I have a small high LED used for mapping out air cells in shipped eggs, looking at tiny veins and MARAN eggs. OH YOU MARAN EGGS!
The one i chose is approx a quarter in diameter and has a lip that allows easy sliding across an eggs surface without bleeding light. After the initial candle and setting the eggs I candle every 48 hrs and mark air cells. I don't find it nessicary to candle everyday as long I watch the air cell growth (or lack there of). Our hatch is on lockdown, I'm slightly worried about 3 saddle eggs but we'll see if the chicks manage to bust out without assist tonight or tomorrow.