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I have a 808nm laser diode with a spot size of 0.4mm and output power = 400mW, Can it burn the skin or hair if the exposure duration = 10ms?

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  • $\begingroup$ To put this into perspective, this would be the equivalent of about $15$ infrared remote LEDs. I'm not an expert, but I seriously doubt this could cause any harm unless shone in the eye. Personally, I would be comfortable shining this on my skin for a few seconds and just seeing if I can feel the heat. But you do so at your own risk. $\endgroup$ – Myridium Jun 4 '17 at 9:39
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    $\begingroup$ In case there is a misconception, it is ultraviolet light which causes sunburn. Infrared light results in a more conventional heating which can then burn if the temperature exceeds safe limits. $\endgroup$ – Myridium Jun 4 '17 at 9:46
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As the comment above states at approx 1um we only need to consider power, not any chemical bond breaking.

The sun's irradiance is around 1300 $W/m^2$, or 1.3m $W/mm^2$.

Your spot is approx 1/5 $mm^2$ so sunlight would give you perhaps 0.2mW, you are supplying 2000x as much power/area as simply being outside.

Another way of estimating this, assume you can concentrate the image of the sun into a 1mm spot using a magnifying glass. Then you would need to collect 400/1.3 = 300 $mm^2$ of sunlight to get the same power/mm on your skin. 300 $mm^2$ is only a circle of radius 10 mm, so an ideal magnifying glass only an inch across could give the same effect.

You have probably experienced burning yourself with a magnifying glass like this (certainly if you have an older brother)

BUT the thing I would worry about is what happens when you point this into your eye / near your eye / have a reflection from the INVISIBLE beam anywhere near your eye - 400mW will blind you instantly.

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The blunt answer is: according to ISO60825 "Safety of Laser Products", 400mW in a 0.4mm spotsize for 10ms is highly hazardous to skin as well as being utterly ruinous to eyesight (see the end of my answer for calculations).

For any question about laser safetly always consult with your organization's laser safety officer!

Most universities have such a person. If you work with a private organization, then you need to have such a person. You are talking about an ISO60825 Class 4 laser that will utterly destroy the sight of any eye it enters even fleetingly and will burn skin. If your organization does not have a laser safety officer when the laser you cite is being used then your management is certifiably insane. By a "laser safety officer", I mean either a fulltime position, or someone who fully understands ISO60825 and how to put all its requirements in place properly. Any competent physicist or optical engineer won't do - you need specific training in the standard because its language and conventions are very foreign to most physicists and even most optical people - it's written in the conventions of radiometry which are highly specialized and arcane. When I - who have studied optics most of my professional life - was asked to be a laser safety officer for my employer ten years ago I found the standard completely incomprehensible and immediately signed myself up for training with the appropriate regulatory body.

Below is the relevant Maximum Permissable Exposure (MPE) table (for skin - eye MPEs are MUCH lower) from ISO60825

ISO60825 Table

whereas below is the table of constants needed to interpret the skin MPEs:

MPE Constants

From the MPE table, the safe limit for 10ms exposure is $MPE = 1.1\times10^4\, C_4\,t^{\frac{1}{4}}{\rm J\,m^{-2}}$. The correction factor $C_4 = 10^{0.002\,(\lambda-700)}$ equals 1.64. Therefore, the MPE is $5.7{\rm kJ\,m^{-2}}$.

However, over an interval of 0.01s, your laser delivers roughly (depending on the exact definition of your beam diameter) $0.01{\rm s} \times 0.4{\rm W} / \left(\pi\times(0.4\times 10^{-3})^2\,{\rm m^2}\right)=8{\rm kJ\,m^{-2}}$. So this is above the MPE.

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  • $\begingroup$ @RodVance: You have taken the power in the final calculation as 400W instead of 400mW and the laser radius as 0.4mm but this is the diameter. With these corrections, the laser is delivering ~32kJ/m^2 which is still higher than 5.7kJ/m^2. If I reduce the exposure duration to 1ms, then the MPE = 2.99kJ/m^2, and our laser will deliver ~3kJ/m^2. Is this safe then? $\endgroup$ – Shil Jun 5 '17 at 8:04
  • $\begingroup$ @Shil Whoops: however, you are still over the 5.7kJ / m^2 threshold. The laser is still not skin safe. $\endgroup$ – Selene Routley Jun 5 '17 at 8:42
  • $\begingroup$ @RodVance: But if we are reducing the exposure duration from 10ms to 1ms, then our new MPE=2.99kJ/m^2 and our laser is delivering 3kJ/m^2. So it should be safe now? $\endgroup$ – Shil Jun 5 '17 at 12:31
  • $\begingroup$ @Shil You're right on the edge there. You need to check your definitions of spotsizes carefully: are you using $1/e$, $1/e^2$ intensity definitions or something else. Generally, one usually adds margin to account for these uncertainties. Given you're so close to the limit, I'd be measuring the beam profile carefully. Then you need to get a laser safety officer to recommend a pulse duration: without seeing the setup and making precise measurements, noone can describe its safety accurately $\endgroup$ – Selene Routley Jun 5 '17 at 12:56
  • $\begingroup$ @RodVance: The source is a fiber coupled source. Therefore, the beam would is not a gaussian beam but would rather be a speckle or top-hat. In our case, we are measuring the total power using a power meter and not using any beam profiler. Is there any problem with this? $\endgroup$ – Shil Jun 6 '17 at 11:02

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