A manual device is useful for one-time set up of a system, and is a near-equivalent to a fixed attenuator, and may be referred to as an "adjustable attenuator". An optical attenuator, or fiber optic attenuator, is a device used to reduce the power level of an optical signal, either in free space or in an optical fiber. Key requirements include minimal effect on the beam profile, low wavelength and polarization dependence, and sufficient power handling capability. The intensity of the signal is described in decibels over a specific distance the signal travels.