Hot hair tools have been around for over a hundred years! Direct heat is the oldest technology. If you have to wait for a good 10 minutes for the tool or appliance to heat up, then it is direct heat. If the tool heats up in 15 seconds or less, then it typically using infrared heat. So your old curling irons, old flat irons, and hot rollers that you had to turn on early to get them hot enough to use are also direct heat. Pictured are two 2 inch irons. One cost $40 retail and the other costs $200 retail. The $40 marcel flat iron must be heated up in or on a heat source. The use of it becomes direct heat. The $200 Chi Turbo flat iron pictured heats up within 15 seconds of plugging up and turning it on.
When to use the infrared heat versus the direct heat? If you are going for bone straight hair without any bend nor curls, then infrared heat from the flat iron works great for styling purposes. Thermal protectant is vital to maintain the health of the hair. However, if you want any type of body or curl without having to use heat everyday to touch it up, then direct heat from even the traditional curling iron seems to hold for days without reapplying heat. The direct heat seems to give more body. Re-emphasizing that direct heat method has been effective for styling and allowing the hair to grow for over a hundred years WITHOUT "thermal protectants." The only things needed for decades were 1) styling spray for maximum hold and 2)shine spray. And yes, pressing oils were needed for pressing comb applications. No rocket science here . . . and we had plenty of "growing hair!" Madame C.J. Walker was living proof that we did not have to have infrared heat to have long hair. We just need "higher end" hair care products and conservative application of heat for styling.
Please remember that hair should be as clean as possible when using any type of hot tool. Also, hair growth is normally a half inch a month, but we have to prevent the strands from breaking in order to see that the hair gets maximum length.
On a side note, Black salons are losing the black haircare business . . . I do not think that it is coincidence that switching from the traditional stove and marcel styling into exclusive flat iron usage just happened around the same time. The traditional methods of marcel irons and pressing combs defined "Black hair."