The visible spectrum of light can be broken down into definite colour bands, each of which has a wavelength or frequency, measured in nanometers ( 0.000000001mtr), and this frequency is to be found between the values 400 to 700 nm

 Welding processes generate radiation hazards for the operator at both ends of the visible spectrum, and it is the function of the welding helmet filter to reduce these harmful UV & IR radiations

UV - ultra violet radiation, whose wave form is to be found below 400nm is the main culprit for the condition generally referred to as "arc eye", and although this condition is fortunately in most cases only a temporary result of excessive UV exposure, it can cause sharp "sand in the eye" pain for 12 to 24 hours thereafter.

IR - Infra Red radiation is perceptible normally as heat, however, this is normally a local source to the arc, and can generally be contained or tolerated in most welding styles.

These recommendations are minimum shade scales, as defined in BS EN 169 BS EN 379, however, the operator may for personal preference reasons prefer to weld on a lighter or darker setting