Light is a strange thing to make into a real substance. It is seemingly without form and, yet, as is seen with the sun, must have form. This contradictory nature is precicesly what makes light such a strange thing to us mortals.
If we were to listen to Thoma Rattius, we would find that light comes strictly from fire, but that cannot be true. Light is largely created by fire, but also by air, as can also be seen by the sun.
Instead, the dragon's breath must be inhaled swiftly, to the point where he almost chokes. This must then be sprinkled with the life-giver's given life to make the breath larger and more dazzling. The dragon will be thirsty during this process, so water must be given every-so-often and allowed to turn to steam else the dragon burn out. A constant, slim drink and plenty of food will allow the dragon to settle into a glowing state.
The result will be a cool and physical flame.