Page 342 - Blog Posts v1

E56 - How many photons are in a laser pulse?
Have you ever wondered how many photons you generate each time you fire a pulse of laser energy? I have!
Fortunately, I can work it out because I'm a physicist. It's the kind of thing us physicists do!! It keeps us off the streets at night......
So, to calculate this number we need to know the energy of each individual photon. This is quite easy to do using a formula from quantum physics:
E= h f
where 'E' is the photon energy, 'h' is Planck's constant and 'f' is the frequency of the light. Max Planck was a very clever physicist who helped to develop quantum physics back in the early 1900s. Using the above simple formula we can calculate the energy of a photon of any wavelength (remember, wavelength = the speed of light / frequency).
So, below I show the energy of photons of a number of wavelengths typically used in dermatology lasers:
Wavelength (nm)
532 - Nd:YAG, 2nd harmonic 585 - pulsed dye laser 694 - ruby laser
755 - alex laser
1064 - Nd:YAG
Photon energy (joules)
3.734 x 10-19 3.396 x 10-19 2.862 x 10-19 2.631 x 10-19 1.867 x 10-19
Notice that the individual photon energy falls as they become 'redder'!
Now, we simply divide the number of photons (above) into the pulse energy used in a treatment. Let's say that the pulse energy is 1 joule; so the number of photons for each wavelength is:
Wavelength (nm)
532
585
694
755
1064
Number of photons
2.6781 x 1018 2.9449 x 1018 3.4936 x 1018
3.8007 x 1018 5.3562 x 1018
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