Quote from: EmilyMK03 on October 13, 2016, 01:27:35 PM
I was in a similar situation as you. I have fair skin and black hair. After about 8 laser hair removal treatments, I still had a gray shadow in my upper lip. But there wasn't any noticeable hair growth (even after several days of not shaving, I didn't notice any hair growing back). I just kept waiting though, and I did eventually have some hairs grow back. I just kept doing more and more laser treatments, and now after 11 sessions, I don't have any more gray shadow. I can finally go out without any makeup, which is incredibly liberating.
I've been doing laser 2,5 years now, and I lost count of the amount of sessions I've had. Here's the thing: the laserlight is transmitted through the melanin (brow pigment) within the hairs to the follicle, which it then burns. Hair grows in cycles, each lasting between 6 and 8 weeks. In each cycle, about 20% of the follicles are active. When you apply laser, as you've already mentioned, you can't get all of the hair in one go, because the follicles grow at several depths. The first passover, you'll be able to 'get' two third to 3/4 of the total depth. Usually not more, because the amount of active follicles also delimits the amount of energy your skin can take. With a full beard, you'll have lots of thick black hairs = lots of melanin. That means you can only apply so much energy without burning the skin.
This energy is measured in Joules/cm² and a typical untreated male beard can take 13-15 J/cm² on the first couple of treatments without burning the skin. You need 30-33 J/cm² to get to the deeper layers, though. One passover therefore requires about five treatmens, which lasts 5 x 8 weeks = 10 months. So, within 5 treatments between 2/3 and 3/4 of the follicles will be gone. You will then still need another pass though, because 1/4 to 1/3 of the follicles (those lying deeper within the skin) won't have been targeted by the laser in the first passover. And since your hair still grows in cyles, this means at least another 5 cycles, equalling at least 10 in total.
These deeper lying hair follicles are usually the reason people develop a 'shadow'. It's simply the leftover of what was there originally, and it means more work is still to be done.
There is one more complication though, and that's the reaction of your skin itself to the laser. As with any light bombarding it (such as the sun in summer), the body's natural reaction is to create pigment, aka a sun tan. In some cases, the body responds to the laser treatment as if it was the sun shining down on it on a summer day and it actually creates isolated spots of melanin in the skin. These usually go away after a while though, the 8 weeks wait between sessions is normally enough.
So, there you have it. Laser 101

Oh, this information is a mix of what my dermatologist has told me and my own experience over the last 2,5 years of doing this