With this optical technique, one measures the difference in light absorption of the species of interest between a maximum and minimum in the spectrum. The technique most often uses the visible or ultraviolet regions of the spectrum. A broadband continuous white light source, generally a high-pressure Xe lamp or incandescent quartz-iodine lamp, is used for this purpose, but light from the sun and the moon have also been used. Differential optical absorption has been used in the atmosphere to detect gas molecules such as ozone, sulfur dioxide, nitrogen dioxide, nitrate radical (NO3), and hydroxyl radical (OH), with sensitivities ranging from the low parts-per-trillion (ppt) level to several hundred ppt.