I bought two high powered LED penlights from
amazon, one tiny, one penlight size. On even the darkest eggs, putting one on Either end of the egg shows me progress or not. They weren't as expensive together as most commercial candlers and they're not hot.
I just let broodies do their thing and never candle under a broody - it's just rude. I candle what I am hatching myself. If an egg explodes under a broody they scrape out the crud and go on. Broodies mean business and they know their work. The buggers will hatch or they won't.
If a hen leaves the nest with chicks and leaves eggs, I haul those in, let them warm for about four hours and check for life and hatch them myself if they're still kicking.
Pissing off and messing with broodies are how things get crushed and hens quit broody and a whole host of bad things happen.
Sure some hens will let you, and not be fussy but every time you pick up an egg you have a percentage chance of dropping it. Ask me how many eggs I've dropped.... ugh.
Watching a broken egg bleed is not good for you or it... It dies and you get the guilt for a long time.