  Empirical relations between major UV and extreme UV spectral lines are one of
the inputs for models of chromospheric and coronal spectral radiances and
irradiances. They are also needed for the interpretation of some of the
observations of the Solar Orbiter mission. We aim to determine an empirical
relation between the intensities of the HI 121.6 nm and HeII 30.4 nm Lyman
$\alpha$ lines. Images at 121.6 nm from the Chromospheric Lyman Alpha Spectro
Polarimeter (CLASP) and Multiple XUV Imager (MXUVI) sounding rockets were
co-registered with simultaneous images at 30.4 nm from the EIT and AIA orbital
telescopes in order to derive a spatially resolved relationship between the
intensities. We have obtained a relationship between the HI 121.6 nm and HeII
30.4 nm intensities that is valid for a wide range of solar features,
intensities, and activity levels. Additional SUMER data have allowed the
derivation of another relation between the HI 102.5 nm (Ly$\beta$) and HeII
30.4 nm lines for quiet-Sun regions. We combined these two relationships to
obtain a Ly$\alpha$/Ly$\beta$ intensity ratio that is comparable to the few
previously published results. The relationship between the HI 121.6~nm and HeII
30.4 nm lines is consistent with the one previously obtained using irradiance
data. We have also observed that this relation is stable in time but that its
accuracy depends on the spatial resolution of the observations. The derived
Ly$\alpha$/Ly$\beta$ intensity ratio is also compatible with previous results.
