1. Relationship of candled appearance of eggs to their quality;Almquist;California Agr. Exp. Sta. Bul.,1933
2. Botsford, Η. E. 1940. Measuring egg quality inside the shell. University of Maryland Mimeograph Release, pp. 1-3.
3. The Haugh unit for measuring egg quality;Haugh;U.S. Egg Poultry Mag.,1937
4. Hauver, W. E., Jr., and J. A. Hamann 1956. Egg Grading Manual, Agriculture Handbook No. 75, U.S. Department of Agriculture, Washington, D.C.
5. The yolk color index;Heiman;U.S. Egg Poultry Mag.,1935