I have difficulties to compare different brightness data of the sky. In older literature I found different measurements of the brightness density of the sky. Some of the results are shown in in m(vis)/deg^2 and others in asb. In order to compare the data in units of m(vis)/sterad, I proceeded as follows: The asb values: 1 Lux / sr = asb * Pi since 1 Lux corresponds to -14.18 m(vis) 1 asb = - 1 / Pi * 14.18 m(vis) / sr for the conversion I calculate m(vis)/sr value corresponds to -14.18 - 2.5*LOG10( Pi * asb value ) For m(vis)/deg^2 values: m(vis)/sr value corresponds to m(vis)/deg^2 value - 2.5*LOG10( (180/Pi)^2 ) = m(vis)/deg^2 value - 8.79 The results show me that doing so is not correct! However, the difference of around. 2.67 m(vis)/sr indicates some where a missing factor of 4*Pi. In the conversion of the m(vis)/deg^2 values, a possible 4*Pi-factor in the LOG10-expression was omitted intentionally since the 'per degrees' are converted in 'per radians' and not for the full sphere. Could some one please indicate me where I am doing wrong. Thank you.