If your psoriasis can't be controlled by topical treatments, your healthcare provider might suggest phototherapy, or light therapy. Ultraviolet light is emitted from the sun and, while it may be invisible to the eye, it can dramatically slow the growth of skin cells, reducing the symptoms of psoriasis. Getting sun is one way of helping your psoriasis. But artificial ultraviolet lamps and lasers are a more targeted way of treating the condition.

• Sunlight. Getting sun is a simple way of helping your psoriasis, but this isn't an invitation to go bake at the beach. Too much sun results in sunburn that can worsen psoriasis, as well as increase your risk of skin cancer. If your doctor suggests that you try to get some sun each day, make sure that you carefully control the amount. Typically, 20 minutes a day should be enough.
  
  
• UVB (Ultraviolet B). While sunlight can help, artificial ultraviolet light can be used on your psoriasis plaques with more precision. UVB treatment involves exposing yourself to lights designed to emit the sort of ultraviolet rays that are most helpful for psoriasis. Usually, you would get UVB treatment at the doctor's office, but you may also do it at home using a phototherapy lamp recommended by your doctor. Typical treatment is for three to five days a week for two or three months. UVB treatment may be combined with other approaches. One psoriasis treatment, called the Goeckerman regimen, consists of UVB therapy combined with coal tar. Another psoriasis treatment, called the Ingram Regime, combines UVB and a coal tar bath with an anthralin-salicylic paste.
  
  Another approach to UVB treatment is called narrow-band UVB, in which the devices are refined to use only the wavelengths of ultraviolet B light that best help psoriasis. Because this approach is more focused, you'll need fewer sessions. As a result, experts believe that any potential risks from the rays are lower. Narrow-band UVB may also offer a safer alternative to PUVA.
  
  
• PUVA (Psoralen plus Ultraviolet A). PUVA uses a different band of ultraviolet light to treat psoriasis -- ultraviolet A -- in combination with psoralen, an oral or topical medication that makes your skin more sensitive to light. Because this approach uses medication as well as light, PUVA is sometimes called photochemotherapy. PUVA is particularly effective at clearing up severe psoriasis quickly and with long-lasting results, but it has some side effects, including nausea, exhaustion, headaches, burning and itching. Most importantly, using PUVA over a long period of time can increase your risk of skin cancer. It's generally only used in extreme cases or when other treatments haven't worked. You would usually get 25 sessions of PUVA at your doctor's office two or three times a week. Scheduling it can be inconvenient.
  
  Because psoralen drastically increases your body's sensitivity to light, it's important to protect your skin and especially your eyes after taking it. You should wear glasses that block ultraviolet light for at least 12 hours following treatment.
  
  
• Lasers. One of the newest tools used in phototherapy are lasers. Lasers emit highly focused beams of light. Their advantage is that the light affects only the psoriatic skin while healthy skin isn't exposed at all. This precision reduces the side effects of phototherapy and may lower the potential risks of developing skin cancer. Laser therapy also requires fewer treatments than conventional phototherapy.
  
  One type of laser, the XTRAC excimer laser, uses focused high-energy ultraviolet B light. Studies have shown that plaques that were treated with XTRAC showed significant improvement faster than conventional phototherapy. Typically, treatment would be done twice a week for four or five weeks.
  
  After treatment with lasers, you should try to avoid exposure to the sun and be careful not to injure the area that was treated. If any blistering appears, call your doctor.
  
  The side effects from laser treatment are generally mild, although some people feel pain. You may also have bruising and scarring at the treatment site. After treatment with lasers, you should try to avoid exposure to the sun and be careful not to injure the area that was treated. If any blistering appears, call your doctor.

Bathing in sea water in combination with sun exposure is used to treat  psoriasis, as is the use of salt water baths with artificial UV exposure (balneophototherapy).