To confirm if an eggs are fertilized or not, this light will let you see then by showing a dark spot and veins. You just can't tell before then. You can mark your eggs with dates in pencil so that you can keep track. This light was very helpful and was really fun to watch the growth of the eggs. You can check about once or twice a week to make sure that they were still growing. Features - ColorRed - Material: Plastic. - Size: 11.70 x 3.50 x 3.50cm/ 4.60 x 1.38 x 1.38 inch. - Come with candler covers that will from large eggs to tiny quail eggs. - By shining a candling torch through the shell, you can often make out signs of as and when they take place. - When checking whether the egg to be incubated or not, just put the bigger side of egg to upside, turn on the led egg candler and aim at the tops of bigger side. - Brighter and cooler light sources are available which do not carry the risk of harming the development of the embryos. - Light egg candler is a product that checks whether the egg is fertilized and grows healthy. Package list: 1 x eggtester