Far infrared named infrared ray, it is an electromagnetic wave whose wavelength is between visible light and microwave. Because it is located in outside the red edge of the parameter, so we called it infrared ray .we can not see it in our vision. According to the wavelength ,we divide the infrared ray into three parts ,the close infrared ray( the wavelength is 0.76-1.4ｕm ),the medium infrared ray (the wavelength is 1.4-3.0ｕm)and the far infrared ray(the wavelength is 3.0-1000ｕm).so the far infrared refers to the electromagnetic wave whose wavelength is  3.0-1000ｕm. It has the functions of reflection and refraction as same as the ultraviolet ray, visible light and microwave. But it is best to heat at the wave band of far infrared.