Attenuation of light beams

Attenuation filters are used to reduce the intensity of a light beam, and ideally this attenuation is independent of the wavelength of the light. They can be divided into two main groups according to the mechanism used to reduce the beam intensity. 

This type of filter physically blocks a fraction of the profile of a light beam. If one assumes, for the sake of simplicity, that the beam is uniform in intensity across its profile, then the percentage of the beam that is blocked by an aperture device represents the percentage reduction of the beam intensity. Of course, rarely are light beams homogeneous in their spatial intensity distributions for example, a good-quality laser beam exhibits a spatial Gaussian intensity profile. 

Laser Chemistry Spectroscopy, Dynamics and Applications Helmut H. Telle, Angel Gonzalez Urena Robert J. Donovan 2007 John Wiley Sons, Ltd ISBN 978-0-471-48570-4 (HB) ISBN 978-0-471-48571-1 (PB) 

Normally, the amount of beam attenuation is given in terms of the optical density OD of the filter. The OD is the degree of opacity of a medium, and it is defined as 

Two common types of ND filter are used, namely absorbing glass filters and metal-film filters  

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