Light therapy became popularised in the early 20th century as part of a healthy lifestyle. During the late 19th century, a range of epidemics spread through the cramped and dark living conditions of much of the population.
Sun lamps like this were designed to give patients access to healthy ultraviolet light from the comfort of their own home. They would simply sit in front of the lamp for a period of three minutes at a time in hopes of achieving the promised health benefits, which were thought to include better sleep, clearing up of acne and treatment of rickets.
Interestingly, at a time when many of the dangers of UV light were not fully understood, it was known to cause eye damage and goggles were sold with the lamp to protect patients.