  We present the first multi-band optical light curves of PSR J1622-0315, among
the most compact known redback binary millisecond pulsars, with an orbital
period Porb=3.9 h. We find a flux modulation with two maxima per orbital cycle
and a peak-to-peak amplitude of about 0.3 mag, which we attribute to the
ellipsoidal shape of the tidally distorted companion star. The optical colours
imply a late-F to early-G spectral type companion and do not show any
detectable temperature changes along the orbit. This suggests that the
irradiation of the star's inner face by the pulsar wind is unexpectedly missing
despite its short orbital period. To interpret these results, we introduce a
new parameter fsd, defined as the ratio between the pulsar wind flux
intercepted by the companion star and the companion intrinsic flux. This flux
ratio fsd, which depends on the spin-down luminosity of the pulsar, the base
temperature of the companion and the orbital period, can be used to quantify
the effect of the pulsar wind on the companion star and turns out to be the
most important factor in determining whether the companion is irradiated or
not. We find that the transition between these two regimes occurs at fsd=2-4
and that the value for PSR J1622-0315 is fsd=0.7, placing it firmly in the
non-irradiated regime.
