ISOPHOT, or PHT, is composed of 3 subsystems, or sub-instruments, optimised for specific photometric modes. Only one sub-instrument can be used at a time. The sub-instruments are:
PHT-P, a multiband, multi-aperture photometer with three single
detectors covering wavelength
range 3 - 120 
m from the near-infrared (NIR) to
the far-infrared (FIR).
PHT-C, two photometric far-infrared (FIR) cameras for the wavelength
range 50 - 240 m.
PHT-S, two grating spectrophotometers, operated simultaneously, for the
wavelength ranges of about 2.5 - 5 m and 6 - 12 m.
The modular subsystem concept minimises mechanisms and results in a compact and rigid instrument. By appropriately setting three rachet wheels a certain subsystem can be selected from a large choice of useful combinations of filters and apertures.
A focal plane chopping mirror is included for beam switching and modulation within the field of view of the instrument. For absolute photometric measurements, the fine calibration sources, FCSs, can be used as standard reference sources.
In addition, PHT is equipped with two sets of three polarisers, one set for
the PHT-P detector group and one for the PHT-C detectors, covering the whole
wavelength region from 3 to 240 m.
Fig. 1 shows a schematic view of the instrument.
