This is used for the coating on radiometers because, when applied in the appropriate manner, it is one of the lowest reflectance surfaces that exist (at least in the visible and near IR part of the spectrum).

This means that we absorb all the incident optical radiation which we are seeking to measure and minimise any corrections and uncertainties due to reflectance losses.

The low reflectance (blackness) is a result of two things:

1. the alloy itself; and
   
   
2. a surface treatment process which etches the surface into a structure that can only be described as a microscopic 'forest' of pine tree like structures which allow multiple reflections and thus absorption.

The material has other advantages: it has good thermal conductivity and is stable under radiation etc. robust to thermal expansion and shock. However a weakness is that it is fragile to the touch as the 'pine trees' are easily damaged.