c © ESO 2001 Astronomy Astrophysics

Abstract. We examined the sky confusion noise in 40 sky regions by analysing 175 far-infrared (90{200 m) maps obtained with ISOPHOT, the photometer on-board the Infrared Space Observatory. For cirrus elds with hBi> 5 MJy sr−1 the formula based on IRAS data (Helou & Beichman 1990) predicts confusion noise values within a factor of 2 of our measurements. The dependence of the sky confusion noise on the surface brightness was determined for the wavelength range 90 200 m. We veried that the confusion noise scales as N / hBi1:5, independent of the wavelength and conrmed N / 2:5 for 100 m. The scaling of the noise value at dierent separations between target and reference positions was investigated for the rst time, providing a practical formula. Since our results conrm the applicability of the Helou & Beichman (1990) formula, the cirrus confusion noise predictions made for future space missions with telescopes of a similar size can be trusted. At 90 and 170 m a noise term with a Poissonian spatial distribution was detected in the faintest elds (hBi 3−5 MJy sr−1), which we interpret as fluctuations in the Cosmic Far-Infrared Background (CFIRB). Applying ratios of the fluctuation amplitude to the absolute level of 10 % and 7 % at 90 and 170 m, respectively, as supported by model calculations, we achieved a new simultaneous determination of the fluctuation amplitudes and the surface brightness of the CFIRB. The fluctuation amplitudes are 7 2 mJy and 15 4 mJy at 90 and 170 m, respectively. We obtained a CFIRB surface brightness of B0 = 0:8 0:2 MJy sr−1 (I = 14 3 nW m−2 sr−1) at 170 m and an upper limit of 1.1 MJy sr−1 (I = 37 nW m−2 sr−1) at 90 m