Red light is visible to the eye. We  use 660nm wavelength for all of our devices. It is a shorter wavelength, widely used in research and clinical settings. This wavelength is short which means it hits the surface of the skin and penetrates about 1-2mm below the skin's surface. Meanwhile, Near InfraRed (NIR) light is nearly invisible to the naked eye, and it penetrates deeper below the skin surface, 2 to 3mm. Rouge lights are always balanced with half our lights at 660nm and the other half at 850nm.