One avenue towards next generation spintronic devices is to develop half-metallic ferromagnets with 100% spin polarisation and Curie temperature above room temperature. Half-metallic ferromagnets have unique density of states, where the majority spins are metallic but the minority spins are semiconducting with the Fermi level lying within an energy gap. To date, the half-metallic bandgap has been predominantly estimated using Jullière’s formula in a magnetic tunnel junction or measured by the Andreev reflection at low temperature, both of which are very sensitive to the surface/interface spin polarisation. Alternative optical methods such as photoemission have also been employed but with a complicated and expensive set-up. In this study, we developed and optimised a new technique to directly measure the half-metallic bandgap by introducing circularly-polarised infrared light to excite minority spins. The absorption of the light represents the bandgap under a magnetic field to saturate the magnetisation of a sample. This technique can be used to provide simple evaluation of a half-metallic film.