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Neutral density filters are optical attenuators which have an approximately constant degree of attenuation (filter loss) in a substantial wavelength range. Attenuation is accomplished by either absorption using light absorbing glass or by a combination of absorption and reflection using a thin-film metal coating. The level of attenuation can be specified from optical density (OD) 0.04 to 4.0 where OD is defined as -log10 (T). By placing several neutral density filters from a large filter kit in series, one can realize many different degrees of total attenuation. A continuous adjustment of absorbance is possible with a rotatable neutral density filter wheel.