I'm confused--East Texaroo, do you mean heat is what matters for a brood light? Because it's my understanding that it's the brightness that matters most for candlers, but heat for brooders.
toxo--I'm sorry, but I don't know the answer to your question. I just use whatever flashlight I can find (which is a challenge in my house with three young boys every one of whom likes a good flashlight but can't seem to keep track of it) in a dark room (usually at night with all the lights off). I cup the egg in my hand, with a narrow opening at the bottom for the light to shine through and a larger opening at the top for me to examine the egg through.
Regular bulb, LED bulb, it doesn't really seem to matter. Just that the brighter the better--make sure you have fresh batteries if you're using a battery-powered bulb. I would think something plugged in would be even better, as it can get better power, but I haven't had a chance to find out.
I don't think light can hurt the embryo--just heat (aha--that must be what you meant, east texaroo--sorry!). Just be careful not to leave it on the candler long enough to change the internal temperature of the egg. It should take less than one full minute to see what you need to see anyway, so any flashlight that's cool enough for you to handle is cool enough not to do the egg damage.
The husband is a Law Enforcement Officer- best flashlights in the world. I "borrow" it, and have been using an old paper towel tube- flashlight in one end, egg on the other. I have white eggs- but it has been working great.
Oh, a paper towel (or toilet paper, right?) tube is a great idea! That will channel the light and keep the egg away from the direct heat. Thanks for that!
I wish I had a decent flashlight, but no flashlight in this house ever survives more than a week before being subsumed into the chaos of three young boys, so I dare not invest more than a couple bucks in a new one. Such a shame! But, even a cheap flashlight does well enough to get the job done.