Explain about Light therapy or Photo therapy.

Light therapy or phototherapy (classically referred to as heliotherapy) consists of exposure to daylight or to specific wavelengths of light using polychromatic polarised light, lasers, light-emitting diodes, fluorescent lamps, dichroic lamps or very bright, full-spectrum light. The light is administered for a prescribed amount of time and, in some cases, at a specific time of day.

One common use of the term is associated with the treatment of skin disorders, chiefly psoriasis, acne vulgaris, eczema and neonatal jaundice.

Light therapy which strikes the retina of the eyes is used to treat circadian rhythm disorders such as delayed sleep phase disorder and can also be used to treat seasonal affective disorder, with some support for its use also with non-seasonal psychiatric disorders.

Medical uses

Skin conditions
The treatments involve exposing the skin to ultraviolet light. The exposures can be to small area of the skin or over the whole body surface like in a tanning bed. The most common treatment is with narrowband UVB (NB-UVB) of 311-313 nanometer. It was found that this is the safest treatment wavelength. Full body phototherapy can be delivered at doctor's office or at home using a large high power UVB booth.

Psoriasis
In psoriasis, UVB phototherapy has been shown to be effective.A feature of psoriasis is localized inflammation mediated by the immune system.Ultraviolet radiation is known to suppress the immune system and reduce inflammatory responses. Light therapy for skin conditions like psoriasis usually use NB-UVB (311 nm wavelength) though may use UV-A (315–400 nm wavelength) or UV-B (280–315 nm wavelength) light waves. UV-A, combined with psoralen, a drug taken orally, is known as PUVA treatment. In UVB phototherapy the exposure time is very short (seconds to minutes depending on intensity of lamps and the person's skin sensitivity). The time is controlled with a timer that turns off the lamps after the treatment time ends.

Vitiligo
One percent of the population suffer from vitiligo, and Narrowband UVB Phototherapy is an effective treatment. "NB-UVB phototherapy results in satisfactory repigmentation in our vitiligo patients and should be offered as a treatment option."

Acne vulgaris
High intensity blue light (425nm) used for the treatment of acne.
Evidence for light therapy and lasers in acne vulgaris as of 2012 is not sufficient to recommend them. There is moderate evidence for the efficacy of blue and blue-red light therapies in treating mild acne, but most studies are low quality. While light therapy appears to provide short term benefit, there is a lack of long term outcome data or data in those with severe acne.

Cancer
According to the American Cancer Society, there is some evidence that ultraviolet light therapy may be effective in helping treat certain kinds of skin cancer, and ultraviolet blood irradiation treatment is established for this application. However, alternative uses of light for cancer treatment – light box therapy and colored light therapy – are not supported by evidence.

Other skin conditions
Phototherapy can be effective in the treatment of eczema, atopic dermatitis, polymorphous light eruption, cutaneous T-cell lymphoma and lichen planus. Narrowband UVB lamps, 311–313 nanometer is the most common treatment.

Wound healing
Low level laser therapy has been studied as a potential treatment for chronic wounds.Reviews of the scientific literature do not support the widespread use of this technique due to inconsistent results and low research quality.Higher power lasers have also been used to close acute wounds as an alternative to stitching.

Mood and sleep related
Seasonal affective disorder

Full sunlight or exposure to bright light from a light box is used to treat seasonal affective disorder (SAD). Light boxes for SAD are designed to filter out most UV light, which can cause eye and skin damage. Mayo Clinic states that light therapy is a proven treatment for seasonal affective disorder.It is considered a first-line treatment. Controlled-trial comparisons with antidepressants show equal effectiveness, with less expense and more rapid onset of therapeutic benefit, though a minority of patients may not respond to it. Direct sunlight, reflected into the windows of a home or office by a computer-controlled mirror device called a heliostat, has also been used as a type of light therapy for the treatment of SAD.

The effectiveness of light therapy for treating SAD may be linked to the fact that light therapy makes up for lost sunlight exposure and resets the body's internal clock. Studies show that light therapy helps reduce the debilitating and depressive behaviors of SAD, such as excessive sleepiness and fatigue, which results lasting for at least 1 month. Light therapy is preferred over antidepressants in the treatment of SAD because it is a relatively safe and easy therapy.

It is possible that response to light therapy for SAD could be season dependent. Morning therapy has provided the best results because light in the early morning aids in regulating the circadian rhythm.

Non-seasonal depression
Light therapy has also been suggested in the treatment of non-seasonal depression and other psychiatric disturbances, including major depressive disorder, bipolar disorder  and postpartum depression. A meta-analysis by the Cochrane Collaboration concluded that "for patients suffering from non-seasonal depression, light therapy offers modest though promising antidepressive efficacy."A 2008 systematic review concluded that "overall, bright light therapy is an excellent candidate for inclusion into the therapeutic inventory available for the treatment of nonseasonal depression today, as adjuvant therapy to antidepressant medication, or eventually as stand-alone treatment for specific subgroups of depressed patients."A 2015 review found that supporting evidence for light therapy was weak due to serious methodological flaws.

Chronic CRSD
In the management of circadian rhythm disorders such as delayed sleep phase disorder (DSPD), the timing of light exposure is critical. For DSPD, the light must be provided to the retina as soon after spontaneous awakening as possible to achieve the desired effect, as shown by the phase response curve for light in humans. Some users have reported success with lights that turn on shortly before awakening (dawn simulation). Morning use may also be effective for non-24-hour sleep–wake disorder, while evening use is recommended for advanced sleep phase disorder.

Situational CRSD
Light therapy has been tested for individuals on shift work,and for jet lag.

Sleep disorder in Parkinson's disease
Light therapy has been trialed in treating sleep disorders experienced by patients with Parkinson's disease.