneous shut-off if the flame was extinguished. His most ingenious contribution was the sun valve: a metal rod with alternating blackened and polished surfaces that expands differentially under sunlight, mechanically opening or closing the gas feed without external power. This innovation allowed lighthouse beacons to operate with a night-duty cycle of about 10 %, greatly extending mantle life and reducing fuel costs. Through AGA AB, thousands of lighthouses and buoys were automated worldwide between 1907 and the 1930s. Although Dalén was blinded by an acetylene explosion during testing, he continued to manage and innovate within the company. His concepts later evolved into photo-cell based electric controls, making him a forerunner of modern automatic lighting systems.

Dalén’s regulator combined a two-stage diaphragm reducer that lowered acetylene from 2.5–3 MPa storage pressure to 20–40 kPa service pressure with a bi-metal actuator for thermal feedback. The sun valve exploited the absorptivity contrast between black-ened (α≈0.95) and polished (α≈0.05) surfaces; incident solar radiation above 600 W m⁻² generated a >2 mm stroke that closed a poppet valve. During operation a stoichiometric 1:11 acetylene-air mixture fed a mantle burner, achieving radiant efficiencies up to 560 lm W⁻¹. The regulator responded to flow disturbances with a gain of 0.8–1.2 s⁻¹, keeping flicker within ±2 % even under 5 m wave-induced accelerations. AGA lanterns employed a scavenging pilot flame that suppressed CO formation and reduced mantle erosion rates below 1 mg h⁻¹. Dalén’s design represents a pinnacle of passive control in a pre-mechatronic era and presaged later piezo-actuated phase-shift valves and photodiode logic controls. In his 1912 Nobel lecture he presented field data showing 92 % average gas savings and service intervals of three years, data that reshaped life-cycle cost analyses for coastal agencies. The rigorous empirical validation and industrial scaling made this one of the rare cases where an engineering invention garnered the Physics Prize.