At the time, lupus vulgaris, a form of cutaneous tuberculosis, often led to facial disfigurement and even blindness. Finsen built devices that filtered sunlight or carbon-arc lamps to concentrate short wavelengths with strong bactericidal power, mainly ultraviolet rays. He evaluated the therapy statistically, documenting ulcer shrinkage and improved scar formation in hundreds of cases. The effect was explained by direct bacterial destruction and improved local blood flow that boosted immunity. Finsen founded the Medical Light Institute in Copenhagen, where many physicians learned the technique. His phototherapy became the forerunner of modern UV sterilization and artificial sunbathing, laying a cornerstone of dermatology.

Finsen’s work applied contemporary optical physics, combining wavelength-selective filters and condenser lenses to deliver high-intensity radiation to lesions. He demonstrated that ultraviolet light in the 300–400 nm band possesses strong antimicrobial activity, and he used glass-and-water filters to tame the carbon-arc spectrum while avoiding thermal burns. Clinically, he established a protocol of 30-minute exposures several times per week for 4–6 months, achieving significantly higher cure rates than control groups. He also proposed that phototherapy enhances local blood flow and oxidative stress, thereby activating macrophages. His achievements fostered interdisciplinary debates with heliotherapy and emerging radium therapy, accelerating the rise of “physical medicine,” where physical energies are repurposed for therapy. Modern UV disinfection, balneotherapy, and photodynamic therapy (PDT) all trace conceptual roots to Finsen’s framework.

Finsen employed a carbon-arc source (~15 A, 55 V) and engineered a double-condenser system delivering ~10 W cm⁻² within a 5 mm spot, while a 0.5 mm water layer and cobalt glass filter attenuated IR (>700 nm). Histological biopsies taken pre- and post-treatment showed granulomatous necrosis with subsequent epithelial regeneration, correlating with Mycobacterium tuberculosis colony reduction in vitro under matched fluence (~500 J cm⁻²). He introduced sequential irradiance escalation governed by the erythema dose, foreshadowing the modern minimal erythema dose concept. Data from 804 cases were scrutinized using early biostatistics, revealing 55 % complete remission compared with 15 % after conventional curettage. Finsen’s mechanistic hypothesis of photochemically induced reactive oxygen species anticipated current photodynamic concepts, albeit without exogenous photosensitizers. His device, detailed in "Om Anvendelse af koncentreret Lys ved Sygdomsbehandling" (1896), inspired subsequent mercury-arc and xenon flash systems. Institute records document dissemination to Berlin, Paris, and St Petersburg, influencing dermatologic oncology protocols until chemotherapeutics emerged. Finsen’s oeuvre thus exemplifies pioneering translational research intertwining optical physics, microbiology, and clinical medicine.