I know fluence and spot size are very important in getting good results, but how important is pulse duration? I’ve asked this question in a previous post called “How safe is numbing cream?” but I decided to move it to a new post since it is a completely different subject.
Here’s some of what was said:
Question: “Why do you recommend a low pulse for treatments? Doesn’t it make sense to increase the pulse for coarser hair and decrease the pulse for thinner, lighter hair? I don’t understand why the Gentlelase’s pulse cannot be changed and is only 3 ms.”
Response: “Lowest pulse width is most effective on all types of hair. Higher pulse is technically just available to protect darker skin when needed (it still reduces the effectiveness of treatment, but there is no option for darker skin and often slightly higher pulse IS enough for the most coarse hair anyway). 3ms and 18mm spot sizes are in part why GentleLASE and GentleYAG have such good feedback.”
I found some information online for why I think pulse duration is an important treatment parameter:
Small objects lose heat more rapidly, so optimal pulse duration will be shorter. Large objects lose heat more slowly, so pulse duration will be longer. The pulse durations listed below are short enough to efficiently heat the target but long enough to provide minimal heating of the epidermis:
fine hair: 10-20 ms
medium hair: 20-30 ms
coarse hair: 40-60 ms
The size of the object does not completely dictate the thermal transfer properties of the object. It depends on many factors. Thermal conductivity properties of hair are the same as any other object. They depend on the material and temperature of the surrounding medium (skin/air in the follicle) and the material, temperature, and surface area of the hair itself.
The only real difference between a thin and coarse hair is the surface area. Surface area for a hair is approximated by the outside of a cylinder, or pi*r^2 * length of hair, so all things being equal the only difference between coarse and thin hair is the radius (and possibly the length, since coarse hairs are often deeper). So more coarse hairs have much more surface area (it increases as the square of the radius).
The point is that total heat transfer of the object to the adjacent medium (skin) increases with the surface area. Thus, larger hairs has more risk of burning the skin if they get exceedingly hot (as they would, because they “see” more photons from the laser). This is probably why coarse hairs are recommended to have longer heating times, in order to allow the hair to release its pent up heat over a longer time (pulse width), allowing the surrounding skin to spread the heat around a bit before taking on more of it from the heated hair.
I think you answered your own question in a way. Basically, what’s the advantage of increasing the pulse for coarse hair? By your description, low pulse is great for fine hair AND better for coarse hair compared to high pulse, i.e. higher pulse will work on coarse hair, but lower pulse will probably work even better on coarse hair.
LAGirl is right, because the mechanism for destroying the matrix that grows hair is heat transfer, and if it is done at high temperatures it is more effective. However, coarse hair runs a higher risk of damaging the surrounding tissue, too, because it is more often in direct contact with it, or there is less surrounding air (a buffer).
Laser is only attracted to pigment. The damage could be to the skin (i.e. a burn) if the skin is not light enough for the settings set. As long as the skin is light enough, the hair will be damaged and the skin will be left untouched. Of course, that is why experience of the tech is very important - to be able to judge what is the max setting (highest joules and spot size and lowest pulse combo) the client’s skin can handle safely (doing some test spots is also not out of the question).
Well, one of the Tria engineers had this to say in their patent filing:
“It was initially believed that pulse durations in the 5-30 ms range are optimum; however, subsequent studies showed that longer pulses (up to at least several hundred milliseconds) can quite effectively achieve hair-regrowth inhibition, and can also reduce epidermal heating”.
It goes on to say that “The thermal relaxation time of the hair follicle is generally at least 10 ms and may be as high as 100 ms to 600 ms for the stem cells surrounding the hair follicle, and so for the reasons provided, it is particularly desired to have pulse durations at least as long as this thermal relaxation time, such that a particularly preferred minimum pulse duration may be 100 ms or more.”
Now the Tria laser just happens to be limited to long pulse durations. So it’s hard to tell if longer pulse durations really are beneficial/effective, if they were just trying to support their device limitations, or a little of both.
Do they provide any links/info on these studies? When this device was discussed at conferences, it was brought up that it won’t be able to destroy the follicles permanently due to the high pulse. Considering most of the people who are involved in various studies with regards to hair removal are at these conventions, I would be curious to see if the studies they’re referring to actually exist.
It looks like one of the referenced studies advocating longer pulse widths apparently comes from “Palomar Super Long Pulse Diode Laser System Clinical Data” which was done for the Palomar SLP1000 (using up to 1000 ms widths).
The marketing speak for the SLP1000 says:
“In hair removal treatments, Super Long Pulses spread heat beyond the hair shaft to the stem cells. Most lasers merely ablate the hair shaft. However, the SLP1000 disables the entire hair follicle. The result: permanent hair reduction.”
And basically reiterates the dark-skin advantage discussed already:
“Super Long-Pulse Technology permits higher fluences to be used on darker-skinned patients”
I couldn’t find the actual text of the Palomar study, but there’s a few other studies that support long pulse width efficacy:
We might be able to surmise that, as long as the resulting fluence is high enough, pulse duration doesn’t matter much except for safety with darker skin tones.
You’re confusing pulse width with pulse in relation to the difference between alexandrites, diodes, and Yag types of lasers. All hair removal lasers are long-pulsed. So this marketing speak doesn’t say anything. They are merely stating that their machine is somehow different even though it’s the same as other diodes, i.e. it has 810nm (vs 755nm for alex and 1064nm for Yag). By their definition, Yag lasers are even better, which they’re not. That’s why Yags are fine on coarse hair, but not good on fine hair at all. They penetrate too deep for most fine hair which is normally more shallow. Pulse width measures how fast the energy is delivered to the follicle.
Here’s what they say before the part you posted: “The Palomar SLP1000 uses Super Long Pulses (up to 1000 ms), which deliver energy to the skin over a longer period of time than shorter pulses (<1 to 100 ms). This keeps the epidermis from overheating and dramatically reduces discomfort and side effects. By providing maximum protection to the epidermis, Super Long-Pulse Technology permits higher fluences to be used on darker-skinned patients than would otherwise be possible.”
Basically, they’re trying to convince you that heating up the follicle less is somehow a benefit to you, when in fact, heat is what you need to destroy the follicle. Remember that lasers are attracted to dark pigment. So if the skin is light, the heat will only be attracted to the hair anyway, which is dark. The problem arises when the skin has pigment too, in which case the pulse needs to be adjusted to avoid heating up the skin along with the hair. However, doing this also results in a less effective treatment since the hair is receiving less heat.
What they are trying to say here is that the diode can treat more skin types than an alex, which is true. However, they don’t mention that you lose efficiency in killing hair, i.e. there is give and take. They’re trying to tell you that it’s a benefit to not heat up the follicle as much, when it’s not. It’s safer on the skin, BUT at the same time it’s less effective.