Candling an egg is when you use a light source to see inside of the shell without breaking it. The method was originally done with candle light giving it the name still used today. Embryologists study embryo formation inside of an egg shell using modern lighting techniques. It is also used in the egg industry to determine egg quality for consumers. Hatcheries use candling for quality assurance monitoring the embryos and ensuring that incubator parameters are the best possible for successful hatches. When a poor hatch occurs candling unhatched eggs helps to diagnose the most likely cause.
The first candling should occur at 7-10 days of incubation. In hatcheries a sample of eggs is taken from the incubator, usually 1/3 of the total eggs. Removing dead or infertile eggs from the incubation cycle helps to have a successful hatch of the remaining embryos. Dead eggs harbor bacteria and can end up bursting contaminating the entire incubator. The air quality will decrease as well due to the gases created by the decomposition process.