We present here the results of a deep survey in a 0.4 deg ^ { 2 } blank field in Selected Area 57 conducted with the ISOPHOT instrument aboard ESAs Infrared Space Observatory ( ISO Based on observations with ISO , an ESA project with instruments funded by ESA member states ( especially the PI countries : France , Germany , the Netherlands , and the United Kingdom ) and with the participation of ISAS and NASA . ) at both 60 \mu m and 90 \mu m. The resulting sky maps have a spatial resolution of 15 \times 23 arcsrc ^ { 2 } per pixel which is much higher than the 90 \times 90 arcsec ^ { 2 } pixels of the IRAS All Sky Survey .
We describe the main instrumental effects encountered in our data , outline our data reduction and analysis scheme and present astrometry and photometry of the detected point sources .
With a formal signal to noise ratio of 6.75 we have source detection limits of 90 mJy at 60 \mu m and 50 mJy at 90 \mu m. To these limits we find cumulated number densities of 5 \pm 3.5 deg ^ { -2 } at 60 \mu m and 14.8 \pm 5.0 deg ^ { -2 } at 90 \mu m. These number densities of sources are found to be lower than previously reported results from ISO but the data do not allow us to discriminate between no-evolution scenarios and various evolutionary models .