In physics,<unk>  is defined as the number of particles that intersect a unit area . Its units are m-2. In particular, it is used to describe the strength of a radiation field.  It is considered one of the fundamental units in dosimetry.
It has two equivalent definitions:
1) Imagine that an infinitesimal sphere of cross sectional area da is impinged upon by dN particles of a certain type.  Then, the<unk>  is:
Φ
=
d
N
d
a
{\displaystyle \Phi ={\frac {{\rm {d}}N}{{\rm {d}}a}}}
.
2) Using the same image of a small sphere as above:
Φ
=
∑
d
ℓ
d
V
{\displaystyle \Phi ={\frac {\sum {\rm {d\ell }}}{{\rm {d}}V}}}
,
where 
d
V
{\displaystyle {\rm {d}}V}
 is the infintesimal volume and 
∑
d
ℓ
{\displaystyle \sum {\rm {d\ell }}}
 is the sum of all the path lengths of the particles that transverse the volume.
Fluence
Fluid
Ionic liquid
Colloid
Biodiesel
Mineral water
Diol
Filter 