In this work we study the spectral properties ( 3600 - 6800 Å ) of the nuclear region of early-type galaxies at low ( |b| < 25 ^ { \circ } ) , intermediate ( including surroundings of the Magellanic Clouds ) and high ( South Polar Cap ) Galactic latitudes .
We determine the E ( B - V ) reddening values of the galaxies by matching their continuum distribution with respect to those of reddening-free spectral galaxy templates with similar stellar populations .
We also compare the spectroscopic reddening value of each galaxy with that derived from 100 \mu m dust emission ( E ( B - V ) _ { FIR } ) in its line of sight , and we find that there is agreement up to E ( B - V ) = 0.25 .
Beyond this limit E ( B - V ) _ { FIR } values are higher .
Taking into account the data up to E ( B - V ) \approx 0.7 , we derive a calibration factor of 0.016 between the spectroscopic E ( B - V ) values and Schlegel et al. ’ s ( 1998 ) opacities .
By combining this result with an A _ { K } extinction map built within ten degrees of the Galactic centre using Bulge giants as probes ( Dutra et al .
2003 ) , we extended the calibration of dust emission reddening maps to low Galactic latitudes down to |b| = 4 ^ { \circ } and E ( B - V ) = 1.6 ( A _ { V } \approx 5 ) .
According to this new calibration , a multiplicative factor of \approx 0.75 must be applied to the COBE/IRAS dust emission reddening maps .