The same char cloth you’d use for traditional flint and steel fire starting is the ideal tinder for optical fire starting. It has a low ignition temperature due to the charring process. It’s also black, so it readily absorbs heat. And it gives you a decent burn time, allowing you to place it into tinder and blow it into flame before it dies out. Char cloth can be made by placing cotton, linen or fluffy tinder in a metal box or can with one tiny vent hole. Toss this can in a campfire and let it cook for 5-6 minutes or until smoke stops jetting out of the vent. Use a stick to roll the can out of the fire and allow it to cool before opening. The resulting material should have become black and it should have shrunk in size a little. It should also be excellent at catching sparks from ferrocerium rods, flint and steel, and optics. Test it with a few sparks, just to be sure. When the char gets red spots from the sparks that hit it, you did everything just right. Store your char in the can, or in another dry container with your fire starting gear.