My apologies for the provocative-sounding title; I just couldn’t help myself. The exposure being referred to is sunlight. The prevalence of rickets increased after the advent of the Industrial Revolution but it was not until the early 20th century that the connection was made between this disease and a deficiency in Vitamin D. It was also learned that the human body produces this nutrient when exposed to adequate sunlight, specifically ultraviolet B (UVB) radiation. Although it is common practice to simply provide supplements in pill form, some people are unable to metabolize this nutrient in this form and sunlamps are sometimes needed. This technique was quite popular in middle of the 20th century, especially for children of more affluent parents or in a clinic where multiple children could be treated concurrently.
A reader provided a link to a photographic collection featuring images of this sunlight treatment. Browsing the images, one can see that these examples come from many countries in more northerly latitudes such as: the Soviet Union, Germany, UK, Canada and the US. Undoubtedly, other northern countries were also represented. I would recommend perusing the many examples that can be found with the above link including vintage advertisements for the lamps and treatment.