After the infrared rays irradiate the body surface, part of it is reflected, and the other part is absorbed by the skin. The degree of skin's reflection of infrared rays is related to the condition of pigmentation. When irradiated with infrared rays with a wavelength of 0.9 microns, the skin without pigmentation reflects about 60% of its energy; while the skin with pigmentation reflects about 40% of its energy. When long-wave infrared (wavelength above 1.5 microns) is irradiated, most of it is reflected and absorbed by the superficial skin tissue, and the depth of penetration of the skin is only 0.05-2 mm, so it can only act on the superficial tissue of the skin; short-wave infrared (wavelength (within 1.5 microns) and the near-infrared part of red light penetrate the deepest tissue, with a penetration depth of up to 10 mm, which can directly affect the blood vessels, lymphatic vessels, nerve endings and other subcutaneous tissues of the skin.