Strömgren, Bengt (1956). "Two-dimensional spectral classification of F stars through photoelectric photometry with interference filters". Vistas in Astronomy. 2 (1): 1336–1346. Bibcode:1956VA......2.1336S. doi:10.1016/0083-6656(56)90060-5.
Crawford, D. L. (1958). "Two-Dimensional Spectral Classification by Narrow-Band Photometry for B Stars in Clusters and Associations". Astrophysical Journal. 128: 185–206. Bibcode:1958ApJ...128..185C. doi:10.1086/146536.
Strömgren, Bengt (1956). "Two-dimensional spectral classification of F stars through photoelectric photometry with interference filters". Vistas in Astronomy. 2 (1): 1336–1346. Bibcode:1956VA......2.1336S. doi:10.1016/0083-6656(56)90060-5.
Crawford, D. L. (1958). "Two-Dimensional Spectral Classification by Narrow-Band Photometry for B Stars in Clusters and Associations". Astrophysical Journal. 128: 185–206. Bibcode:1958ApJ...128..185C. doi:10.1086/146536.