  The structure of the excited $2^{3}$S and $2^{3}$P triplet states of $^{3}$He
and $^{4}$He in an applied magnetic field B is studied using different
approximations of the atomic Hamiltonian. All optical transitions (line
positions and intensities) of the 1083 nm $2^{3}$S-$2^{3}$P transition are
computed as a function of B. The effect of metastability exchange collisions
between atoms in the ground state and in the $2^{3}$S metastable state is
studied, and rate equations are derived, for the populations these states in
the general case of an isotopic mixture in an arbitrary field B. It is shown
that the usual spin-temperature description remains valid. A simple optical
pumping model based on these rate equations is used to study the B-dependence
of the population couplings which result from the exchange collisions. Simple
spectroscopy measurements are performed using a single-frequency laser diode on
the 1083 nm transition. The accuracy of frequency scans and of measurements of
transition intensities is studied. Systematic experimental verifications are
made for B=0 to 1.5 T. Optical pumping effects resulting from hyperfine
decoupling in high field are observed to be in good agreement with the
predictions of the simple model. Based on adequately chosen absorption
measurements at 1083 nm, a general optical method to measure the nuclear
polarisation of the atoms in the ground state in an arbitrary field is
described. It is demonstrated at $B\sim$0.1 T, a field for which the usual
optical methods could not operate.
