Visible dye penetrant is typically red and uses a white developer to draw the dye back to the surface from inside the discontinuities by 'wicking' or capillary action; this is often referred to as the 'colour contrast' method. It is highly visible under natural light and may be used on a variety of materials. It is generally not as sensitive to very small discontinuities as the fluorescent method. However, it does not require special lighting conditions, therefore crews do not have to carry UV-A Lamps, blackout shades or power generators.