At present, there are two kinds of light sources used and contacted by human beings, namely artificial light sources (including lighting, electro-optic and industrial light) and natural light sources, namely sunlight. The latter is an important light source for human survival and life. Sunlight is not only a light source but also brings warmth to the world, so it is also a heat source. But also because of its photosynthesis, plant chlorophyll uses this function to synthesize water and carbon dioxide into carbohydrates, forming one of the important nutrients for human beings.

In addition, ultraviolet rays in the sun can promote the body to produce endorphins (natural opium), which is also the reason why sunbathing makes people feel comfortable and happy. Ultraviolet rays can also convert 7- dehydrocholesterol contained in skin into vitamin D, which plays an important role in children's bone growth and maintaining normal human bone structure. Therefore, it is of great benefit to bask in the sun regularly and properly in winter.

However, sunlight also has a negative side to human body. The skin contains a Langerhans cell, which has skin immune function. But it is sensitive to ultraviolet rays. If the skin receives excessive ultraviolet rays. It can inhibit cellular immune function, damage skin cells and make them age. Too much sunlight can also make people with genetic qualities suffer from kidney cancer, which is more likely to occur in whites. So, be careful when sunbathing.

In addition, skin exposed to intense sunlight for a long time can also cause photosensitive dermatitis, commonly known as sun erythema. However, due to the release of some prostaglandin substances from the skin after irradiation, local capillaries dilate and appear erythema, and even some epidermal cells appear necrosis, blisters and desquamation, and the skin is white and tender in the sun, or when ultraviolet physiotherapy is excessive or allergic, this phenomenon is more common.

In summer and autumn, it is also common for people to eat plant porphyrins such as small root vegetables, grey vegetables, amaranth, Sophora japonica or alfalfa. This is because these plants contain photosensitive porphyrin substances. Due to the differences of people's individual qualities, under the irradiation of sunlight, the skin will be flushed and swollen, and then the skin will be peeled off and eroded.

Cosmetic dermatitis is also a common photosensitive skin disease in women and literary and art circles, and it is more common to use some fluorescent substances. Through the interaction of sunlight, oxygen and dyes, "cytotoxic substances" are formed, which damage skin cells and cause erythema, swelling, papules or necrosis. Such cosmetics often contain citrus oil, lemon oil, sandalwood oil or fluorescent raw materials.

Drug-induced dermatitis is a strong light-induced reaction caused by people's external use of some drugs or oral administration of drugs such as hibernating spirit, chlortetracycline, amiodarone and psoralea corylifolia, which is more common in daily life. It is characterized by skin bleeding, flushing, edema, blistering or peeling.

In a word, the basic function of sunlight is beneficial and indispensable to human health and even the survival of human life. But it also has its negative side, which should be paid attention to.