Abstract: The reason of using yellow lights as fog lights is that it has better penetration power than any other single color lights. The wavelength range of yellow light is about 15nm. A method to generate man-made fog is presented. The reason that dry ice is not suitable for making the man-made fog is described. The factors for selecting the wavelength ranges to be measured are considered. The experiment equipment is introduced, with six key issues stressed, radiant power of light source should be higher, width of the I/O slits of the monochromator should be reasonable and both of them should be adjusted to an equal width, the focal length of the convergent lens should be longer, the exit of humidistat should be square, the function of vacuum pump and fog collecting hood is important, and the wavelength adjustment of the monochromator and the auto-record of the measuring signal should be controlled by a micro-computer. It is found that the optimal wavelength of LED taken as fog lights is 578nm.