# Explanation of laser intensity

I've become interested in lasers and this question came to me today. Let's say we have two 10 mW lasers. One has a spot size of 1mm and one has a spot size of 10mm. Therefore, the first laser has a greater intensity. In classical physics we are told the wave amplitude increases. In modern physics we have more photons. Do either of these concepts relate to spot size (cross sectional area of a laser beam)? Also, what determines the spot size of a laser from a functional point of view? - Thanks

Andy Resnick
Since the lasers have equal power output, increasing the beam diameter decreases the intensity (units [W/m^2]). Note that in photometry, the quantity of [W/m^2] is *irradiance* (or emittance, or exitance) while 'intensity' is [W/sr].

Laser beams are often approximated as "Gaussian beams", and so the minimum spot size is inversely proportional to the beam spread angle (divergence). I'm not sure what sets the beam diameter at the laser aperture, but it may be related to the resonator geometry and manufacturing constraints.

Since the lasers have equal power output, increasing the beam diameter decreases the intensity (units [W/m^2]). Note that in photometry, the quantity of [W/m^2] is *irradiance* (or emittance, or exitance) while 'intensity' is [W/sr].

Yes, I realize this. Let me clarify as my title was misleading. I interpret light as a particle traveling in the x-direction yet oscillating in the y-direction. The "particle" hits within an area of known probability. Do this thousands of times and you have a dot, right? I was wondering if this had anything to do with spot size.

Andy Resnick