David F. Gray (1992), The Inferred Color Index of the Sun, Publications of the Astronomical Society of the Pacific, vol. 104, no. 681, p. 1035 – 1038 (November 1992).
Michael S. Bessell (1990), UBVRI passbands, Publications of the Astronomical Society of the Pacific, vol. 102, Oct. 1990, p. 1181 – 1199.