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 -Color:Black -Material:Aluminum alloy, ABS -Size:11.30X3.80X3.80cm/4.44X1.49X1.49in -Power supply: 3 AAA batteries (not included). -Professional and practical design with beautiful appearance, easy to carry and use. -The egg uses high-penetration and high-brightness lamp beads. -After the incubator has been running for a few days, it's possible to scan the eggs with a bright light to see inside the shells. -When the lamp is lit, the hatching degree in the egg can be clearly seen, and the hatching degree of the egg can be easily judged. -Light egg candler is a product that checks whether the egg is fertilized and grows healthy. Package list: 2 x egg incubator lamp