How much power does a laser need before you feel heat on your skin from it?

As mentioned 2 days ago, I now have the Amazon laser 3 pack, including:

1. 405nm laser of estimated 30-40mW or so(estimated by members here in reviews).
2. 532nm laser estimated at 20-30mW here
3. 650nm laser estimated at <>100mW

Now I know that divergence will play a role in this, so I've measured the diameter of each laser at 2 feet(24 inches). Here's the list:

1. 405nm- 6.5mm diameter beam at 2 ft
2. 532nm- 3.5mm at 2 ft
3. 650nm- 4.5mm at 2 ft

The red and green lasers seem pretty tight, but the 405 violet laser is far from tight. at about 10 feet distance, its beam is already out to about 25mm diameter!

Now, I still dont have my safety glasses yet, but I dont mess with the lasers indoors hardly at all, and dont shine them on reflective surfaces. I mainly use them outdoors at night, and I dont shine them off anything reflective thats closer than 100 feet or so, a stop sign. By the way, at about 100 ft, the 405nm laser covers the whole stop sign!

So anyway, those specs now listed, I have tried to shine each of these lasers onto my arm. At first I did it only for less than 1/2 second to be safe. Then I gradually tried it for longer amounts of time, til it became obvious that no matter how long I shine any of them onto my arm, I cannot feel ANY heat.

Assuming that 2 of these 3 lasers have decent beams, with minimal divergence, what does that tell you about these lasers and their supposed power levels. They were 'advertised' at <5mW, but they are clearly above 5mW, but maybe not by much......:cryyy:

How much wattage(or milliwattage) does a laser need before you can actually feel heat from it, if its beam stays together well, like 2 of these 3 seem to do?:thinking:

I assume that none of these 3 will pop even a black balloon..... I tried to heat up a section of a black plastic bag, but felt no heat, no hole melted through.

well at 30-40 mW it will not be even close to generating heat, even in dark objects. the red might, but it's probs not even that powerful. I believe it has to get over 100mW to generate heat, and even then only in dark objects.

It depends on your skin coloration and thermal sensitivity and the energy density of the beam (power x diameter).

As little as 25mW can be felt on the skin by most people if the wavelength is one that is heavily absorbed (such as 405nm or 532nm) and the energy density is high enough (100um spot).

All light striking matter generates heat, how do you think thermopile LPMs work? They can feel even 1mW of light striking them. It is all a matter of sensitivity and energy density.

What the nerve endings in the skin measure is temperature differentials - that's why we feel comfortable on a 60F day and on an 80F day so when you apply heat to your arm, you will only sense that heat if you put more heat in than the body can absorb and pull away from the area.

It's like soldering with too small an iron - the metal absorbs the heat of the iron and so doesn't get hot enough for the solder to melt.

So your question doesn't have an accurate answer - if the beam is highly focused into one spot it would need less power than if it covered a wide area. Wavelength changes the equation too.

It is also a matter of power density indeed, perhaps even more so then of total power. If you have a narrow beam (say 1-2mm diameter) you can feel a 532 nm laser sometimes on a sensitive area like the back of the hand.

Red lasers arent felt that easily at all since most of the energy goes through the skin (in white people) and is aborbed in deeper tissues that are quickly cooled down by blood circulation.

I was taking pics of a 200 mW single mode 445 and didn't notice that I had moved far enough into the beam that it was focused on my hand. I FELT that. It didn't blister, but it stung good!

It really just boils down to wavelength... I've shown a 200 mW red at my hand, and felt nothing.

Shine a 100 mW 405 nm at my hand, that friggin hurts.

Shine a 1 watt 445 nm unfocused at my hand that really really hurts.

It's just what's more readily absorbed. The lower the wavelength the more energy from the laser is absorbed. It's why I can burn a white piece of paper with 200 mW of purple, but get absolutely nothing with 200 mW of red.

200mW for sore does something on my hand. Unfocused it is just warm, but when focused for burning it simply .. burns ..

I burned my hand with my 333mw laser (unfocused) and it hurt like hell. Try and keep your hands away from the beam lol.

300mw focused red on a white person with usually only sting if you get the root of a hair follicle, or a freckle. My friend who is black said he felt the sting instantly on his skin just. in my experience, the lower the nm, the easier it is to burn anything, including humans. I have a pretty wicked burn on my leg from my primary tests with high power lasers(i know MUCH MUCH better now). A 1W 445 focused will burn a hole through skin into muscle about 20mm in a minute, and after it heals you will not have feeling where the scar is for years to come. i also had a wickedlasers E3 "200mw" and the unfocused beam just felt warm like sunlight on more sensitive areas like finger tips or lips (no i do not mean finger, tips , or lips). Lower wavelength+ higher beam density(assuming the power is the same) = better burning

bdgreenb said:

I have a pretty wicked burn on my leg from my primary tests with high power lasers(i know MUCH MUCH better now). A 1W 445 focused will burn a hole through skin into muscle about 20mm in a minute, and after it heals you will not have feeling where the scar is for years to come.

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This sounds REALLY interesting. Was alcohol involved?

tsteele93 said:

This sounds REALLY interesting. Was alcohol involved?

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Must've been, burning down 20mm through your skin would be incredibly painful I would think, so it's either alcohol or Novocaine in that spot!

correct, Alcohol in my mouth, once you get through the initial layers of skin the laser actually kills the nerves before you really feel anything, it's more itchy than painful. zero blood, much like a lightsaber wound, it instantly cauterizes, as to be expected.