The infrared wavelength range has borrowed units from the surrounding optical and radio (submm) regimes. Both magnitude and flux representations are widely used. For the ISO proposals (scaled) SI units are required.
Flux density 
of a source is defined as the flux per unit
frequency. Flux density is the physical unit used for point sources.
The unit of flux density is Wm-2 Hz 
. For practical purposes
in the infrared it is often convenient to scale this SI unit and use
Jansky instead.
For extended objects, the surface brightness 
is used instead
of flux density. Also background emission for e.g. point sources is
expressed in brightness units. Brightness is defined as the flux
density per unit solid angle. The unit of brightness is
Wm 
Hz sr , but in practice it is often scaled to
MJysr .
Flux density of a point source, in particular for a stellar object, is
often given in magnitudes m. In the ISO wavelength range these units
are mostly used for ground based measurements below 20 
m.
The conversion to or from magnitudes is performed via zero magnitude
flux density 
, which depends on the wavelength and the
photometric system used. Zero magnitude flux densities for ESO infrared
filters J through Q are listed in Table 5.
Magnitude m of a ground based observation can be converted into flux
density with formula
| Filter |  |  | |
| [ m] | [ m] | [Jy] | |
| J | 1.24 | 0.2 | 1587 |
| H | 1.64 | 0.3 | 1074 |
| K | 2.18 | 0.4 | 653 |
| L' | 3.76 | 0.7 | 253 |
| M | 4.69 | 0.5 | 150 |
| N | 10.3 | 5.2 | 29.4 |
N  | 8.38 | 0.8 | 48.7 |
N  | 9.67 | 1.6 | 34.9 |
N  | 12.9 | 3.7 | 19.7 |
| Q | 18.6 | 5.6 | 9.5 |
For spectroscopic measurements, the total flux F of a line is often
the most interesting quantity. A flux calibrated spectrum may be
presented in the form of flux density 
as a function of
wavelength . Flux F of a line in interest can be obtained by
subtracting an appropriate continuum flux density level of the spectrum
before integrating over the entire line in wavelength . The
unit of flux is Wm-2.