The parameter brief (with its actual uncertainty sbrief)
is the mean effective surface brightness, i.e. the mean surface
brightness inside the effective aperture (the circular aperture
enclosing one-half the total flux). This mean surface brightness
is expressed in B-
.
Two equivalent measurements of brief were derived: i) from the
apparent diameter 
enclosing a mean
surface brightness of 20.75 
(Dressler et al. 1987),
ii) from the mean surface brightness inside the effective isophote
(elliptical isophote enclosing one-half the total flux) measured by
Lauberts & Valentijn (1989; LV).
where 
, 
is the standard deviation,
is the correlation coefficient and n the number of galaxies
used for the comparison.
Similarly we have:
where m'(LV) is the average blue central surface brightness
within half total Blight (noted 
in LV) , from Lauberts &
Valentijn (1989). The correction for inclination is in good agreement
with the predicted one 
(see RC3 p50, Rel.71). The final value brief
is calculated as the weighted mean of each determination.
Another estimate of the mean surface brightness is bri25, the mean surface
brightness inside the isophote 
.
This brightness must be corrected for inclination effect (Bottinelli et al. 1995). We then obtain the corrected mean brightness bri25:
where
Notations are those used for the calculation of ai (Sect. 7).