There is a general use of this term and it's concepts which do not especially adhere to the scientific foundation.
There is a very definite definition more often said as coefficient of absorbtion or mass attenuation factor. The units there are length squared/ mass. This is used in astrophysics, optics, and plasma research.
Where it runs into trouble is that it generally is defined for a certain frequency applied to specific medium.
As such in general public use; such as police checking car windows, they are simply provided with a set of samples and must rely on judgement, much like someone trying to match a paint color in the home.
There is a very definite definition more often said as coefficient of absorbtion or mass attenuation factor. The units there are length squared/ mass. This is used in astrophysics, optics, and plasma research.
Where it runs into trouble is that it generally is defined for a certain frequency applied to specific medium.
As such in general public use; such as police checking car windows, they are simply provided with a set of samples and must rely on judgement, much like someone trying to match a paint color in the home.
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No specific unit, only and no., from 1 to 10.