The unit of intensity or the irradiance of the light is $\mathrm{W/m^2}$. So what's the intensity of a light pointer?

Searching on the web, the laser pointer's power is about $5\mathrm{mW}$, imagine the spot is $1\mathrm{mm^2}$, then the power is about $5\mathrm{kW}/m^2$, the same magnitude as sunlight on earth.

I read a paper on floquet topological phase, the light intensity required is about $10^{17}\mathrm{W/m^2}$, I can't imagine how strong this light intensity is. Please give some real physical situation where this strong light intensity can occur.

I would assume this strong light intensity would "destroy" any material, is it true? If just shed the light for a femtosecond, I believe the material will still be OK.

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    $\begingroup$ Solar constant on Earth is about 1kW/m^2. You can focus that laser on a spot of less than (100um)^2, so the laser pointer would get you easily to 1MW/m^2 (do not point at your eyes with a 5mW laser!). If you want much higher, you can use a 1J laser focused on approx. 10um^2 with a picosecond pulse. That's 1e23W/m^2. $\endgroup$
    – CuriousOne
    Jan 6, 2015 at 12:32
  • $\begingroup$ Really depends on the pointer. We have one at work that is in the "hazard to aircraft" category. For a couple of hundred dollars you can get a handheld pointer that is more like a small lightsaber. $\endgroup$
    – paul
    Jan 6, 2015 at 12:47
  • $\begingroup$ @CuriousOne Is your 1J laser experimentally realizable? Will it "burn" the material a hole? $\endgroup$ Jan 6, 2015 at 13:33
  • $\begingroup$ That's a tabletop laser these days. Yes, it will "burn". :-) $\endgroup$
    – CuriousOne
    Jan 6, 2015 at 13:38

2 Answers 2


The system with the greatest intensity I know of is at the National Ignition Facility. This generates a peak power of around 500TW and I think the target area is around 10 mm$^2$ (I can't find a detailed description of the target). That means the power density is around $5 \times 10^{19}$ Watts per square metre, but allow a factor of ten either side due to the uncertainty in the target size.

The effect of this high power density is to compress and heat deuterium and tritium until they undergo nuclear fusion.

  • $\begingroup$ What do you think such a high intensity $10^{17}$ will do when shed on the carbon? I don't know how short the laser pulse can be, but theoretically can you estimate the largest time before doing damage to the carbon. $\endgroup$ Jan 13, 2015 at 6:45

Power is energy per unit time, so in the case of the laser it would be the rate that the photon energy leaves the laser. The intensity is power per unit area and would be the power divided by the aperture area. So if the aperture is $1 mm^2$ then you are correct. This is why laser pointers warn against pointing them at eyes.

Intensity is a density function. Just because the sun may have the same intensity as a laser doesn't mean the total power is the same. Spread the laser light across half the surface of the earth and you'll see and feel nothing, but concentrate just a few square cm of sunlight to the size of a laser beam using a magnifying glass and you can burn holes in things. Try to burn a hole with a laser pointer and you'll just be staring at a colored dot for a really long time.


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