ISOPHOT is one of the four focal plane instruments (ISOPHOT, ISOCAM, LWS, SWS) of the European Space Agency (ESA)'s Infrared Space Observatory (ISO). ISO was the first versatile infrared space observatory, allowing diffraction limited imaging and spectroscopy and it opened a new wavelength region in the far infrared (110…240 µm). ISO was cooled by 2300 litres of superfluid helium to a temperature near absolute zero (-273°C). It achieved a sensitivity 100 times better than the exploring IRAS-mission in the year 1983. ISO's scientific goals were the exploration of the cold and dust hidden universe as well as the spectroscopic analysis of the interstellar and circumstellar matter.

ISOPHOT is an imaging photopolarimeter for the wavelength range 2.5 ... 240 µm and includes a low-resolution spectrometer for the 2.5 ... 12 µm range. ISO was launched on 17 November 1995 into a highly excentric 24h orbit with an apogee at 70 000 km well outside the earth's radiation belts. The mission ended on 08 April 1998; data were gained for 29 months, 11 months longer than originally predicted.

ISOPHOT was designed and built by a consortium of various science institutes and industrial enterprises in Europe under the leadership of the Principal Investigator (PI), Prof. Dietrich Lemke at the Max-Planck-Institut für Astronomie (MPIA). For more information on the ISOPHOT instrument, its observing modes, the calibration, the data reduction and the data products see The ISO Handbook, Volume IV: PHT - The Imaging Photo-Polarimeter.

The observations ranged from comets and dust in the solar system to deep surveys of galaxies in the young universe. In April 2002, i.e. 4 years after the end of the ISO-mission about 30% of the scientific data obtained had been reduced. This resulted in 770 papers in journals with referee system and more than 1000 conference contributions. More scientific impact from the huge data base can be expected within the next few years by "archive observations" during "ISO's Active Archive Phase". ISO's Legacy Archive is open to all astronomers.