# Linear attenuation coefficient

**Linear attenuation coefficent** **(µ)** is a constant that describes the the fraction of attenuated incident photons in a monoenergetic beam per unit thickness of a material. Its complement is the transmitted portion of the beam. It is expressed numerically in units of cm^{-1}.

###### Calculating µ

The intensity of the beam at distance x (cm) within a material is calculated using the following equation ^{1}:

*I _{x}*=

*I*

_{0}.

*e*

^{−μx}

Where I_{x} is the intensity at depth of *x* cm, I_{0} is the original intensity, and µ is the linear attenuation coefficient.

Rearrange and take the log of both sides gives the equation for µ.

*μ = ln(I _{o}/I_{x})/x*

###### Variants

The mass attenuation coefficient is a normalisation of the linear attenuation coefficient per unit density of a material producing a value that is constant for a given element or compound ^{2}.