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UV Light therapy has been used for a very long time: First the Egyptians treated psoriasis by rubbing contents from a plant on psoriasis patients before sending them out into the sun. The plant had photosensitizing properties (meaning it makes the skin much more sensitive to UV rays from the sun). 

Modern treatment started with applying psoralen (ingredient from plants; also photosensitizing) to the skin before going into a light booth. It is still used today, but, whilst very effective, is now for more selected cases as it does come with risk of skin cancer. Then followed broadband UVB treatments, that were safer, but still had risk. Eventually we settled on narrowband UVB (Ultraviolet B) therapy. In these booths, you are exposed to ultraviolet light B, but a very specific small narrow subset of it. This wavelength is not only more effective, but also much safer overall. You can say we have cleaned up the sun by removing the worst of it and keeping it's beneficial properties!