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Attenuation coefficient
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<p>The linear attenuation coefficient, attenuation coefficient, or narrow-beam attenuation coefficient characterizes how easily a volume of material can be penetrated by a beam of <a href="/facts/Light/jeofpMn4">light</a>, <a href="/facts/Sound/DlKXlPuF">sound</a>, <a href="/facts/Particle/JHKr2w6h">particles</a>, or other <a href="/facts/Energy/wd8PrQM7">energy</a> or <a href="/facts/Matter/osyc04UN">matter</a>. A coefficient value that is large represents a beam becoming 'attenuated' as it passes through a given medium, while a small value represents that the medium had little effect on loss. The (derived) <a href="/facts/International_System_of_Units/S7YLglNz">SI unit</a> of attenuation coefficient is the <a href="/facts/Reciprocal_metre/mlrDdsFD">reciprocal metre</a> (m−1). Extinction coefficient is another term for this quantity, often used in <a href="/facts/Meteorology/xblFr28q">meteorology</a> and <a href="/facts/Climatology/bYS7FktY">climatology</a>. Most commonly, the quantity measures the <a href="/facts/Exponential_decay/wLB1zYDP">exponential decay</a> of intensity, that is, the value of downward <a href="/facts/E-folding/wU5J9CWu"><i>e</i>-folding</a> distance of the original intensity as the energy of the intensity passes through a unit (<i>e.g.</i> one meter) thickness of material, so that an attenuation coefficient of 1 m−1 means that after passing through 1 metre, the radiation will be reduced by a factor of <i><a href="/facts/E_(mathematical_constant)/coEtSBk8">e</a></i>, and for material with a coefficient of 2 m−1, it will be reduced twice by <i>e</i>, or <i>e</i>2. Other measures may use a different factor than <i>e</i>, such as the <i>decadic attenuation coefficient</i> below. The broad-beam attenuation coefficient counts forward-scattered radiation as transmitted rather than attenuated, and is more applicable to <a href="/facts/Radiation_shielding/5LaNNgXe">radiation shielding</a>. The <i><a href="/facts/Mass_attenuation_coefficient/vGQshk05">mass attenuation coefficient</a></i> is the attenuation coefficient normalized by the density of the material. </p>