equivalentthickness

Equivalent thickness is a concept used to express the overall effect of a material, layer, or composite as if it were a single, homogeneous layer of a reference material with known properties. It provides a convenient way to compare diverse materials and configurations by reducing their complex behavior to a single thickness value.

In radiation shielding, equivalent thickness relates a material’s attenuation to that of a reference material, typically lead. For a photon energy where the material has an attenuation coefficient μ and thickness x, the transmitted fraction is exp(−μx). The equivalent thickness of reference material with coefficient μref is xeq = (μ/μref) x. This approach allows designers to report shielding effectiveness in terms of an easily understood thickness of the reference material.

In thermal engineering, equivalent thickness translates a multilayer insulation into a single layer with the same thermal resistance. The total thermal resistance is R = sum(di/ki). If this is matched by a reference material with conductivity kref, the equivalent thickness is deq = kref × sum(di/ki). Equivalently, the condition R = deq/kref expresses the same resistance with a single thickness of the reference material.

In optics and coatings, an equivalent or optical thickness often refers to the product of refractive index and physical thickness, n d, which determines phase delay and interference effects. This is a common way to compare thin-film stacks without detailing every layer’s properties.

See also: half-value layer, shielding materials, thermal resistance, optical thickness.