Have a look at:

1) http://www.ucl.ac.uk/~ucapahh/research/crystal/cao.htm

2) Calcium oxide (CaO) band structure Landolt-Börnstein - Group III Condensed Matter 41B (II-VI and I-VII Compounds; Semimagnetic Compounds)

http://materials.springer.com/lb/docs/sm_lbs_978-3-540-31359-5_220 10.1007/10681719_220 (Springer-Verlag Berlin Heidelberg © 1999)

3) https://www.researchgate.net/publication/245167517_Electronic_band_structure_of_calcium_oxide

4) https://www.researchgate.net/publication/235002143_The_valence_band_structures_of_BeO_MgO_and_CaO

Article Electronic band structure of calcium oxide

Article The valence band structures of BeO, MgO, and CaO

Band gap of CaO is the separated energetic distance between valence band(O2p), and the conduction band(Ca 3dxy). It can be meseaured spectrophotometrically using diffuse reflactence spectroscopy. Each of VB and CB can be measured using cyclic voltammetry as VB refers to HOMO level, and CB refers to LUMO.

Thank you Arnulf Jäger-Waldau , Abbas Jassim Lafta