Eggs about to hatch are candled to find out the stage of development of the chick inside. An eggshell is very easy to see through when held up to a light. This lets the farmer know the approximate time when the chicks will hatch. It also lets the farmer know which eggs are not going to hatch.
A person shines a very bright light through an egg to see through the shell, this is called candling. What they are looking for when candling are the embryos in hatching eggs or defects in eggs going to market.
Candling is a process where a bright light is shined through an egg to see inside. By examining the development of the embryo and the presence of blood vessels, candling can help determine the fertility of eggs.
Candling is done to check on the contents of the egg. Candling eggs reveals blood spots and when done to fertilized eggs can verify the growth of the embryo.
Eggs hatching
I see that the eggs are hatching now. I am hatching an idea about that.
The eggs break and the babies come out. This is called hatching.
Eggs Hatching - 1899 was released on: USA: September 1899
It very much depends on the egg and your experience. Many people hatching many eggs a year can accurately candle pale eggs at just 3-4 days but 6 days for white eggs and more than 8 days for brown eggs is a lot easier. The longer you wait the easier it is to see the chick inside.
The hatching time for robin eggs is typically around 12-14 days after they are laid.
The typical hatching time for budgies' eggs is around 18-21 days.
put them in a brooder
You can try candling quail eggs at about eight days. Though it is often hard to see due to the coloration of the shell.