How Important is the "Repetition Rate" in Hz ?

I Know the most important things are the size of the Spot, the Joules, and pulse duration but, how about the Repetition Rate?

What is exactly the repetition rate? Is automatic or can be chosen? I know that lightsheer ET repetition rate is up to 2hz, is that ok or it has a very low repetition rate?

My second question is, what is the “peak power” and how important is it? For example, candela gentlelase plus has a peak power of 17,666 watts and Lightsheer ET has a peak power of 1600 watts. Are better higher watts? Watts are fixed or can be choosen? For example, if lightsheer has a peak power of 1600, if i use this kind of laser, it will work at 1600watts or can be used with lower settings?

Thanx!!! :slight_smile:

Hopefully Chris will respond to this with more details, but I can tell you one thing: you can’t compare diode to an alex. Settings are not comparable between types of lasers. You can compare alex to alex lasers or diode to diode lasers.

thanx for you answer :slight_smile:

Repetition rate would only come into play when discussing speed of treatment, not efficacy. Units of hertz are simply inverse seconds, so a laser with a repetition rate of 2 Hz means the laser can be fired, at a maximum, every 1/2 seconds. (Example: If you had a laser whose repetition rate was 0.004 Hz, you would have to wait over 4 minutes between firing each pulse.) In general, however, most if not all lasers used for hair removal have decent repetition rates. The actual repetition rate is simply how fast your technician goes.

Not sure what you mean by “automatic.” A laser with an “automatic” repetition rate would imply that the laser would automatically fire pulses without human control. (Think semi-automatic gun!)

Peak power is somewhat self-explanatory. Power, when expressed in watts, is the amount of work done (here work = radiant energy) over a period of time. 1 watt = 1 Joule/second.

GentleLase has a higher maximum power because it can do more work (ie, deliver more radiant energy) in a smaller amount of time (3 milliseconds). While the radiant energy amount can be changed on both machines, the pulse width on GentleLase is set at 3 ms. With LightSheer, though the pulse width is adjustable, the shortest time is 5 ms.

You’ve probably noticed that most of the contributers on this forum don’t usually compare lasers by power. That’s because power doesn’t describe the complete picture. To do that, we refer to the fluence, which is radiant energy per unit area. Bigger area covered = faster treatment time.

Nothing further to add. Speed does not effect the efficacy of the treatment.

This is correct. The key is the different wavelengths used. The longer wavelengths of the diode (800nm) and Nd:Yag (1064nm) = deeper penetration. However, melanin has a peak absorbance at around 335nm, so alex and ruby will be absorbed better.

Brilliant!!! Thanx for the answer :slight_smile: Very very well explained :slight_smile:

I’m confused though, I can’t find the post but when I went to my last clinic they used a higher repetition rate and the joules were lower, as there’s a max joules available based on repetition rate. I think it was something like with Apogee elite mpx 18mm whatever Hz they used 15 joules was the maximum. A lower Hz would do a slower treatment but could go higher in terms of joules. Just confused when you say speed doesn’t affect the efficiency of the treatment when clearly it should since there’s different joules available based on the repetition rate. You had told me previously Chris that the best treatment with Apogee would be with a super low repetition rate but that the treatment would take forever so that places wouldn’t go that low. So just confused now?

It sounds like you are confusing pulse length with repetition rate. The repetition rate is typically not a parameter that can be directly adjusted. It will, however, be influenced in machines where the pulse length can be adjusted, because selecting a shorter pulse length will decrease the rate of repetition. Think of it as a type compensation. The science behind the explanation for this is too detailed to explain here, but a useful, though not entirely accurate analogy is a cell phone: watching a 30 minute HD video depletes energy faster than a 30 minute conversation, so to return to full charge after the HD video you’d have to compensate by charging the phone longer.

Keep in mind, though, that in laser hair removal applications, usually only one pulse is fired at a given target. While the Apogee Elite can deliver up to 5 pulses per second (depending on the wavelength), doing so at a decent fluence would cause severe epidermal heating. That is not desirable except for applications like skin rejuvenation or removal of vascular lesions, where ablation of the skin is the ultimate goal.

In other words, since hair removal is typically one shot per target, the speed of the technician is not going to approach the maximum theoretical repetition rate of the machine.

This is correct. The key is the different wavelengths used. The longer wavelengths of the diode (800nm) and Nd:Yag (1064nm) = deeper penetration. However, melanin has a peak absorbance at around 335nm, so alex and ruby will be absorbed better. [/quote]

I should add that comparing output parameters like power or fluence is legit when talking about lasers in general. So the claim in my previous post that GentleLase has a higher power than LightSheer is still valid.

Setting comparisons between lasing media (alex, diode, Yag, etc.) are not valid when discussing hair removal because of the different wavelengths used. Output doesn’t equal absorbance due to wavelength.

No I’m not confusing pulse width I know the difference. This was the Apogee Elite MPX. I was treated at whatever joules and spot size as well as pulse width which seemed low. The tech told me it was at the maximum which seemed off so I asked Chris here about it and it was due to the repetition rate. If they lowered it then the joules would go higher.

Found it here, was 18mm spot size 13 joules 2Hz repetition rate and 15ms pulse width. I told her she can go higher so she debated on going to 15 joules which is the max, or lowering the pulse width and decided on that so she went to 10ms. She told me I was 2 joules away from the max that 2Hz can handle and would go higher next time.

I was told to hit something like 22 joules with a 10ms pulse it requires a 1Hz repetition rate but that that would be an incredibly slow treatment.

So the joules definitely do change based on what repetition rate there is, she even showed me the machine wouldn’t go higher than 15j when pushing the button it would stop there due to the repetition rate.

1 Hz = one pulse a second. GentleLase Plus has a repetition of rate of 1 Hz, and I wouldn’t call that “incredibly slow.”

Apogee Elite MPX can be operated in continuous wave (“automatic”) mode which is why the repetition rate can be adjusted. With laser hair removal, there’s no need to fire pulses faster than 1 to 1.5 Hz. The operator can only move the guide so fast, so too much overlap and epidermal heating would occur at the highest fluence and fastest repetition. Hence, for safety, the fluence is lowered at the faster repetition.

By the way, saying “joules change” is a misnomer. Joules are a unit of measurement. The property (energy) is what changes.

I’m going by the email I have from Romeo that said she probably isn’t using 22 joules because the repetition rate for that would make my treatment take 2 hours which no one would want to do. When I say incredibly slow I’m not talking about gentle lase.

I’m saying joules change in regards to repetition rate meaning at 2Hz on elite MPX 15 joules is the max where other rates can make the joules go higher. 15 joules to 22 joules is a change in joules so not sure what you’re trying to suggest.

The average human has 1.7 square meters of skin surface area. Do the math. Whatever the machine, at 1 Hz with a 15 mm spot size, you’d treat your whole body in 2 hrs. I’m pretty sure your treatment area did not encompass every mm of skin on your body.

Read my post again. The highest fluence at the fastest repetition rate would cause too much epidermal heating. Cynosure limits this for safety. That’s why you can’t treat higher than 15 J at 2 Hz with that machine.

I’m not suggesting anything – except that your statement was incorrect. Again, joules are a unit of measurement. Saying “joules change” is incorrect because it’s not a physical quantity. The fluence (energy) is what is changing, and that’s what you are describing. A mile is 5280 ft. You can’t “change” it to 6000 ft. This is kind of belaboring the point; it shouldn’t be that difficult to grasp.

Sorry buddy but having the machine at 15 joules and switching it to 22 joules is CHANGING the joules on the machine. It’s a unit of measurement per CM^2 so 15 joules per CM^2 or 22 joules per CM^2. I never said that 1Hz vs 2Hz at 15 joules is different, I said that at certain Hz you can’t physically go higher in joules. What part of switching the machine from a fluence of 15 j / cm^2 to 22 j / cm^2 isn’t a change to you?

As for the taking 2 hours for treatment, that came from one of the experts on this forum which to be honest probably has a lot more knowledge about LHR than you do considering you just joined and have a total of 6 posts on here with all of them being from thread alone.

Only 6 posts…so what? I only recently found this forum. I have an MD PhD in medical physics and I’ve done lots of research on LHR. Please stay on topic and avoid ad hominem arguments.

No, a joule is a unit by itself. Fluence is a measurement of joules per cm^2.

I’ve already mentioned the reason for this: it’s a safety limit. It has nothing to do with going “physically” higher, because the technology is otherwise capable of doing so.

Yes! Fluence was being changed. I see you’ve used the correct terminology this time. Your original statement was incorrect, and that’s all I was pointing out. No need for recalcitrance.

That doesn’t make it correct. Perhaps he or she was confused with your question.
In any case, you don’t need a PhD to calculate this for you. If you know the machine has a 15 mm spot size, the average person has 1.7 m^2 of body surface area, and the rate of repetition for pulse firing is 1 Hz, then with simple unit conversions you can calculate how long it would take to theoretically lase your entire body. (Hint: 15 mm spot size refers to the diameter of the circle.)

Of course there are limits, but not at a fluence of 15 J/cm^2, 15 mm spot size, a pulse width on the order of ms, and 2 Hz repetitions. The solid state lasers in our lab can definitely do better, and they are by no means state-of-the-art. But specs of those lasers are not really necessary for LHR. (By the way, BMW does the same thing with their cars in many countries: the speed is electronically limited to something like 155mph, even though they are capable of going much faster :frowning: )

In any case, fluence and spot size should be the only parameters that limit efficacy. Instead of operating in CW mode on the Apogee MPX, one could manually fire each pulse at the largest spot size and fluence. Doing so would not come close to the maximum repetition rate, and would not significantly increase the total treatment time.

All I’m saying is I’ve been told by a few people as well as my technician that it’s a big difference in treatment time. When I was treated at 18mm, 10ms, 2Hz, 15 joules which is the max joules for that rate, I was told that they won’t use 22 joules because the repetition rate required to hit 22 joules would make the treatment take way too long and that we don’t want to be there all day doing one treatment.

I realize you were told that, but I was trying to point out to you why the claims of lengthy treatment time at a lower repetition don’t make sense. I’m a research physician scientist, so my only interest here is to help people educate themselves–especially when given dubious claims.

I’ve already explained why you can’t be treated at 22 joules given the other parameters. Your clinic should have adjusted the repetition rate to 1.5 or 1.0 Hz. Other Alex lasers such as GentleLase treat in this range, and treatments with them certainly do not take all day.

It sounds like unless you wanted treatment with the dual Alex/Yag mode on the MPX, another Alex laser or a different clinic would have been a better choice.

Just curious: Were you happy with the outcome of the treatments with the MPX? I know people who got great results with the dual mode.