Shafey, T.M., et al. “Effect of Pigment Intensity and Region of Eggshell on the Spectral Transmission of Light That Passes the Eggshell of Chickens”. International Journal of Poultry Science, vol. 3, no. 3, Feb. 2004, pp. 228–233, https://doi.org/10.3923/ijps.2004.228.233.