  Given a field vacuum state in a black hole spacetime, this state can be
analysed in terms of how it is perceived (in terms of particle content) by
different observers. This can be done by means of the effective-temperature
function introduced by Barcel\'o et al. in [1]. In Barbado et al. [2], this
function was analysed in a case by case basis for a number of interesting
situations. In this work, we find a general analytic expression for the
effective-temperature function which, apart from the vacuum state choice,
depends on the position, the local velocity and the acceleration of the
specific observer. We give a clear physical interpretation of the quantities
appearing in the expression, and illustrate its potentiality with a few
examples.
