One can create an index based on the difference between measurements in narrow-band (30 Angstrom) and wide-band (150 Angstrom) filters centered on H-beta (4861 Angstroms). This is the so-called β index.
For very hot stars, there is a simple relationship between the value of the β index and luminosity class -- and so absolute magnitude.
Copyright © Michael Richmond. This work is licensed under a Creative Commons License.