text: The White cell was first described in 1942 by John U. White in his paper Long Optical Paths of Large Aperture, and was a significant improvement over previous long path spectroscopic measurement techniques. A White cell is constructed using three spherical, concave mirrors having the same radius of curvature. The mirrors are separated by a distance equal to their radii of curvature.   The animation on the right shows a White Cell in which a beam makes eight reflective passes or traversals. The number of traversals can be changed quite easily by making slight rotational adjustments to either M2 or M3; however, the total number of traversals must always occur in multiples of four. The entering and exiting beams do not change position as traversals are added or removed, while the total number of traversals can be increased many times without changing the volume of the cell, and therefore the total optical path length can be made large compared to the volume of the sample under test. The spots from various passes can overlap on mirrors M2 and M3 but must be distinct on mirror M1.  If the input beam is focused in the plane of M1, then each round trip will also be focused in this plane.  The tighter the focus, the more nonoverlapping spots there can be on M1 and thus the higher the maximum pathlength.
  At present the White cell is still the most commonly used multipass cell and provides many advantages. For example,
  * The number of traversals is easily controlled
  * It allows for high numerical aperture
  * It is reasonably stable (but not as stable as the Herriott cell)
  White cells are available with path lengths ranging from less than a meter to many hundreds of meters.